Design of Small Diameter Frictional Piles and Cases Study

Information Paper

Design of Small Diameter Frictional Piles and Cases Study

STRUCTURAL ENGINEERING BRANCH ARCHITECTURAL SERVICES DEPARTMENT March 2013

Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

Page 1 of 71

File Code: Friction Pil Piles.doc es.doc CTW/MKL/CYK/KWK/LPL Issue/Revision Issue/Revisi on Date : March 2013

CONTENTS Content Page 1.

Objectives............................................. ................................................................... ............................................ .......................................... .................... 3

2.

Background ............................................ .................................................................. ............................................ ...................................... ................ 4

3.

Load-Carrying Load-Carryi ng Capacity of Frictional Piles - a Summary .............................6

4.

I n-Situ n-Situ Measurements in ArchSD Projects .....................................................30

5.

.................................................................. ........................................... ..................... 48 Summary of Findings ............................................

6.

Loading Tests ............................................ ................................................................... ............................................. ................................ .......... 50

7.

Pile Group Settlement ........................................... ................................................................. ........................................... ..................... 51

8.

Method of Procurement ........................................... ................................................................. ........................................ .................. 53

References

Annex A

Estimation of the Length of Piles

Annex B

Sample Particular Specification for Design and Construction Construction of Frictional Mini-Piles


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1.

Objectives

1.1

Traditional small diameter frictional piles, which are usually drilled and cast in situ piles, are especially suited as foundations for sites with difficult access, congested or where minimal disturbance to the existing structure is required. Though Clause 5.3.2 of Code of Practice for Foundations 2004 (“ Foundations Code 2004”) 2004 ”) issued by Buildings Department states that the allowable bearing capacity for such non-driven piles may be determined by the allowable bearing pressure and bond or frictional resistance of the ground, it further states that unless for piles socketted into rock, the load-carrying capacity of the piles should not be derived from a combination of the shaft resistance and end bearing resistance of the piles unless it is justified that the settlements under working load conditions are acceptable and adequate to mobilise the required shaft resistance and end bearing resistance of the piles simultaneously. As such, traditional mini-piles in Hong Kong are designed to be socketted into rock, and their allowable capacity is derived solely from the average bond strength between the grout and rock. An average bond strength is usually adopted for piles socketted into rock, and the study carried out by the University of Hong Kong (Department of Civil Engineering 2009) confirmed that most of the axial load transmitted to the rock socket is dissipated dis sipated at the top portion of the socket.

1.2

This Information Paper, besides reviewing the design and construction of different types of small diameter frictional piles, introduces a non-traditional piling system – the “frictional mini-piles mini-piles””. All such small diameter frictional piles, unlike traditional mini-piles, derive its load-carrying capacity from shaft friction from the soil. The frictional mini-piles are constructed constructed with steel Isection or a group of reinforcement bars in a pre-bored hole with a temporary steel casing and then injected with cement grout. Similar type of frictional piles, constructed with steel I-section in a pre-bored hole by continuous flight augering (CFA) and then injected with cement grout, had successfully been employed in four ArchSD sites in the 1990s, including a site in Tung Chung, a site in Yuen Long Long and two sites in Ma On Shan. A distinctive feature of the frictional mini-piles is that the pre-bored hole, instead of forming by CFA, is constructed with Odex or similar method with a temporary steel casing. Recently, ArchSD has successfully employed such frictional mini-piles as foundations in two projects – one in Mid-Levels and the other in Central. Instrumented piles were also installed these two projects to monitor the stress distribution along the piles. This Information Paper provides: a) a literature review on the design of piles deriving their load-carrying capacity from shaft resistance from soil; b) summary of the results of instrumented piles in some ArchSD projects; c) the design shaft friction to be adopted for the design of the different types of small diameter frictional piles (including the frictional mini-piles); and d) a particular specification on the design and construction of the frictional mini-piles.


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2.

Background

2.1

Typical sub-soil sub-soil profile in Hong Hong Kong consists of a layer layer of loose fill overlying marine deposit and alluvium. Completely weathered soil then follows before reaching Grade III or better better bedrock. For low-rise development, shallow foundation in the form of of pad and/or raft footing is usually adopted. adopted. For medium-rise or high-rise development, founding the building on topmost fill or alluvium will result in excessive settlement of the building. building. Deep foundation in the form of piled foundation is therefore required.

2.2

Approved systems of piles in ArchSD can be classified into replacement piles or displacement piles. Replacement piles include non-percussion cast in-situ concrete piles (e.g. PIP piles); large diameter bored piles; pre-bored rocksocketted steel H-piles, barrette piles; mini-piles founding on bedrock; and hand-dug caissons (which has been banned) may only be used for public works under the conditions imposed by Works Branch Technical Circular No. 9/94 (available: www.devb.gov.hk/ www.devb.gov.hk/)). Displacement piles include: precast concrete piles; precast prestressed tubular piles (e.g. Daido, SS piles); driven steel H piles; and percussion cast in-situ concrete piles (e.g. Frankie piles, Vibro piles).

2.3

Among these approved systems systems of piles, driven steel H-piles H-piles (a small displacement piling system) are one of the most popular and economical piling options in Hong Kong due to the quick installation time and tidy site condition. However, noise and vibration are particular concerns for some sites, and for some sloping sites the driving operations also require the construction of heavy temporary platforms. Replacement non-percussion piling systems are then adopted. Common systems of non-percussion end-bearing piles include: large diameter bored piles, pre-bored rock-socketted steel H-piles, and mini-piles. In fact, these piles derive the resistance mainly from their end-bearing on hard stratum and partly from the shaft friction between the soil and the pile shaft. However, there are uncertainties on the shaft friction between the soil and the pile shaft. Furthermore, if the rock end bearing stratum is available at a reasonable depth, the shaft friction component is small when compared with the end bearing component in the overall load carrying capacity of a pile. As such, for traditional replacement non-percussion piles (e.g. pre-bored rock-socketted steel H-piles, mini-piles, or large diameter bored piles) the shaft friction component is usually neglected.

2.4

However, on some sites in Hong Kong (e.g. in Mid-Levels, Tung Chung, or some newly reclaimed land) sound bedrock can only be found at very deep level, e.g. more than 60m from the ground level. In such cases, a cost effective solution is to adopt replacement piles relying on the shaft friction component. Shaft friction can be developed after small relative displacements between the soil and the pile shaft though may reach ultimate shortly after that, and hence shaft friction component often contributes the bearing capacity in the working load situations.


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2.5

Besides PIP piles, frictional mini-piles have now been employed commonly in Hong Kong for those sites with bedrock at great depth. Frictional mini-piles are a hybrid system combining traditional mini-piles with replacement non percussion cast in-situ piles. Traditional mini-piles consist of a steel permanent permanent casing with internal diameter not greater than 400 mm, with a group of reinforcement bars in the middle as the load bearing element and the remaining cavity filled with cement grout. grout. They are required to be socketted into bedrock, and hence derive their load carrying capacity from end-bearing on the bedrock. The frictional mini-pile is constructed with steel I-section or a group of reinforcement bars in a pre-bored hole with a temporary steel casing and then injected with cement grout. The frictional mini-pile is not required to be socketted into bedrock, and as such it behaves similar to the other non percussion cast in-situ piles deriving their load carrying capacity from the shaft friction along the length of the piles. Moreover, steel casing is only temporarily provided for pre-drilling and will be removed during the subsequent grouting work.

2.6

The advantages of frictional mini-piles are that they are especially suited as foundations for loading that is not high and for the sites that are with difficult congested access (Figure 1), or requirements for minimal disturbance to the existing structure, or bedrock can only be found at very deep level.

Figure 1 Congested site with difficult access access


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3.

Load-Carrying Load-Carrying Capacity of Frictional Piles – Piles – a Summary

3.1

Figure 2(a) shows the forces acting upon an axially loaded pile. Figure 2(b) shows the typical relationship of shaft resistance R s and end bearing R b components of the pile founded founded on soil with the settlement of the pile. pile. In theory, by integrating the mobilised shaft friction fric tion f s over the surface of pile can give the total shaft resistance R s.

Figure 2(a) Stresses and and forces on an an axially loaded pile

Figure 2(b) Typical shaft resistance and end bearing versus displacement displaceme nt in a pile founded on soil


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Because relatively large displacements are required for piles founded on soil to mobilise the end bearing capacity in normal range of acceptable settlement criterion (Figure 2(b)), the ultimate bearing capacity of such a frictional pile may develop up to 80 – 90% of its capacity through shaft friction (Holt et al 1982, Kwok 1987). However, the design parameters for the shaft resistance along the length of pile also show great variation. The pile-soil interface shear friction, besides determined by the stress history of the soil, is affected by the following key parameters (Brown et al 2007): al 2007): a) the construction method; b) the shear displacement of the soil at the pile-soil interface; c) the in-situ soil properties (e.g. soil composition, water content, saturation, stiffness and strength); d) the concrete/grout properties (e.g. composition, viscosity, pressure, stiffness and strength). 3.2

Effect of construction methods on shaft friction

3.2.1 Among the parameters listed in the above paragraphs, construction method affects the shaft friction substantially. That is, despite of the same soil and grout, piles constructed with the different construction method can have significantly different shaft friction (Kay and Kalinowski 1997; Lo Lo and Li 2003). 2003). The construction method affects the relative volume of soil displaced in proportion of the pile volume, the magnitude of the increase in the effective horizontal stress at the pile-soil interface, the relative roughness of the pile-soil interface (Figure 3(a)), and the effective diameter of the pile (Brown et al 2007). In Hong Kong, pile driving by means of hydraulic hammer is commonly adopted to install displacement piles such as driven steel H-piles. For replacement piles, the following three methods are commonly employed to form the holes: a) for large diameter bored piles, various excavating tools such as grabs, chisels are used to form the holes within a temporary casing; b) for PIP piles, CFA is used to form the hole; c) for mini-piles or pre-bored rock-socketted steel H-piles, Odex method is usually used to form the holes.

(a) smooth interface (b) rough interface Figure 3(a) Roughness at pile-soil interface interface (Source: Source: Rollins et al 2005) al 2005)


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3.2.2 Besides the difference in the method to form the holes, Armour et al (2000), al (2000), as illustrated in Figure 3(b), classifies the different concreting/grouting operations for replacement piles. In Type Type A, concrete/grout concrete/grout is placed under gravity head head only. In Type B, grout is placed into the hole under pressure at around 0.5 to 1MPa as the temporary steel drill casing is withdrawn. In Type C, a two-step process of grouting is employed with cement grout placed under gravity head as with Type A and prior to hardening of the primary grout (after approximately 15 to 25 minutes), injection of grout via a sleeved grout pipe at a pressure of at least 1MPa. In Type D, a two-step process of grouting is employed similar to Type C with grout in the second step injected via a sleeved grout pipe at a pressure of 2 to 8MPa. A pair of double packers is usually used inside the sleeved pipe so that specific horizons can be treated several times. Among the different construction methods, it can be expected that the shaft friction increases from Type A to Type D construction.

Figure 3(b) Classification of construction construction method method for replacement replacement piles (Source: Source: modified from Armour et Armour et al 2000) al 2000)

3.3

Shaft friction for different types of frictional piles

3.3.1 Section 3.2 describes the differences in forming the hole and concreting/ grouting operations operations for installing replacement piles. It should be be noted that the different construction methods affect the effective horizontal stress on the pilesoil interface, and hence the shaft friction. Figure 4 summarises their loadsettlement behaviours of piles relying on pile-soil friction, including driven steel H-piles, augered piles and bored piles (O’Neill 2001). Two lines have been added to Figure 4 to show the expected load-settlement behaviour of frictional mini-piles with and without post-grouting along the pile shaft.


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Figure 4 Load-settlement Load-settlement graphs of common common frictional frictional piles (Source: Source: Modified from O’Neill 2001) 2001)

3.3.2 Driven piles and bored bored piles As expected, large displacement piles (e.g. (e. g. driven Daido piles) are of the highest shaft friction. During driving, the surrounding soil is displaced laterally, causing an increase in the effective horizontal stress (Brown et al 2007). For large diameter bored piles, the soil is removed by grabs, and there are temporary casing throughout throughout the installation. Concreting is of Type A (Figure 3(b)). Therefore, the effective horizontal stress at the pile-soil interface tends to reduce or remains unchanged during the construction (Brown et al 2007). al 2007). Hence shaft friction should be the lowest. The behaviour of small displacement driven piles (e.g. driven steel H-piles) will lie between that of bored piles and that of large displacement piles. 3.3.3 CFA piles For augered piles using CFA (e.g. PIP piles), the grouting operation is similar to Type A ( Figure 3(b) ). Yet, shaft friction for some augered piles (e.g. (e.g. displacement CFA piles (or termed “DD piles” in the US)) can achieve the highest shaft friction among the replacement piles (O’Neill (O’Neill 1994) if suitable plants are properly used. It is due to its distinct construction method of such DD piles, where a large diameter CFA with greater torque is used for forming the hole. The large displacement by the large diameter CFA with greater greater torque displaces the soil laterally, and therefore tend to increase the stresses in the surrounding soil (similar to driven piles). piles). However, in the traditional CFA pile construction, the shaft friction at the pile-soil interface is far more complicated than those of driven piles and large diameter bored piles. The continuous augering operation can maintain the effective horizontal stresses near the value Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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that existed before the installation, and may in some cases can achieve higher effective stresses than the preconstruction state. However, it should be noted that the rate of auger penetration is extremely important when installing augered piles (Brown 2005). Prolonged augering at one depth without penetration will flight the soil surrounding the CFA (a phenomenon called “Archimedes pump”), resulting in excessive removal of the surrounding soil by loosening and allowing the adjacent soil to fall into the hole. In such case, the soil surrounding the pile is decompressed and the effective horizontal stresses are then decreased, and thus the shaft shaft friction cannot achieve the the theoretical values. PSE should therefore note that though Figure 4 shows that augered piles can achieve the highest shaft friction among the replacement piles, in reality traditional CFA piles may not be able to achieve such high shaft friction and may sometimes be smaller than that for frictional mini-pile with post grouting. grouting. For details, PSE may refer SEB Information Paper Review of PAKT-IN-PLACE Piles Installation (available: http://asdiis/sebiis/2k/resource_centre/ http://asdiis/sebiis/2k/resource_centre/)). 3.3.4 Frictional mini-piles For the frictional mini-piles without post grouting along the pile shaft, the construction method lies between Type A and Type B ( Figure 3(b)). Their load-settlement behaviour is therefore expected to be better than (though close to) that of large diameter bored piles, as the grout filling the pre-bored holes is placed under a small pressure. Improvement of the shaft friction for the frictional mini-piles can be achieved by post grouting along the pile shaft, and the construction method will then follow Type C or D ( Figure 3(b)). The post grouting is expected to increase the diameter of the mini-piles and hence the effective horizontal stress so as to increase the shaft friction. However, it should be noted that pre-bored holes for frictional mini-piles in Hong Kong are usually formed by Odex method, and hence the densification effect is not predominant.


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3.4

Idealised pile-soil interface shear friction

3.4.1 Numerous studies (e.g. Misra and Chen 2004; Frizzi and Meyer 2000) have been carried out to study the relationship of shear friction fricti on with shear movement for the soil at pile-soil interface. Figure 5 shows one of such studies plotted in dimensionless axes of developed shear to ultimate shear resistance (f/f max max) against the shear displacement to diameter of the pile (W/D).

Figure 5 Relationship of shear shear resistance with shear shear displacement at at pileSource: modified from Frizzi and Meyer 2000) soil interface (Source:

3.4.2 Theoretical model of development of shaft friction with displacement Although the actual load-transfer mechanisms developed along the pile-soil interface are highly complicated (Mayne and Harris 1993; Paik et Paik et al 2003; al 2003; Yang et al 2006), an idealised model for the soil at the pile-soil interface is usually adopted (Misra and Chen 2004). Figure 6(a) plots the relationship between the idealised pile-soil interface shear friction  and shear movement u. In the idealised model,  is assumed to vary linearly with u in the elastic zone, and is then assumed to be constant once the soil movement exceeds critical shear displacement uc. Mirsa and Chen (2004) derive the following governing ODE for the shear displacement u(x) at a distance x from the pile tip in the pile of diameter D D and length l : 2

d u ( ) d 

2

  2 u ( )  0 for 0     e

2

and and

d u ( ) d 

2

  2 u c  0 for  e    1


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where

and

k

=

stiffness factor (Figure 6(a)),

λ

=

l

k m =  =  e =

k k m

,

axial stiffness of the pile, non-dimensionless length = x /l , transition from elastic to plastic zone = l e /l ( /l (Figure 7)

Misra and Chen (2004) then give the general solutions to this governing ODE with the following shear displacement u( )along )along the pile shaft with an applied axial load of P : P : P cosh  elastic zone: for 0    1 u     P u  sinh  elasto-plastic zone:

u

where  = and P u =

P

cosh 

P u

 sinh 

for 0     e

 P  1   e2   1     e   2 for  e    1 2   P u   1 2  P  1   1 for 0    1 u            2   P  2   u    u

and plastic zone:

   

   

1



2

P ul/k m, ultimate load carrying capacity of the pile =  Dl   c.

The location of transition from elastic to plastic zones  e at a given load P can be solved from the following equation: equation: tanh  e 1 P  e  1    0 cosh  e  P u For the value of uc, Figure 6(b) shows the summary of Luo et al (2000) al (2000) of the test results by Cartier and Gigan (1983), Lim and Tan (1983), Murray et al (1980), Billam (1972), Chang et al (1977), al (1977), and Taylor (1948). Their summary indicates that, for silty soil, uc lies between 0.8mm to 5.6mm, and 2.5-5.6mm is the mode. Thus, a shear movement movement of around 3 to 6mm may already cause full mobilisation of the pile-soil interface friction.

Figure 6(a) 6(a) Idealised pile-soil pile-soil interface shear friction with shear displacement (Source: Source: Misra and Chen 2004)


Figure 6(b) Frequency distribution table of the test data of uc (Source: Source: Luo et al 2000) al 2000)

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3.4.3 Zoning in a frictional pile When a frictional pile is subjected to vertical load, the movement at the pile head will be larger and the movement decreases along the length of the pile. Figure 7 shows the variation of shaft friction along a pile in a homogenous uniform soil medium using the idealised model (Wong 2003; Misra et al 2004). al 2004). When the pile is in elasto-plastic stage, the shaft friction along the length may be idealised into two zones: plastic and elastic (Figure 7) (Misra and Chen 2004), depending depending the shear movement at the pile-soil interface. Elastic zone is at the lower portion of the pile where the shear displacement at the interface is still less than uc. Plastic zone occurs at the top portion of the piles where the shear displacement at the interface exceeds uc. Unlike an end-bearing pile, not the whole length of the pile (especially the portion near the pile tip) for a frictional pile will be mobilised.

Figure 7 Zoning of soil soil along a frictional pile (Source: Source: modified from Misra et al 2004) al 2004) Figure 8 predicts the load-settlement curve in a black line of a frictional pile obtained from the above general solution against the actual load-settlement of three mini-piles measured by Misra and Chen (2004). The elastic, elasto-plastic and plastic behaviour at increasing shear displace ment are clearly shown.

Figure 8 Calculated shear shear displacement against against measured measured displacement (Source: Source: Misra and Chen 2004) Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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3.5

Methods to calculate shaft friction The mobilised unit shaft friction (f s) along the pile shaft is theoretically determined by the sum of pile to soil cohesion and friction components in the following equation: f s = ca + σ’h tan tans where ca and s are respectively the adhesion and friction parameters between the soil and the pile shaft, and σ’h is the effective horizontal stress due to overburden. Numerous theoretical methods (e.g. Nordlund method (1963), αmethod (Tomlinson 1971), -method (Burland 1973; Fellenius 1991), Nottingham and Schmertmann CPT method (1975, 1978), method based on SPT-N values (Meyerhof 1976)) have then been developed to compute the shaft resistance along a pile shaft.

3.6

α (total stress) method α (total stress) method suggests that the ultimate capacity of the pile is be determined from the undrained shear strength (c u) of the cohesive soil (Tomlinson 1971). This method further assumes that the shaft resistance is independent of the effective overburden pressure, and the unit shaft resistance f s is therefore given by the following equation: f s = ca = αcu where α is an empirical empirical adhesion factor to relate the average undrained shear strength along the pile length. The factor α depends on the nature and strength of the cohesive soil, pile dimensions, method of installation, and time effects. Typical values of α range from 1.0 for soft clay to 0.30 for very stiff clays (Kulhawy and Jackson 1989). α method, however, assumes slow dissipation of water and is therefore not applicable in most of of the soils in HK. Fellenius (2011) further commented that the load transfer between a pile and the soil is governed by effective stress rather than the undrained shear strength. In Hong Kong, GEO Publication No. 1/2006 - Foundation Design and Construction (GEO 2006) published by the Geotechnical Engineering Office therefore recommends the use of either -method or method based on SPT- N SPT- N values (“N-value (“N-value method”), method”), which are applicable to both cohesive and cohesionless soil.

3.7

 (effective stress) method

3.7.1  (effective stress) method models the long-term drained shear strength conditions of piles using the effective stress, and the ultimate unit shaft resistance f s is calculated by Coulomb’s friction law using the following equation: f s =  σ’v where  (a dimensionless coefficient) = K stan tan, σ’v = average effective overburden pressure along the pile shaft, K s = lateral earth pressure coefficient, and  = friction angle between the soil and the pile shaft. Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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The ultimate unit shaft resistance f s is limited to 200kPa (O’Neill ( O’Neill and Reese 1999), whilst GEO (2006) recommends a limit of 150kPa unless a higher value is substantiated by site specified tests. 3.7.2 Table 1(a) summarises the typical values of  obtained from back analysis of field test data for different methods of installation and types of pile given in GEO (2006). The range of  given is similar to those specified in overseas research, which gives a minimum of 0.15 (Fellenius 2011) and a maximum value of 1.20 (O’Neill and Reese 1999 ; Caltrans 2008). Besides summarising the back analysis of field test data, GEO (2006) also gives the field data of each site in its Appendix, and it should, however, be noted that the test data ( Figure 9) show very high variability. Besides the construction method,  also depends on the types of soil. Fellenius (2011) gives approximate range of values of  (Table 1(b)) for different types of soil; but cautions that the values of  can “deviate significantly from [those] values.” Table 1(a) 1(a) Typical values values of of for different types of piles Pile Type Soil Type

Small displacement driven piles (e.g. driven steel H pile) Large displacement driven piles (e.g. driven Daido SS piles)

CWG

0.1 - 0.4

Loose to medium dense sand

0.1 - 0.5

CWG

0.8 - 1.2


0.2 - 1.5

CWG

0.1 - 0.6


0.2 - 0.6

CWG

0.2 – 0.2 – 1.2 1.2

Large diameter bored piles Shaft-grouted bored piles

(Source: Source: GEO 2006) Table 1(b) 1(b) Typical values values of for different types of soil Soil Type 0.15 - 0.35 Clay (25o    30o) o o 0.25 - 0.50 Silt (28    34 ) o o 0.30 - 0.90 Sand (32    40 ) o o 0.35 - 0.80 Gravel (35    45 ) (Source: Source: Fellenius 2011)


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(a)

(b)

Figure 9

Replacement piles without shaft grouting

Replacement piles with shaft grouting

(c) Displacement piles 1 values for piles installed in saprolites in Hong Kong (Source: Source: GEO 2006)

1

GEO (2006) (2006) defines “saprolite” as “mass that retains the original texture, fabric and structure of the parent rock”. In Hong Kong, saprolite may be used interchangea bly interchangea bly with “decomposed granite” (Lo and Li 2003). Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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3.7.3 The following two approaches are usually used to evaluate the value of β adopted in Drilled in Drilled Shafts: Construction Procedures and LRFD Design Methods Me thods issued by the US Federal Highway Administration (Brown et al 2010): al 2010): 1) 2)

the depth-dependent depth-dependent β method, which establishes an empirical relationship of β versus depth is determined from field load tests; and the rational method based on soil mechanics theory, which expresses β in terms of K s and δ.

3.7.4 Depth-dependent Depth-dependent β method The depth-dependent depth-dependent β method was first introduced in 1978, and was claimed to provide conservative estimates of side resistance given the uncertainties associated with construction effects (O’Neill and Reese 1978; 1978 ; Brown et al 2010). Since then, there have been a lot of field tests confirming its applicability. Kulhaway and Chen (2007) calculated the values of  from the available field data for replacement piles piles in gravels and cobble soils. Their results are shown in Figure 10. Figure 11(a) shows the variation of  of  summarised by Rollins et al (2005) al (2005) together with two lines inserted by Fellenius (2011) for piles in sand the values recommended by CFEM (1992) and GEO (2006). All results show that peak value of  occurs at the pile head decreasing with depth, corresponding to the larger shear displacement at the top portion of the pile as predicted in Section 3.4.

Figure 10 Variation of for replacement piles in cohesionless soil (Source: Source: Kulhaway and Chen 2007)


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Figure 11(a) Variation of for replacement piles in sand (Source: Source: Fellenius 2011)

Design and Construction of Continuous Flight Auger (CFA) Piles published by the US Federal Highway Administration (Brown et al 2007) recommends the use of the depth-dependent depth-dependent β method of either O’Neill either O’Neill and Reese (1999) method or Coleman and Arcement (2002) method. O’Neill and Reese (1999) method was based on a design trend line for  related to the depth of the soil layer by data fitting. As  also depends on the types of soil, the method tries to relate the types of soil to their standard penetration test blowcount (SPT-N values) by scaling down  by the ratio of N/15 for loose sand layers with N 15. The following equations for calculating the value of  at depth z (in m) from the ground level are derived: 0.5

sand with N N15:

= 1.5 – 1.5 – 0.245 0.245 × z

soil with N N15:

=

N 15

0.5

×{1.5 – ×{1.5 – 0.245× 0.245× z 

Coleman and Arcement (2002) method is derived from loading tests on a number of CFA piles in mixed soil conditions consisting of alluvial, loessial deposits and inter-bedded sands and clays in Mississippi and Louisiana, the US, and the following set of equations for for calculating the value of  at depth z (in m) from the ground level as follows: -0.67

silty soils:

= 2.27× z

sandy soils:

= 10.72 × z


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-1.3


Figure 11(b) shows the plot by Coleman and Arcement (2002) comparing their derived equations equations against that by O’Neill and Reese (1999) for sand with their data for silts and clays. The main difference between those of O’Neill and Reese (1999) and Coleman and Arcement (2002) is that  in the latter equations has larger value at the pile head, which decreases rapidly with depth.

Figure 11(b) Comparison of O’Neill and Reese (1999) method with Coleman and Arcement (2000) method (Source: Source: Coleman and Arcement 2002)

Using these equations, Figure 12 shows the variation of  with depth for homogenous soil profiles with N N 15. It shows that  will be much greater for the top soil, and will approach zero at depth around 30m. Brown et al (2010) account for the variation of  with depth by arguing that the values of K s are higher near the surface, where many soil deposits are overconsolidated as a result of burial, erosion, fluctuations in the water table, capillary rise, desiccation, etc. Brown et al (2010) al (2010) argue that the effect of preconsolidation is to increase the in-situ horizontal stress and, ther efore, efore, β, and with with increasing depth, most soil deposits trend toward a normally consolidated state, a lower value of K s and therefore a lower value of β. However, the validity of such argument is to be further substantiated, especially the fact that the topmost soil in Hong Kong is relatively loose and is seldom overconsolidated, and that though there is preconsolidation effect due to fluctuation in the water table etc, the increase in effective vertical stress would not be significant. On the other hand, the variation of  along the length of the pile tallies with the zoning soil profile in Figure 7. Hence, this Information Paper suggests that the variation of  with depth may be due to the different relative pile movement at the pile-soil interface along the length of the pile. That is, soil at the top portion of the pile will become fully mobilised with larger relative pile movement, and it is difficult to mobilise soil fully at lower portion.


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Figure 12 Variation of with depth in a homogenous uniform soil with N 15

3.7.5 Rational method , being the lumped shaft friction parameter including K s and , is a function of the soil strength, soil stress state and its change, and the soil-shaft interface characteristics (Kulhawy and Chen 2007). The rational method aims at evaluating separately the parameters the parameters that t hat are ar e lumped into β using theory of soil mechanics. Firstly, the friction angle between the soil and the pile shaft  is related to the friction angle  of the soil. Kulhawy (1991) (1991) found that for cast in situ concrete piles with good construction techniques, a rough interface can develop, giving /  equal to 1.0; but with poor slurry construction this ratio can be 0.8 or lower. Secondly, K s is found to depend on soil displacement, pile installation method, pile geometry, and the stress changes caused by construction, loading, and desiccation. Analysis of field load tests has shown that K s can range from about 0.1 to over 5 (Kulhawy and Mayne 1990). Kulhawy (1991) reported his study of the effect of installation installa tion methods on a number of piles, and found that t he values of K s related their values to the effect with respect to Rankine at rest earth pressure K o within the ratio of 0.67 and 1. Their findings findings are as summarized in Table 2. The range shows that the shaft friction developed in large displacement driven piles is the highest, followed by small displacement driven piles, and the lowest shaft friction should s hould be that of large lar ge diameter bored piles. piles . Their findings tally with the load-settlement behaviours for these types of piles in Figure 4. The results further shows shows that the soil at the pile-soil interface for Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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large displacement driven piles behaves in passive stage and K s approaches Rankine passive earth pressure coefficient K p, and that for large diameter bored piles behaves in active stage and K s approaches Rankine active earth pressure coefficient K a (Vesic 1977; Rollins et al 2005). al 2005). Table 2 K s/K o for different types of piles Pile Type Small displacement (e.g. driven H-pile) Large displacement (e.g. driven Daido SS piles) Drilled shafted piles (e.g. large diameter bored piles) (Source: Source: modified from Kulhawy 1991)

K s/ K o 0.7-1.2 1.0-2.0 0.67-1.0

It was also in the past limited by the lack of data from load tests to correlate β with parameters, such as K s and δ. Given the uncertainties and difficulties in relating K s and  with K o and  respectively, recent studies as published in FHWA (Brown et al 2010) and Kulhawy and Chen (2007) have tried to relate them with SPT-N value of the soil. The correlation of  and N value is a common practice in Hong Kong and presumably the US as well. The following set of equations is then proposed to calculate the value of β of β (Brown et al 2010): al 2010): ≈  = 27.5 + 9.2 log N 60 sin  σ' p    tan   K p tan  β  (1  sin  ) σ'  v  where ’ p is the effective vertical preconsolidation stress = 0.47×pa× N600.8 (for silty sands) or 0.15×p a× N60 (for gravelly soils); N60 is the SPT-N value corrected for field procedures and apparatus ;2 and pa = atmospheric pressure. Brown et al (2010) reported that in-situ tests have shown generally good agreement with these correlations. However, factors such as cementation, aging, structuring, desiccation, etc that may affect K s and δ have not been accounted in the above equation. The rational method, though is better than the depth- dependent β method from a soil mechanics perspective, has not addressed the relative pile movement between pile and soil. 3.7.6 Applicability of  of  method Section 3.1 has identified the key parameters affecting the shaft friction, which include construction method (including the workmanship), the shear displacement at the pile-soil interface, site specific soil properties and the 2

N60 assumes that the SPT hammer has about 60% efficiency, and to convert the measured blowcount SPT-N value to N 60, the equation N 60 = N×C N×CE×CB×CR ×CS×CA×CBF×CC, where C N = overburden correction factor; C E = energy correction factor; C B = borehole diameter correction factor; C R = rod length correction factor; C S = sampling method correction factor; C A = anvil correction factor; C BF = blow count frequency correction factor; and C C = hammer cushion correction factor. For details, see Skempton (1986). Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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concrete/grout properties. Two methods – depth-dependent depth-dependent β and rational – have been used to evaluate the values of β. The equations based on the depthdependent β method only take into account for its variation with the depth (the variation presumably due to the different shear displacement along the pile shaft). The effect of other site specific soil properties (including soil types and degree of consolidation) is only partially considered by classifying them into sand, gravelly sand and soil with N N 15. Brown et al (2010) al (2010) tried to provide the rationale behind on why load test results indicate β is depth-dependent by saying that the forming of the hole disturbs the soil, reducing its density and allowing relaxation of horizontal stress. They viewed that “detailed evaluations of in-situ strength and state of stress are not warranted because the in-situ properties are changed by construction and the changes cannot be predicted reliably”. With this limitation, the depth-dependent depth-dependent β method assumes that the soil disturbance can reduce the soil friction angle to a lower-bound value corresponding to the critical state void ratio. The practice of lumping K s and δ into a single parameter (β) and then evaluating β solely as a function of depth therefore neglects the influence of geology, material type, and stress history. Its use is therefore restricted to site-specific ground conditions (Brown et al 2007). al 2007). Should such relationship in one specific site be applied to other sites, the pile length for different sites with the same load carrying capacity will be the same, as the variation of  with depth will be the same. same. Kulhaway and and Chen (2007) remarked that they “do not believe that a mean line should be drawn through these data, which then would be used as a ‘desi gn line’.” In order to include the effect of site specific soil properties, the rational method is an improvement of the depth-dependent β method by method by relating the value of β to the site specific SPT-N values of the soil. However, the typical soil profile in Hong Kong shows increasing SPT-N values with depth, and using the above equations based on the rational method will give increasing values of β with depth and this does not match β diminishing with depth as shown by load test results. This Information Paper Paper suggests that, as discussed earlier, relative pilesoil movement should be considered. Hence, a practical way recommended by this Information Paper is to consider the site specific SPT-N values of the soil, which can take into account of the combined effect of site specific soil properties and the overburden effective pressure, and this together with considerations of pile-soil movement will be discussed in the subsequent paragraphs.


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3.8

N-value method

3.8.1 N-value method (Meyerhof 1976) is more commonly adopted in Hong Kong to calculate the unit shaft friction and base resistance of piles. It estimates pile capacity based on semi-empirical correlation between SPT-N values results and static pile load tests. The method correlates SPT-N value directly to shaft friction, with different coefficients depending on whether the foundation is a replacement or driven pile. As SPT-N values have been included in the calculation, the type of soil (including the degree of consolidation and its strength properties) and overburden pressure have indirectly been incorporated into the method. method. As the SPT-N values are readily available in every project, project, this method is very easy to use and provides a quick way to calculate the shaft friction. The following following paragraphs paragraphs will discuss the semi-empirical correlation values to be used for different construction methods. 3.8.2 Shaft friction for piles without post-grouting 3.8.2.1 Most of the test data ( Figure 13) available to date in Hong Kong have been summarised in GEO Publication No. 1/2006 . 1/2006 . These data were obtained by load-test frictional piles to calculate the f max max/N ratio. These data give a mean value of 0.89 for replacement piles, which is lower than values reported elsewhere (e.g. Chang and Wong 1995; Tan et al 1998). Moreover, like the data for  for , they have a high variability with a coefficient of variation (COV) of 61%.

(a) Replacement piles without shaft grouting


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(b) Displacement piles Figure 13 f max max/N values for piles installed in saprolites in Hong Kong (Source: Source: GEO 2006)

3.8.2.2 Meyerhof (1976) provides the correlation factor of the average avera ge maximum mobilised shaft friction f max max (in kPa) with SPT-N values to be 2 for driven pile and 1 for bored piles. For augered piles, the earlier paragraphs have pointed out that their shaft friction lies between driven piles and bored piles. In the design of PIP piles which was a proprietary piling system patented by Intrusion Prepakt before the 21 st century in Hong Kong, owing to their special construction method, the average maximum shaft friction (in kPa) had traditionally been taken as f max max = 4.8×N with limiting value for SPT-N values at about 40. The proprietary method is in fact very similar to CFA pile. However, recent studies found that such high correlation factor (4.8N) is only recommended for relatively large displacement DD piles, and for conventional CFA piles the correlation factor may not be able to achieve such high values (Brown et al 2007). al 2007). In view of the experience over the years (summarised in SEB Information Paper Review of PAKT-IN-PLACE Piles Installation (available: http://asdiis/sebiis/2k/resource_centre/ http://asdiis/sebiis/2k/resource_centre/)), ArchSD SEI 04/2010: Particular Specification for Non-Percussion Cast In-situ Concrete Piles (available: http://asdiis/sebiis/2k/MAIN%20TOPIC/technical%20paper/frame.htm)) now http://asdiis/sebiis/2k/MAIN%20TOPIC/technical%20paper/frame.htm specifies that the design shaft friction (in kPa) with a FOS of about 3 for non percussion cast in-situ concrete pile (including PIP Piles) is now taken as varying from a maximum of 1.6×N for CFA piles to 0.7×N for piles formed by boring with an auger and temporary casing with limiting value for SPT-N values at about 40, and specifies further that the adopted design values have to be further verified by trial piles before construction. 3.8.2.3 For other frictional piles, GEO (1996, 2006) provides the correlation factor of the average maximum shaft friction f max max in (kPa) with SPT-N values for different types of piles and methods of construction as follows: f max max= Fgeo×N (kPa)


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where Fgeo is taken as 1.5 to 2 for small-displacement piles (e.g. driven H-piles) for N up to about 80, and 4.5 for large-displacement driven piles (e.g. Daido SS piles) with a limiting average shaft resistance of 250 kPa. For replacement piles formed by boring, Fgeo is taken as 0.8 0.8 to 1.4 1.4 for N up to 200. 200. GEO (1996, 2006) further recommends the base resistance to be ignored in calculating the load carrying capacity of the pile. 3.8.3 Shaft friction for piles with post-grouting 3.8.3.1 Shaft friction of a cast in-situ pile can further be increased by post-grouting along the pile shaft. Post-grouting is a pressurised process that injects cement grout to the interface between the surface of the installed pile and soil through tubes embedded within the pile. The installed pile surface must be cracked open by injecting either water or grout under high pressure. This is done after the concrete or grout of the pile has set, but before it has gained significant strength. An early application of the shaft grouting method overseas was in 1975, where loading tests on six shaft grouted 660mm bored piles showed that there was an increase in shaft friction of 2.5 times that of piles without post-grouting (Gouvenot and Gabaix 1975). With regard to the long term durability of the effect of the shaft grouting, it was reported in a Bangkok site that there was no loss of shaft resistance for two shaft grouted piles in alluvial sand and clay when reloaded one year after the first load test (Littlechild et al 1998). 3.8.3.2 Shaft-grouted technique has been employed in Hong Kong for mi ni-piles since the early 1990s (Lui et al 1993) and for barrettes and large diameter bored piles since the late 1990s. In Hong Kong, post-grouting is carried out using tube-a-manchette in stages after casting the piles. The system adopted for shaft grouting consists of 50mm diameter mild steel tube-a-manchette pipes, with manchettes spaced at about 1m intervals along the pipes ( Figure 14). The tube-a-manchette pipes are fixed, using normal tie wire, to the outside of the reinforcement cage and within the zone of the concrete cover for large diameter bored piles, or to the steel H-piles. Within 24 hours after concreting/grouting, water is injected through the tube-a-manchette pipes at the perimeter of the shaft with a pair of double packers in stages to crack the green grout/concrete (Figure 15), which can then be followed by shaft grouting operation.


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(a) Tube-a-manchette Tube-a-manchette pipe without manchette

(b) Tube-a-manchette Tube-a-manchette pipe with manchette

(c) Double packers and pumps

(d) Inflated packer inserted in the Tube-a-machette Tube-a-machette pipe

(e) Tube-a-manchette Tube-a-manchette pipe installed adjacent to the flange of steel section

(f) Pressure gauge for post-grouting

Figure 14 Tube-a-Manchette Tube-a-Manch ette pipe


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(a) Arrangement of Tube-a-manchette pipe along perimeter of shaft

(b) Post-grouting by means of tube-a-manchette Figure 15 Cracking of green grout and post-grouting

3.8.3.3 For shaft-grouted mini-piles, the horizontal soil stress around the pile perimeter will increase due to the soil modification resulting from pressure grouting and this effect may be considered as similar to a pile without postgrouting but with an enlarged pile diameter. In-situ measurements have been carried out to correlate the shaft friction at the pile-soil interface after the shaft grouting operation. Littlechild et al (1998) al (1998) reported that the f max max/N value was 5 with a maximum value of 260kPa and 200kPa for shaft-grouted piles respectively in sand and clay in Bangkok, whilst Stocker (1983) reported maximum values of 250 250 to 400kPa 400kPa in sands and 200 200 to 270kPa in clays. In Hong Kong, Chan et al (2004) used f max max/N of 2.85 for a frictional mini-pile with post-grouting construction method in a project for the former KowloonCanton Railway Corporation (the “KCRC”) in Tuen Mun, Hong Kong, and Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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found satisfactory performance in the subsequent loading test to twice the design working load. Littlechild et al (2000), al (2000), based from a number of loading test results on the foundations in a KCRC project, reported that the f max max/N values range from 1.3 to 3.6. Their study further noted that shaft grouting is more effective for completely weathered materials with SPT-N values less than 60, and that for soil with SPT-N values greater than 60, the percentage of increase in shaft friction is less marked. 3.8.3.4 GEO (2006) summarized the data of loading tested on shafted grouted replacement piles in Hong Kong ( Figure 16), and found that the f max max/N values can range from 1.4 to 5.5. The highest value was reported reported by Lui et al (1993) al (1993) in a project in Mid-Levels, at which an average mobilized shaft friction f max of 5.5N was recorded in an instrumented pile in completely weathered granite with a maximum value of of 270kPa. GEO (2006), unlike that for piles without post-grouting, does not recommend the range of f max max/N for piles with postgrouting, and only suggest the f max max/N values should be limited to 5 with a limiting N-value of 100.

Figure 16 f max max/N values for replacement piles with shaft-grouted installed in saprolites in Hong Kong (Source: Source: GEO 2006)


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3.9

Section Summary

3.9.1 In Section 3, three of the key parameters affecting the shaft friction along the length of the pile have been discussed, including: a) the construction methods, b) the shear displacement at the pile-soil interface, and c) the site specific soil properties. This Information Paper then summarises  and N-value methods in estimating the shaft friction along the length of a pile. The depth-dependent  method, has the advantage of relating the shaft friction with the depth, with greater values at the pile head tallying with the greater shear displacement at the pile-soil interface at such region. However, it does not correlate with the construction methods and soil properties, e.g. soil strength and degree of consolidation. The rational method  method has included the effect of soil properties of a specific site by relying to the SPT-N values values of the soil. Though it is better than the depth-dependent depth-dependent β method from a soil mechanics perspective, it has not addressed the relative pile movement between pile and soil. N-value method, on the other hand, has the advantage of relating the shaft friction with the SPT-N values, and hence incorporates indirectly the soil properties in its calculation. Moreover, the effect of overburden pressure has also been indirectly taken into account in the SPT-N values. It further takes into account of the various construction methods by using different c orrelation factor for different types of piles. This is why N-value method is widely widely used in calculating the shaft friction for a frictional pile. To incorporate the variation of shear displacement at the pile-soil interface along the pile length, an average correlation factor Fgeo is usually adopted for calculating the pile capacity. 3.9.2 Upper bounds of shaft friction In Section 3, available literature has been reviewed, and it was noted that there has been a consensus that there are upper upper bounds of the shaft friction. For example, GEO (1996, 2006) gives an upper bound for N to about 80 for smalldisplacement piles and limits the shaft friction for large-displacement piles to 270kPa. Similarly, for shaft-grouted shaft-grouted piles Littlechild et al (1998) al (1998) reported that the maximum shaft friction in sands lies between 250 to 400kPa, and further reported that shaft grouting is more effective for soil with N less than 60. Micropile Design and Construction Guidelines published by the US Federal Highway Administration (Armour et al 2000) al 2000) summarises the values of typical shaft friction with reference to the different construction methods in described in Section 3.2 and different types of soil. Table 3 is an extract of the summary for sandy soil, and it can be seen that the shaft friction increases from Type A to D construction methods and also increases from with increasing percentage of gravel. Furthermore, the ranges of of shaft friction are in line with those observed by literature. liter ature. Post grouting along the shaft (i.e. Type C or D) is very effective eff ective for soil with weak soil; but for good soil, the percentage of the increase in the shaft friction is less obvious. This observation tallies with the observation observation of Littlechild et al (1998). al (1998). Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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Soil types

Table 3 Typical range of shaft shaft friction friction Typical range of shaft friction (kPa) for different construction methods Type A Type B Type C Type D 70 – 70 – 145 145 70 – 70 – 190 190 95 – 95 – 190 190 95 – 95 – 240 240

Sand with silt Sand with silt and 95 – 95 – 215 215 120 – 120 – 360 360 145 – 145 – 360 360 145 – 145 – 385 385 gravel Gravel with sand 95 – 95 – 265 265 120 -360 145 – 145 – 360 360 145 - 355 (Source: Source: Micropile Design and Construction Guidelines (2000))

4.

I n-Situ n-Situ Measurements in ArchSD Projects

4.1 In-situ measurements in the 1990s Numerous in-situ measurements have been carried out since the 1990s on the shaft friction along the length of frictional piles in both private and public sector projects. These in-situ measurements have then been published (e.g. Ng and Lei 2003; Li 2000; Ng et al 2001; Yau 2000; Lei and Ng 2007; Littlechild et al 2000). In ArchSD, Dr H Y WONG (our ex-SGE/NP) carried out in-situ measurements on eight instrumented piles of the four sites (two for each site) during the 1990s (Wong 2003). All piles on the four sites were installed by Intrusion Prepakt using PIP pile pile system. On three out the four sites, steel sections have been inserted so that the load carrying capacity of the piles was in the range of 2200kN to 2700kN, and on the remaining site, the load carrying capacity of the piles was 1461kN (the typical load-carrying capacity of  of 610mm PIP pile). These eight instrumented piles were loaded to twice their working working capacity, and the pile-head settlement and strains along the length of piles were measured. Raw data and more details of his his instrumentation works can be found in Wong (2003). 4.2 In-situ measurements in recent ArchSD projects 4.2.1 Two ArchSD projects have recently employed the frictional mini-piles as the foundations, and instrumented piles (details in Figure 17(a)) were installed in order to verify the shaft friction along the pile length. length. The first project is “A permanent planning and infrastructure exhibition gallery at City Hall Annex” (Inform no. 7195U) (the “City Hall Annex Project”). Project”). 14 nos. of the the frictional mini-piles were installed to a new annex, and each of the frictional mini-piles consisted of a Grade S355JR 152×152×37 kg/m UC installed in a pre-bored hole formed into soil with a temporary steel casing c asing with internal diameter of 305 mm and then injected with cement grout followed by extraction of temporary steel casing before the setting of grout. The load carrying capacity of the frictional mini-pile was 580kN. During the installation, the casing of the upper portion was accidentally left in to a depth of 22m below the ground level, due to the early setting of the grout before extraction, whilst the temporary casing of the remaining piles was withdrawn successfully. As such, negligible friction was expected from the upper portion of the pile, where the temporary casing remains.


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4.2.2 The second project second project is “Transformation of the former police married quarters site on Hollywood Road into a creative industries landmark” (Inform no. 7 955V) (the “Hollywood Road Project”). Project”) . 34 nos. of the the frictional mini-piles were installed to a new annex, and each of the frictional mini-piles consists of a 152×152 built-up I-section from Grade S355JR 20mm thick steel plates installed in a pre-bored hole formed into soil with a temporary steel casing with internal diameter of 305 mm and then shaft grouted with cement grout followed by extraction of temporary steel casing before the setting of grout. Post-grouting using tube-a-manchette in stages after casting the piles was carried out to increase the shaft friction along along the length of the the pile. The load carrying capacity of the frictional mini-pile was 1300kN. Typical details of these frictional mini-piles are shown in Figure 17(b).

Figure 17(a) Details of Instrumented Instrumented Piles at City Hall Annex Project Project


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Figure 17(b) Typical details of the frictional frictional mini-pile at City Hall Annex Project

4.3

Shaft friction for PIP Piles

4.3.1 From the paper reported by Wong (2003) on the 4 ArchSD sites using PIP piles, this Information Paper carry out an analysis of the instrumentation data. Figure axial force along the depth of the instrumented piles. In the 18 plots the axial following paragraphs, both  and N-value methods will be used to calibrate the relationship among the measured shaft friction, SPT-N values and  for these instrumented piles.

(a)

Yuen Long Site


(b)

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Tung Chung Site


(c)

Ma On Shan Site A

(d)

Ma On Shan Site B

Figure 18 Axial load along the length of instrumented instrumented PIP piles in four four ArchSD projects (Source: Source: Wong 2003)

4.3.2  method In Section 3, the following equations for calculating the value of  at depth z from the ground level have been quoted O’Neill and Reese (1999): 0.5

sand with N N15:

= 1.5 – 1.5 – 0.245× 0.245× z

soil with N N15:

=

N 15

0.5

×{1.5 – ×{1.5 – 0.245× 0.245× z 

Coleman and Arcement (2002):

-0.67

silty soils:

= 2.27× z

sandy soils:

= 10.72 × z

-1.3

Figure 19 shows the relationship of the value of  at depth z from the ground level for all the eight instrumented piles, with the best-fit trend line in black shown alongside with the equations e quations of O’Neill and Reese (1999) and Coleman and Arcement (2002). (2002). It correlates with the above equations with the greatest value at pile head decreasing with depth depth z. The data of all sites show similar trend and values, indicating that  in these sites generally follows the same trend with depth z. As the construction method of all these PIP PIP was being the same, it is reasonable to deduce that a generalised generalised equation can be derived. The best-fit line is thus: -1.2

= 7.5× z


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It can further be seen that the best-fit line lies close to that equation of Coleman and Arcement (2002) for sandy soil, indicating the soil types for these eight sites match closely closel y with the soil type used in that equation.

Figure 19 Variation of with depth z for PIP piles

To investigate the value of  of  with the shear displacement of the soil at the pilesoil interface, a plot of  against the measured displacement at the interface is shown in Figure 20 and with the trend line, though not a good correlation, added. The plot confirms that soil behaves elastically with small shear displacement; but will then behave elasto-plastically with the increase in the shear displacement exceeding about 4mm. The soil will behave behave in plastic stage with a shear displacement exceeding 10mm.


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Figure 20 Varation of with shear displacement movement at pile-soil interface

Though for the eight instrumented piles the similar values of  with depth z, Section 3 has already pointed out that such correlation does not consider the soil types (including its properties and the degree of consolidation) at different sites. 4.3.3 N-value method Figure 21 plots the relationship of f max max/N and shear displacement at pile-soil interface with the depth along the instrumented piles for the four selected sites in the study of Wong (2003). (2003). The maximum mobilised mobilised f max/N range from 7 to 25. It was further noted that most most of the shaft friction was developed in the top 10m, and the shaft friction at deeper depth was not fully mobilised. The observations by Wong (2003) tally with the theoretical load profile ( Figure 7) of Misra and Chen (2004) and Misra et al (2004). al (2004). Plastic zone can be seen with a shear displacement exceeding 6mm. Wong (2003) accounted for the variation along the length of the pile by dividing the maximum friction f max = 4.8 N with different FOSs as follows:

FOS = 1 for top one-third (i.e. at depth 0 to H/3) FOS = 2 for middle one-third (i.e. at depth H/3 to 2H/3) FOS = 3 for bottom one-third (i.e. at depth 2H/3 to H). However, as per the discussion in Section 3, the pile head moves more than the pile tip, and the soil behaves plastically as the shear displacement exceeds uc. Therefore the shear resistance at that portion can be fully mobilised, and the FOS of 1 proposed by Wong (2003) represents the shaft friction of the soil in the plastic zone. The degree of of mobilisation will then be decreased as shear displacement at the pile-soil interface decreases, and Wong (2003) used FOS of 2 or 3 to represent the decrease, as it is difficult to have the pile-soil interface to Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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behave fully plastically plas tically unless there is a plunge of the pile into the ground such that mobilisation of the friction of the whole pile length is obtained. In practical design, an average mobilised shaft friction can be adopted, which can be obtained by summing the area under the f max/N graph and then averaging the sum by the mobilised mobilised length of of the pile. That is, instead of dividing the shaft friction along the pile length into three zones, it is a common practice to average the mobilised shaft friction and then apply a single FOS to obtain the safe working load for the pile. This Information Paper therefore calculates the average mobilised shaft friction, which is obtained by averaging the area under the plot of f max max/N against the depth of the pile by the length where the shaft friction has been mobilised. Table 4 shows the detailed calcualtion of the average Fgeo (=f max max/N) with the mobilised length for these instrumented piles on these sites. In summary, summary, the following following avearge f max max/N values are calculated as: Site 1 – 1 – 3.57 3.57 Site 2 – 2 – 3.66 3.66 Site 3 – 3 – 2.70 2.70 Site 4 – 4 – 5.15 5.15 The average f max max/N for the eight instrumented piles of the four sites is 3.77. Applying a FOS of 3 or 2, the design shaft friction can be taken as 1.25 to 1.88N respectively.


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(a) Yuen Long Site

(b) Tung Chung Site

(c) Ma On Shan Site A

(d) Ma On Shan Site B

Figure 21 Variation f max max/N values for frictional piles formed by CFA (Source: Source: Wong 2003)


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Table 4

Depth z from pile head (m) -2 -4 -5 -6 -7 -8 -10 -11 -13 -14 -16 -17 -18 -19 -20 -22 -23 -25 -26 -28 -29 -32 -35 -38 -45 Average Fgeo

f max calculation of average average value max/N against pile depth and calculation f max max/N (kPa) Tung Chung Ma On Shan Ma On Shan Yuen Long Site Site Shan Site A Shan Site B pile no 1

pile no 2

pile no 1

pile no 2

pile no 1

pile no 2

pile no 1

pile no 2

7.83 6.17 2.51 2.69 0.58 4.25 0.14 -

5.22 12.57 4.11 0.79 1.23 1.02 0.88 -

11.67 4.06 4.83 3.99 1.74 1.45 4.08 0.93 1.19 0.39 -

13.39 8.70 5.31 3.26 1.96 1.86 3.53 0.47 0.40 0.10 -

1.74 6.79 6.96 5.57 1.34 0.79 4.06 0.38 4.04 1.09 0.18 1.09 0.49

1.74 8.78 6.40 3.71 2.68 0.79 2.90 1.51 3.73 1.52 0.72 1.09 0.24

18.48 13.44 17.39 4.89 1.63 3.16 4.89 1.45 1.24 1.30 0.61 1.11 0.19

13.05 25.30 5.80 6.52 0.54 1.58 2.72 3.62 1.24 1.96 1.01 0.67 0.06

3.57

3.66

2.70

5.15

Figure 22 plots the relationship of f max max/N and shear displacement at the pile-soil interface. An approximate linear correlation is noted noted between the shear displacement and the value of f max max/N (Figure 22) when the shear displacement is less than 4mm, as the soil behaves elastically. For a shear displacement exceeding 4mm, the soil starts to behave elasto-plastically, and for a shear displacement exceeding 6mm, the soil behaves behaves plastically. The observations observations coincide with the idealised zoning in Figure 7 with the top portion of the piles in plastic zone and the lower portion in elastic zone.


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Figure 22 Relationship between f max max/N values and shear displacement

4.4

Shaft friction for frictional mini-piles without shaft-grouted at City Hall Annex Annex Project

4.4.1 In order to investigate the shaft friction for the frictional mini-piles without shaft-grouted, a pile in the City Hall Annex Project was instrumented to measure the shaft friction along the pile by load-tested it to twice its working capcaity of 560kN. Figure 23 plots the axial force along the depth of the instrumented pile, and Figure 24 plots the shear displacement at the pile-soil interface movement along the length of the pile. Figure 24 shows that up till a depth of 22m from the pile head, only about 300kN out of the total load of 1100kN was taken up by the soil friction. Limited friction was developed at the top 22m despite that there have been substantial movement at the top portion of the pile. This was due to the fact that the temporary casing was not removed, removed, and hence the friction along this portion of the pile shaft is realtively smaller. Focus will therefore be data for the lower portion of the pile to calibrate the relationship among the measured shaft friction, SPT-N values and  for these instrumented piles.


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Figure 23 Axial load for the the frictional pile at City Hall Annex site site

Figure 24 Shear displacement displacement along the length length of pile


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4.4.2

 method Figure 25 shows the relationship of the value of  at depth z for the instrumented pile with the best-fit trend line shown alongside with the equations of O’Neill and Reese (1999) for sand and gravelly sand. The bestfit line is calibrated as: 0.50

= 1.2 – 1.2 – 0.175× 0.175× z This equation differs substantially from that given by O’Neill and Reese (1999) for CFA piles installed in gravelly sand, though it lies close to that given by O’Neill and Reese (1999) (1999) for sand. This equation also differs from that obtained from the eight instrumented PIP piles by Wong (2003). Moreover, the values of  (and hence the shaft friction) are less than those for PIP piles.

Figure 25 Variation of with depth z for instrumented frictional minipile without shaft-grouted

Again, to investigate the value of  with the shear displacement of the soil at the pile-soil interface, a plot of  against the measured shear displacement at the interface is shown in Figure 26, and a linear correlation can be observed.


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Figure 26 Varation of with soil movement at pile-soil interface

4.4.3 N-value method Figure 27 plot the variation of f max max/N and shear displacement at the pile-soil interface with the depth. A linear correlation is noted noted between the shear displacement and the value of f max max/N (Figure 28 ), as the lower portion of the pile is the elastic ela stic zone, where soil at the pile-soil interface behaves elastically as that predicted in the idealised pile-soil interface model. model. The average value of f max max/N for the tested frictional mini-pile is 0.80. Table 5 shows the detailed calcualtion of the average Fgeo(=f max max/N) starting from a depth of 22m from the pile head for the instrumented pile. Applying a FOS of 2, the design shaft friction can be taken as 0.40N.

Figure 27 Variation of f max max/N values and shear displacement with depth


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Table 5 Calculation of average f max max/N in mobilised length of instrumented instrumented pile at City Hall Annex Project

4.5

Depth z from Pile Head (m) -27.22 -29.22 -31.22 -35.22 -39.22 -43.22

f max max/N (kPa) 1.46 0.97 0.56 1.01 0.66 0.16

Average Fgeo

0.80

Shaft friction for frictional mini-piles with shaft-grouted shaft-grouted at Hollywood Road Project

4.5.1 Two shaft-grouted instrumented piles at Hollywood Road Project were test loaded to twice its working capacity of 1300kN and maintained for 72 hours. Figure 29 plots the axial force along the depth of the piles at test load 650kN, 1300kN, 1950kN and 2600kN.


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(a) Pile no. 19 (a) Pile no. 22 Figure 29 Axial load for the instrumented instrumented piles with shaft grouted grouted along their lengths

4.5.2  method Figure 30 shows the relationship of the value of  at depth z from the ground level for two instrumented piles at Hollywood Road Project, with the best-fit trend line in black shown alongside with the equations of O’Neill and Reese (1999) and Coleman and Arcement Arcement (2002). Again, it correlates with the above equations with the greatest greatest value at pile head decreasing with depth z. The data of these two instrumented piles show similar trend and values, indicating that  in this particular site generally follows the same trend with depth depth z. The best-fit line is thus: 0.58

= 2.0 – 2.0 – 0.3× 0.3× z


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Figure 30 Variation of with depth z for instrumented frictional minipiles with shaft-grouted shaft-grouted

As the construction method is different from CFA piles, the equation therefore differs substantially from those given by O’Neill and Reese (1999) and Coleman and Arcement (2002), and also differs from that obtained from the eight instrumented piles by Wong (2003). Thus, it confirms the observation of Kulhaway and Chen (2007) that a single design equation could not be derived, as  is affected by shaft geometry, soil particle size, soil properties, the degree of consolidation, construction method, etc, especially the fact that the construction method in the present case is completely different from CFA. Notwithstanding such limitation, li mitation, as compared with that for PIP piles the values of  (and hence the shaft friction) in the present case shows greater value at the top portion; but the trend line shows shows steeper slope. That is, it decreases at a faster rate with depth as compared with that for PIP piles. Figure 31 plots the variation of  against the measured shear movement at the interface and with the trend line added. The plot confirms that soil behaves elastically with small shear displacement; but will then behave elasto-plastically with the increase in the shear displacement at the pile-soil interface exceeding about 4mm. Full plastic behaviour behaviour could not be achieved in the instrumented piles, when the shear displacement is not adequate for full mobilisation of its shear strength.


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Figure 31 Varation of with soil movement at pile-soil interface

4.5.3 N-value method Figure 32 plots the relationship of f max max/N and shear displacement at the pile-soil interface with the depth along the piles. Again, a strong linear correlation is noted between the shear displacement and the value of f max max/N, as the lower portion of the pile is the elastic zone, where soil at the pile-soil interface behaves elastically as that predicted in i n the idealised pile-soil interface i nterface model. A linear correlation is noted between the shear displacement and the value of f max max/N (Figure 33) when the shear displacement is less than 2mm, and with a shear displacement exceeding 2mm, the soil starts to behave elasto-plastically, and for a shear displacement displacement exceeding 6mm, the soil behaves behaves fully plastic. The maximum f max max/N is 9.78, and the average f max max/N is 4.22 for the first instrumented pile and 3.89 for the second instrumented pile. Table 6 shows the detailed calcualtion of the average Fgeo(=f max max/N) with the mobilised length for these two instrumented piles. Applying a FOS of 2 to 3, the design shaft friction can be taken as 1.9N or 1.2N respectivel y.


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(a) Pile no. 19 (b) Pile no. 22 Figure 32 Variation f max max/N values for frictional piles with shaft grouted



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Table 6 Calculation of average f max l ength of instrumented max/N in mobilised length piles at Hollywood Road Project Depth z from Pile Head (m)

-0.95 -5.95 -10.95 -15.95 -20.95 -25.95 -30.95 -35.95* -40.95* -45.95*

f max max/N (kPa) Pile no. 19 Pile no. 22 4.99 4.29 5.35 6.25 8.25 9.78 7.62 5.02 2.52 1.22 0.56 0.49 0.27 0.17 0.15 0.04 0.05 0.00

0.01

-0.01

Average F eo 4.22 3.89 * The values of f max max/N are neglected in calculating the average value, as this portion is assumed not have been mobilised under the loading test.

5.

Summary of Findings

The above paragraphs are a literature review together with the in-situ measurements of the instrumented piles in ArchSD projects of the shaft friction for cast in-situ piles. Table 7 summarises the average mobilised shaft friction over the length where shear resistance has been mobilised for different types of cast in-situ replacement piles and the suggested maximum design shaft friction for different types of frictional piles with or without post-grouting along pile shaft. It is suggested that N-value method using SPT-N value is used for the design for the frictional capacity. It should however be noted that shaft friction along a pile is hard to be estimated accurately as different construction method, concreting/grouting operation etc will have different effects of modifying or remoulding the soil along the pile shaft, and hence the suggested values have to be further verified by trial piles before construction.


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Table 7 Suggested average shaft friction for different types of # replacement piles for design in sandy soil

Pile type

Average mobilized shaft friction fmax (kPa) N-value method

values from the studied sites*

Bored piles Frictional f s =  σ’v  150kPa [2] mini-piles where without σ’v = average effective postoverburden pressure, grouting and

0.8 to 1.4N [1]

-

0.8 to 2.0N  150kPa [1]

0.4 to 0.7N  50kPa (FOS 3.0) or  60kPa (FOS 2.5) [3]

1.4 to 5.5N  270kPa [3]

1.0 to 1.5N  90kPa [5]

3.0 to 4.8N

1.0 to 1.6N, where N N40 [7]

0.50

= 1.2 – 1.2 – 0.175× 0.175× z Frictional f s =  σ’v  270kPa [4] mini-piles where with postσ’v = average effective grouting overburden pressure, and

Suggested average design shaft friction f (kPa)

0.58

PIP piles installed by CFA

= 2.0 – 2.0 – 0.3× 0.3× z f s =  σ’v  200kPa [6] where σ’v = average effective overburden pressure, and -1.2

= 7.5× z The values quoted in Table 7 are only applicable to sandy soil, and are not applicable to clayey soil. * PSE should particularly note that the equations derived for method are -specific for the studied sites and the values are different from the values obtained from literature. It is not recommended for use as the method is only related to depth and does not correlate with construction methods and soil properties, e.g. soil strength and degree of consolidation. consolidation. It can be used only when it has been calibrated for the specific site and construction method. [1] The range of values is suggested by GEO (2006). [2] f s is limited to 150kPa with reference to Micropile Design and Construction Guidelines (2000). [3] A factor of safety of 3 has been included to get the average design shaft friction; but a smaller FOS can be adopted. [4] f s is limited to 270kPa with reference to Micropile Design and Construction Guidelines (2000) and Lui et al (1993). al (1993). [5] A factor of safety of 3 has been i ncluded to get the average design shaft friction. [6] f s is limited to 200kPa with reference to O’Neill and Reese (1999) and Brown et al (2007). al (2007). [7] Limit of N-value is quoted in SE Instruction No. 04/2010, 04/2010 , which is based on the PIP piles patented by Intrusion Prepakt (Far East) Limited.


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6.

Loading Tests

6.1

Trial pile In order to verify the design assumptions and parameters, loading test on a trial pile is required prior to t o the installation works. The intent of the loading test on a trial pile is to: a) b)

6.2

establish and/or verify installation means and methods of the contractor, and verify the design parameters and hence the load carrying capacity of the pile.

Installed piles After the completion of the installation of the piles, loading tests on a number of the completed piles are required to verify that the contractor is producing acceptable piles. In the General Specification of Building 2012 of ArchSD, 1% of the piles are required to be to be load tested to twice the theoretical safe loading capacity. However, it should should be noted that the the variation of of the load carrying testing of frictional piles is expected to be higher than that for end bearing piles, as the design assumptions and parameters are only provided by the ground investigation and initial loading test on on the trial pile. Moreover, even for sites with uniform soil properties, the integrity of the piles is affected by the workmanship of the contractor. Thus, it is prudent to adopt a higher testing frequency for installed piles. Micropile Design and Construction Guidelines published by the US Federal Highway Administration specifies 5% of the installed micropiles to be subjected to loading tests; whilst EN 14199: Execution of Special Geotechnical Works - Micropiles Micr opiles (BSI 2005) specifies 2% of the first 100 installed micropiles to be tested and 1% for each next 100 installed piles thereafter. This Information Information Paper therefore suggests specifying 3% of the first 100 installed piles and 1% for each next 100 installed piles thereafter for loading test.


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7.

Pile Group Settlement

7.1

Settlement of pile is always a concern for floating piles with their toes stop in the overburden soil. The increase of stress in the underlying soil below the pile toe may cause undue settlement; the thicker the soil layer, the larger the settlement will be. It is therefore specified in our GS that a settlement analysis is required for such such piling systems. systems. Frictional mini piles, PIP piles are considered as floating piles.

7.2

Piles installed in a group to form a foundation can give rise to interaction between individual piles. The overlapping of stress and strain fields can result in the pile-soil-pile interaction and this will not only affect the capacity of the piles but also the settlement set tlement behaviour of the pile group. Interference between zones z ones of influence causes a pile within a group to settle more than a single isolated pile, as a result of pile-soil-pile interaction. Figure 34 shows the zone of influence for a single pile and a pile group. Further details on pile group settlement are discussed in the following paragraphs.

Figure 34 Zone of Influence for a Single Pile and a Pile Group (Source: Source: Brown et al 2007) al 2007)

7.3

If the building or or column load is not high, high, the supporting piles are not closely spaced. However, if piles are closely spaced, the pile groups behave differently from single isolated piles because of pile-soil-pile interactions that take place in the group. For a group of closely spaced frictional piles which is required to support loading from heavily loaded columns, the pile group will form a network of reticulated piles that create a system of confined soil composite with the piles acting as reinforcing elements. Unlike a single pile system where the soil at the pile-soil interface is modified and the lateral stresses may also be increased during the pile installation stage, such effect cannot happen along the perimeter of the pile group ( Figure 35). In this case, the block settlement of a group with closely spaced piles should be considered which may often exceed that predicted from a single pile analysis of an individual pile at the same load. This pile group settlement is sometimes a few times more than a single pile; that means the settlement obtained from load testing of a single pile cannot give the settlement of the pile group/building. The settlement of pile group can be assessed by various methods such as equivalent raft methods or computer modelling by PLAXIS 3D FOUNDATION. SEB promulgated SEB Guidelines SEBGL-PL14: Guidelines on Assessment of Pile Group Settlement (available:


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http://asdiis/sebiis/2k/resource_centre/) on the methods to assess the pile group settlement. If necessary, it is recommended to carry out settlement monitoring after a building is constructed.

Figure 35 Pile Group with Closely Spaced Piles

7.4

For drilled placement placement (DD) piles or shafted grouted grouted frictional mini-piles, the required pile length will be shorter than that those ordinary CFA piles or frictional mini-piles without post-grouting under the same required loading. Figure 36 illustrates the equivalent raft model for the estimation of pile group settlement. It can be observed that the settlement of the pile group of friction mini-piles with shaft grouted will be greater than those piles without postgrouting due to the shorter pile length and hence a thicker depth of compressible soil beneath the base of the pile group.

Figure 36 Equivalent Raft Model for Estimation of Pile Group Settlement

7.5

Thus, it should be noted that the advantages of friction mini-piles in achieving achieving high loading capacity at shallow depth may be offset by settlement considerations. PSEs should therefore make an assessment of the effect of pile group settlement on the superstructure if frictional mini-pile is considered to be a feasible piling option. This is especially the problem for high-rise buildings or under heavily-loaded columns, not only because of inadequate total pile capacity, but also the resulting settlement may be too excessive leading to the infeasibility of using frictional mini-piles as the foundation.


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8.

Method of Procurement Procurement of Frictional Piles

8.1

Either an engineer-design or the traditional deign-and-build arrangement can be adopted for procuring the frictional mini-piles. For an engineer-design, the design in the form of the location, sizes, founding depth and method of installation of the piles will be specified in the contract, and the contractor is only required to carry out the works according to the specified design. However, this Information Paper has shown that one of the key parameters affecting the load carrying capacity of frictional mini-piles is the construction method (e.g. during the post grouting works and operating the CFA for PIP piles). The parameters used in the design of the length of piles also show that the length of the piles is very site-specific, and the only means to check the assumed parameters is through the initial trial pile. Thus, adopting an engineerdesign arrangement cannot demarcate the defaults in design and in workmanship should a dispute arise. It is therefore recommended that the traditional design-and-build arrangement should still be adopted.

8.2

Adopting the traditional design-and-build arrangement, the contractor should then be held responsible to design and construct each frictional pile to a capacity to meet the contract requirements (including the loading specified in the loading schedule), according to his own construction method. PSE should specify the loading points, and and minimum length and size of the piles in the contract. The contractor can then design the number of piles, the size of the piles and the length of the piles based on their chosen design shaft friction, which can vary depending on his proposed plant and experience of the crew; but such chosen values shall be validated validated on site by trial piles and loading test. To specify the minimum length for the piles, the PSE should normally adopt the N-value method for assessment the length. In case the  method is used, the limitations of  method as stated in Table 7 should be noted. Annex A provides an example to estimate the minimum length of piles in a typical site. Annex B provides a sample particular specification for the design and installation works of frictional mini-piles with steel sections in pre-bored holes formed by the Concentric or Symmetrix system (i.e. a system with the pilot bit set back from the ring bit during drilling or other drilling systems) and PSE may vary it to suit individual site and project, and when steel rebars are used in lieu of the steel section.


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Developments in Deep Foundations (Houston: University of Houston), pp.33-45 (available: http://ascelibrary.org/proceedings/,, accessed: 7 October 2011). http://ascelibrary.org/proceedings/ Lo, S C R and Li, K S (2003), “Influence of a Permanent Liner on the Skin Friction of Large Diameter Bored Piles in Hong Kong Granitic Saprolites”, Canadian Geotechnical Journal , 40(4), pp. 793-805. Lu, Y L (2009), Experimental Analysis for the Bearing Capacity of Screw Piles (Shanghai: Jiaotong University). Lui, S P Y, Cheung, S P Y and Chan, A K C (1993), “Pressure grouted minipiles for a 12-storey residential building at the mid-levels scheduled area in Hong Kong ”, Proceedings of International Conference on Soft Soil Engineering, 8-11 November, Guangzhou , pp. 419-24. Luo, S Q, Tan, S A and Yong, K Y (2000) , “Pull-out resistance mechanism of a soil nail reinforcement in dilative soils”, soils ”, Soils and Foundations, Foundations , 40(1), pp. 47-56. Mayne, P W and Harris, D E (1993), Axial Load-displacement Behaviour of Drilled Shaft Foundation in Piedmont Residuum (Washington, DC: Federal Highway Administration). Meyerhof, G G (1976), “Bearing Capacity and Settlement of Pile Foundations”, Journal of the Geotechnical Engineering Division, Division , 102(3), pp. 195-228. Taylor, D. W. (1948), Fundamentals of soil mechanics (New York: John Wiley and Sons). Misra, A, Chen, C H, Oberoi, Oberoi , R and Kleiber, A (2004), “ Simplified Analysis Method for Micropile Pullout Behaviour ”, Journal of Geotechnical and Geoenvironmental Engineering , Engineering , 130(10), pp. 1024-33. Misra, A and Chen, C H (2004), “Analytical “Analytical solution for micropile design under tension and compression,” compression,” Geotechnicaland Geological Engineering , Engineering , 22, pp. 199 – 199 – 225. 225. Murray, R T, Inst, H E, Carder, D R and Krawczyk, J V (1980), Pullout tests on reinforcement embedded in uniformly graded and subject to vibration (London: Department of Transport). NeSmith, W M and Siegel, Siegel, T C (2009), “Shortcomings “ Shortcomings of the Davisson Offset Limit Applied to Axial Compressive Load Tests on Cast-in-Place Piles ,” Proceedings from the International Foundation Congress and Equipment Expo, Orlando, Florida, 15-19 March 2009, 2009 , pp. 568-74. Ng, C W W and Lei, L ei, G H (2003), “P erformance of long rectangular barrettes in granitic saprolites ”, Journal of Geotechnical and Geoenvironmental Geoenvironmental Engineering , Engineering , 129(8), pp. 685 – 685 – 96. 96. Ng C W W, Li J H M and Yau, T L Y (2001), “Behaviour “Behaviour of large diameter floating bored piles in saprolitic soils”, Soils and Foundations, Foundations, 41(6), pp. 37 – 52. 52. Nordlund, R L (1963), “Bearing Capacity C apacity of Piles in Cohesionless Soils” Soils ”, Journal of the Soil Mechanics and Foundations, Foundations , 89(SM3), pp. 1 – 35. 35. Nottingham, L C (1975), Use of Quasi-Static Penetrometer Data to Predict Load Capacity of Piles (Gainesville: University of Florida). O’ Neill, Neill, M W (2001), (2001), “Side Resistance in Piles and Drilled Shafts ”, Journal ”, Journal of Geotechnical and Geoenvironmental Engineering , 127(1), pp. 3 – 16 16 (available: http://ascelibrary.org/journals http://ascelibrary.org/journals,, accessed: 7 October 2011). O’Neill, M W (1994), “Review “Review of Augered Pile Pra ctice Outside the United States”, States” , Transportation Research Record No. 1447: Design and Construction of Auger Cast Piles, and Other Foundation Issues (Washington, DC: Federal Highway Administration), pp. 63-9.


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O'Neill, M W and Reese, L C (1999), Drilled (1999), Drilled Shafts: Construction Procedures and Design Methods, Methods , (Washington, DC: Federal Highway Administration). Paik, K H, Salgado, R, Lee, J, and Kin, B (2003), “Behaviour “Behavio ur of open and closed ended piles driven into sands”, sands”, Journal of Geotechnical and Geoenvironmental Geoenvironmental Engineering , Engineering , 129(4), pp. 296 – 306. 306. Paikowsky S G and Tolosko, T A (1999), Extrapolation of Pile Capacity from Non-Failed Load Test (Washington, DC: Federal Highway Administration) (available: http://ntl.bts.gov/lib/16000/16000/16053/PB2000102368.pdf .; accessed: 4 October 2011). Poulos, H G (1968). “Analysis of the Settlement of Pile Groups”. Groups”. Geotechnique, Geotechnique, 18, pp. 449-71. Poulos, H G (2006), “Pile Group Settlement Estimation Est imation – Research to Practice”, Proceedings of Sessions of GeoShanghai, 6-8 June 2005, Shanghai, China , pp.1-22. Poulos, H G and Davis, E H (1974), Elastic Solutions for Soil and Rock Mechanics (New York: John Wiley & Sons). Poulos, H G and Davis, E H (1980), Pile Foundation Analysis and Design (New York: John Wiley & Sons). Rollins, K M, Clayton, R J, Mikesell, R C and Blaise, B C (2005) , “Drilled Shaft Side Friction in Gravelly Soils”, Soils”, Journal of Geotechnical and Geoenvironmental Engineering , Engineering , 131(8), pp. 987 – 1003. Schmertmann, J H (1978), FHWA-TS-78-209 Report: Guidelines for Cone Penetration Test, Performance and Deign (Washington, DC: Federal Highway Administration). Skempton, A W (1986), “Standard Penetration Test Procedures and the Effects in Sands of Overburden Pressure, Relative Density, Particle Size, Aging and Overconsolidation ”, Geotechnique, Geotechnique, 36(3), pp. 425-47. Stocker, M (1983), “The Influence of Post-grouting Post -grouting on the Load-Bearing Load- Bearing Capacity of Bored Piles”, Proceedings of the 8th European Conference on Soil Mechanics and Foundation Engineering, 23-26 May 1983, Helsinki, Helsinki , pp. 167-70. Tan Y C, Chen C S and Liew S S (1998), “Load transfer behaviour of cast -in place bored piles in tropical soils of Malaysia”, Proceedings of the 13th SE Asian Geotechnical Conference, Taipei , pp. 563-71. Taylor, D W (1948), Fundamentals (1948), Fundamentals of Soil Mechanics (New York: John Wiley and Sons). Tomlinson, M J (1971), “Some Effects of Pile Driving on Skin Friction”, Friction ”, Proceedings of Conference on Behaviour of Piles, Piles , pp 107 – 14. 14. Tomlinson, M J (1994), Pile (1994), Pile Design and Construction Practice (London: E & FN Spon, 4 th ed). Yang, J, Tham, L G, Lee, P K K, Chan, S T, and Yu, F (2006), “Behaviour of jacked and driven piles in sandy soil”, Géotechnique, Géotechnique, 56(4), pp. 245 – 259. 259. Yiu, T M and Lam, S C (1990), (1990) , “Ultimate Load Testing of Driven Piles in Meta-Sedimentary Decomposed R ocks”, ocks”, Proceedings of the Conference on Deep Foundation Practice , Singapore, pp 293-300. Yau, T L Y (2000), Capacity and Failure Criteria of Bored Piles in Soils and Rocks (Hong Kong: University of Science and Technology) (Unpublihsed MPhil Thesis).


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Wong, H Y (2003), “Design and Construction of Friction Bored Piles in Hong Kong with Particular Reference to Marble Areas,” Presented at the Hong Kong Institution of Engineers Annual Seminar on Case Histories in Geotechnical Engineering in Hong Kong, 9 May 2003, Hong Kong . Kong . Vesic, A S (1977), Synthesis of Highway Practice No. 42: Design of Pile Foundation (Washington, DC: Transportation Research Board).


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Annex A Estimation of the Length of Frictional Mini-Piles


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Table A1 calculates of the length of a frictional mini-pile of 305mm using N-value method with load carrying capacity of 1000kN to be installed in a hypothetical hypothetical site by using the SPT-N values of the soil shown. The water table is at 34mPD. Two cases will be considered: frictional mini-pile with and without shaft grouted. For frictional mini-pile without shaft grouted, the design average average shaft friction of 0.4N is adopted, whilst an average value of 1.0N is adopted for frictional mini-pile with shaft grouted. grouted. The calculation shows that the required required lengths of the pile without and with shaft grouted are 42m and 28m respectively. Table A1 Calculation of length of pile using N-value N-value method

Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No. : 1/

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File Code: Friction Piles.doc CTW/MKL/CYK/KWK/LPL Issue/Revision Date : March 2013

Annex B Sample Particular Specification for Design and Construction of Frictional Mini-Piles


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1.

General

A Frictional Minipile for the Contract is defined as a pile which is built-up H section from steel plates, or rolled steel H-section [delete [delet e where appropriate] to be installed in a prebored hole formed into soil with a temporary steel casing with minimum internal diameter of [insert dimension] and then filled with cement grout. The requirements on the construction of the pile are as shown in drawing no. [insert drawing number]. number] . The Contractor is required to engage a specialist contractor to design and construct the piling works, who shall be in the List of Approved Suppliers of Materials and Specialist Contractors for Public Works in the Category of Land Piling Group II and eligible to carry out the registered piling system of Rocksocketed Steel H-pile in Pre-bored Hole or Minipile.

2.

Piling Design

2.1

Design Requirements Requirements

The theoretical safe loading capacity of the individual Frictional Minipile shall be the allowable axial force of the built-up H-section from steel plates, or rolled steel H-section [delete where appropriate]). appropriate] ). The maximum allowable axial working stress of the built-up H-section from steel plates, or rolled steel H section [delete where appropriate]) appropriate] ) shall be 45% of the yield stress, and the combined stresses due to axial load and bending moments shall be limited to 50% of the yield stress. When the calculations of stresses are based on all loads including wind loads, the permissible stress shall be increased by 25% of the above stresses. The theoretical safe loading capacity of each individual pile shall not exceed [insert number]kN. number] kN. The Contractor shall be responsible for the design and construction of the Frictional Minipiles including the length and the number of Frictional Minipiles to support the loading in the loading schedule of drawing no. [insert drawing number]. number] . The founding level shall be at [insert number]m number] m below the cut-off level or ground level [delete as appropriate]. appropriate] . [Guidance notes: the minimum length of the pile is to be calculated according to the GI, and refer to the example in Annex A of this information paper for paper for a worked example to estimate the minimum length.] The frictional resistance between the pile shaft and the soil above [insert number]mPD number] mPD shall not be considered. End-bearing capacity of the pile shall be ignored. The Contractor shall satisfy himself that his method of calculating the design frictional resistance, which shall provide a sufficient factor of safety in his design.


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Pile design submission shall be in accordance with Clause 5.02 of the GS. 2.2

Pile Head Details

The Contractor shall provide capping plate and dowel bars in accordance with the detail as given in the GS for steel H piles.. 2.3

Cover

The minimum clearance (cover) between casing and the steel section shall be 40mm. The Contractor shall submit his proposed spacer details with his pile design submission. 2.4

Minimum Segment Length of Steel Sections

The minimum length of each segment of steel sections forming the whole length of Frictional Minipile shall be 10 m except the uppermost section. 2.5

Provision of Shear Key

The Contractor shall provide shear bars to steel s ections in accordance with the details shown in Appendix. 3.

Submissions for Piling Works

In addition to the submissions stated in GS Clause 5.02, the Contractor shall submit 2 copies of each of the following information with the design submissions: a. details of built-up H-section from steel plates, or rolled steel H-section [delete where appropriate]; appropriate] ; b. details of grout mix; c. method of installation including equipment to be used, sequence of operations, drilling methods, temporary casing installation and extraction, and time of grouting; d. details of grouting operation, taking into account of the subsoil condition and ground water fluctuation during the day; da y; e. method of piling operation to overcome underground obstruction, if encountered; f. spacers details; g. a report on the existing conditions of the adjacent structures (including existing underground pipe, existing retaining walls and any other structures nearby). This initial condition survey report shall include information and record photos of these existing structures with particular attention to those aspects that may be adversely affected by the proposed works. Foundation types of the adjacent structures shall also be included; h. proposal on precautionary measures and actions to be taken so as to prevent the above adjacent structures from being adversel y affected by the works; i. any other requirements specified in this particular specification. Structural Engineering Branch, ArchSD Information Paper on Small Diameter Frictional Piles Issue No./Revision No./Revisi on No. : 1/

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No piling works shall commence on site unless the submissions are approved by the SO in writing. 4.

Drilling Method

Unless otherwise agreed by the SO, Frictional Minipile shall be formed with a temporary casing used to stabilise the surrounding soil, and shall not be installed with the use of bentonite slurry or other drilling muds. Temporary casing of approved quality shall be lowered at the same time when the hole is made. Unless otherwise approved by the SO, the Concentric or Symmetrix system or other drilling systems shall be used to form the pile hole of the Frictional Minipiles. Temporary casing shall be free from distortion, internal projections and hardened grout. 5.

Flushing Medium

Air or water shall be used as the flushing medium during the drilling operation. The Contractor’s attention is drawn to the formation process of the pile shaft using air flushing where special care shall be taken to avoid disturbance to adjacent ground of soil during forming of the pile shaft. 6.

Tolerances

The maximum deviation of the centre of the head of each finished Frictional Minipile from the designed centre point shall not be more than 50 mm in any direction. The maximum deviation from from the vertical axis of the pile through through the centroid of the cross section at the cut off level at any level of the finished pile shall not be more than 1 in 75. [Guidance note: If a group of reinforcement bars, instead of steel section, is used, the maximum deviation of the centre of the head shall be limited to 15mm and the maximum deviation from the vertical axis shall be li mited to 1 in 100.] 7.

Founding Level

Founding level of each Frictional Minipile shall not be higher than those specified in Clause 2.1.

8.

Cutting Off of Pile Heads

All Frictional Minipiles shall be grouted to a minimum level of 300 mm above the specified pile cut-off level.


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9.

Grout for Piling Work

9.1

Grout Material

Grout shall consist of ordinary Portland cement and water with an approved non-shrinkage additive. Where PFA is used, the maximum maximum PFA content shall not exceed 35% of the total cementitious content in the grout. Other admixtures can be used when when approved by the SO. The manufacturer’s guidance shall be strictly followed. Cement sand mix is is not allowed. Grout shall have minimum cube strength of 30 MPa at 28 days. Measurements for bleeding shall be taken at 15-minute intervals. The amount of bleeding shall not exceed 2% at the end of the first 3 hours and no interim readings shall exceed 4%. In addition the water must be be reabsorbed by the grout within 24 hours after mixing. mix ing. Free expansion of grout when measured at the end of 24 hours after mixing shall have a figure between 0% and 5%. A negative percentage figure figure shall not be accepted. Any approved admixtures shall be chloride-free and comply with BS EN 934. The maximum total chloride content, expressed as a percentage relationship between the chloride ion and the cementitious content by mass in the grout shall be 0.1%. Water for grout shall be clean fresh water having a temperature not exceeding 30 30C nor less than 5 5 C. 9.2

Grout Mixing

Grout material shall be mixed by weight batching. The amount of water used shall be measured by a calibrated flowmeter or a measuring tank. The mixing time in high-speed mixers shall be appropriate for the type of mixer used. After mixing, the grout shall be continuously agitated in a holding tank and screened before injection. The grout grout shall be placed within the time limits specified by the manufacturers of the additives. 9.3

Pressure Grouting and Extraction of Temporary Casing

9.3.1 Grout Pipes

Grouting shall be carried out with two non-flexible grout pipes, one at each side of the web of the steel section.


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9.3.2 Grouting Methods

Before grouting, the bottom of the hole shall be cleaned by airlifting or an alternative method approved by the SO. [Delete either of the following options]

Either: The hole shall be grouted grouted by shaft grouted method. That is, upon extraction of temporary casing after the completion of initial grouting to the body of the whole Frictional Minipile, the surface of the shaft of each Frictional Minipile shall be cracked by water within 24 hours of the initial grouting via Tube-a Manchettes or similar method approved by the SO. Subsequent pressurised post grouting shall then be followed such that the pressurised grout expels itself into the surrounding ground. The Contractor shall submit to the SO for approval the detailed proposal of his shaft grouted method. Or: Grouting shall be carried out in an upstage sequence from the bottom of the hole. After the initial pressure grouting of the bottom of the pile, the temporary casing shall first be partially extracted upward to a predetermined level as approved by the SO and then followed by pressure grouting. The bottom level of the grout pipes shall in no case be higher than the bottom level of the temporary casing. The above grouting sequences shall be repeated until the completion of the grouting of the pile. Unless otherwise permitted by the SO, grouting shall be carried out by injecting the grout under pressure into each grouting stage of the hole until the grouting stage refuses to take further grout. The initial grouting shall be carried out in such a way that the lowest part of the grout pipes shall be as close to the pile toe as possible. 9.3.3 Pressure Grouting

Grouting pressure shall be determined by the Contractor and shall in no case be less than the overburden pressure unless otherwise approved by the SO. Holes shall be grouted in a continuous operation at each grouting stage and pressures as approved, and except during subsequent post grouting, shall not be left partially grouted. If, in the opinion of the SO, grouting of any hole or grouting stage has not been completed due to low pressures, excessive leakage when compared to the performance of the trial pile as required in Clause 11 or other causes, the hole shall be redrilled or flushed out with water and re-injected with grout.


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The Contractor shall agree with the SO the method to measure the grout intake volume. Newly grouted piles shall be properly covered and and fenced off.

9.4

Testing of Grout

The Contractor shall employ an approved laboratory to carry out the tests for Bleeding, Free Expansion and Flow Cone Efflux and Crushing Strength of grout. 9.4.1 Definition of Batch

A ‘batch’ of grout is any quantity of grout used for grouting in one continuous operation in one day. 9.4.2 Test for Bleeding Bleeding and Free Free Expansion

The Contractor shall provide one sample of the grout from each Frictional Minipile after mixing and shall protect from changes in moisture content before tests are carried out. Each sample shall be divided into 3 specimens. Each specimen is to be placed in a covered cylinder with a diameter of 100 10mm to a depth of 100 5 mm and the amount of bleeding and free expansion is measured by a scale fixed to the outside of the cylinder. Bleeding = 100% x

H2 - Hg H1

Free Expansion = 100% x where

H2 – H – H1 H1

H1- initial height of grout sample H2- height of sample measured at upper surface of water layer or hardened grout surface if water is full y absorbed Hg- height of grout portion of sample at upper surface of grout

The Contractor shall submit preliminary test results to the SO within 48 hours after the mixing of grout. If the result of the bleeding test of the grout for any pile does not comply with the specified requirements or the free expansion of the grout for any pile is greater than the specified upper limit, the Contractor shall propose changes to improve the materials, grout mix or method of production, though the failure does not constitute a failure of the pile.


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If the free expansion of the grout for any pile has a negative figure, the Contractor shall carry out test(s) at their own expense to demonstrate that the pile can fulfil the original design requirements. 9.4.3 Flow Cone Efflux Test Test

At least one sample from each pile shall be taken and tested in accordance with ASTM C939 to determine the Flow Cone Efflux time. Agree with the SO the frequency of the test. Except with SO’s prior agreement for grout mixes containing additives, grout having an efflux time of less than 15 seconds shall be rejected. 9.4.4 Test for Crushing Strength

The Contractor shall provide one sample of the grout for each Frictional Minipile after mixing and shall protect it from changes in moisture content before making test cubes. Cubes shall be prepared using 100mm cube moulds. The Contractor shall make two cubes from the sample. Strength compliance requirements shall follow GS Clause 6.55. 10.

Steel Sections

GS Clause 5.18 (iii), (v) and (vii) shall apply appl y to Frictional Minipiles. The Contractor shall employ an approved specialist firm to carry out and interpret the inspection and testing of welds, and shall provide any necessary labour and attendance. attendance. The Contractor Contractor shall submit evidence proving that operators carrying out the inspection and testing have been trained and assessed for competence in the inspection and testing of welds. In addition, the Contractor shall submit certificates of competence from a recognised authority for operators carrying out ultrasonic examination. The welded joints of steel sections shall not be lowered into the pile shaft within one hour after they are completed. The maximum length of spliced steel sections in horizontal or inclined positions shall be 24 m. 11.

Trial Pile

After the approval of the design submission and before the commencement of pile installation, one of the piles selected by the SO shall be installed as trial pile to validate the design parameters, method of installation and grouting operation proposed by the Contractor. Contractor.


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The trial pile shall be subjected to a static loading test as included in Clause 12. If the trial pile fails in the static loading test, the Contractor shall revise his piling proposal and re-submit his revised design to the SO for approval. A new trial pile shall then be installed in accordance with the revised proposal by the Contractor and then subjected to the static loading test. No installation works for the t he remaining piles shall be commenced until the trial pile has passed the static loading test. 12.

Static Loading Tests

Loading tests shall be required as instructed by SO in accordance with GS Clause 5.28. Notwithstanding GS Clause 5.28, the SO may order 3% of the installed piles or [insert drawing number] nos. of piles installed, whichever is more, to be load tested to twice the loading capacity of the respective piles. In determining the cross sectional area (A) of pile, the transformed section (comprising the grout and steel section) shall be used. The Young’s modulus of grout shall be taken as that of concrete of the same strength as given in the GS. [Guidance note: Allow for 3% (instead of 1%) of the first 100 installed piles and 1% for each next 100 installed piles thereafter to be load tested to twice the theoretical safe loading capacity] 13.

Piling Records

The Contractor shall keep records of the installation of each pile and submit two signed copies of these records to the SO not later than noon of the next working day after the pile was installed. The record shall give the following information in an approved format:a. b. c. d. e. f. g h. i. j k. l. m. n. o. p. q.

Pile reference number; Date and time of boring; Soil samples taken and in situ test carried out, if any; Date pile installed; Pile type and size; Date and time of drilling; Date of grouting; Position of pile in the works and ground level at pile position; Working level; Drilling rates and material encountered; Depth from working level to pile toe; Toe level; Depth from working level to pile head level; Length and toe level of temporary casing; Length of steel section; Grout mix; Volume of grout in pile (actual and theoretical);


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r. s. t. u.

Details of obstructions, delays and other interruptions to sequence of work; Flow rate and total time required for the grouting operation; Grouting pressure used in each stage; Any other data requested by SO.

On completion of all piling works, the Contractor shall submit to the SO two copies of the record piling plans showing, as appropriate, the position, identity number, size and top and bottom levels of each pile installed.


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Appendix Typical Details of Shear Bars


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Design of Small Diameter Frictional Piles and Cases Study

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