04-NSF Design and EC7 (Prof Harry Tan)

EC7 for Deep Foundations (NSF ISSUES)

Prof. Harry Tan Department of Civil and Environmental Engineering National University of Singapore GeoSS/BCA EC7 Seminar 24 April 2015 4/27/2015

1

Motivations of the Lecture  Brief Introduction to pile design based on EC7  Correct understanding of piled foundation design subjected to dragload. Dragload (negative skin friction) does not diminish pile geotechnical capacity; therefore the factor of safety will not reduce

 Pile design with NSF is a settlement issue rather than capacity issue  Demonstration of dragload cases using Unified Pile Design concept and finite element analysis

4/27/2015

2

Outline  Pile Design using EC7  Problems with BS 8004, CP4, and EC7 on dragload  Design example of dragload using EC7  Unified pile design concept

 FE simulation of single pile and groups of piles subjected to dragload  Summary

4/27/2015

3

Pile Design based on EC7 (EN1997-1:2004) Section 7 Pile Foundations 7.1 General 7.2 Limit states 7.3 Actions and design situations 7.4 Design methods and design considerations 7.5 Pile load tests 7.6 Axially loaded piles 7.7 Transversely loaded piles 7.8 Structural design of piles 7.9 Supervision of construction

4/27/2015

4

7.2 Limit States (1)P The following limit states shall be considered and an appropriate list shall be compiled: ─ loss of overall stability; ─ bearing resistance failure of the pile foundation; ─ uplift or insufficient tensile resistance of the pile foundation; ─ failure in the ground due to transverse loading of the pile foundation; ─ structural failure of the pile in compression, tension, bending, buckling or shear; ─ combined failure in the ground and in the pile foundation; ─ combined failure in the ground and in the structure; ─ excessive settlement; ─ excessive heave; ─ excessive lateral movement; ─ unacceptable vibrations.

4/27/2015

5

7.3 Actions and design situations 7.3.1 General ► axial load ► transverse (horizontal) load (7.3.2.4) 7.3.2 Actions due to ground displacement ► consolidation Downdrag (negative skin friction) (7.3.2.2) downdrag load as an action [7.3.2.2(1)P] calculated based on upper bound (max. downdrag load) [7.3.2.2(3)] this is the big issue: Should NSF force be treated as an ACTION or otherwise???… ► swelling heave (7.3.2.3) treated as an action ► landslides or earthquakes ► ground displacement due to adjacent construction

4/27/2015

6

7.4 Design methods and design considerations 7.4.1 Design methods (1)P The design shall be based on one of the following approaches

─ The results of static load tests, which have been demonstrated, by means of calculations or otherwise, to be consistent with other relevant experience; ─ Empirical or analytical calculation methods whose validity has been demonstrated by static load tests in comparable situations; ─ The results of dynamic load tests (PDA and CAPWAP) whose validity has been demonstrated by static load tests in comparable situations; ─ The observed performance of a comparable pile foundation, provided that this approach is supported by the results of site investigation and ground testing Other methods Dynamic impact tests (7.6.2.4); Pile driving formulae (7.6.2.5); wave equation analysis (7.6.2.6); Re-driving (7.6.2.7) 4/27/2015

7

7.6 Axially loaded piles Clause 7.6 is the core of the section of EN 1997-1 on pile foundations

7.6.1.1 Limit state design (1)P the design shall demonstrate that exceeding the following limit states is sufficiently improbable: ─ ULS of compressive or tensile resistance failure of a single pile; ─ ULS of compressive or tensile resistance failure of the pile foundation as a whole (pile group); ─ ULS of collapse or severe damage to a supported structure caused by excessive displacement or differential displacements of the pile foundation; ─ SLS in the supported structure caused by displacement of the piles Ultimate resistance or “failure” of compression piles [7.6.1.1(4)P] For piles in compression it is often difficult to define an ultimate limit state from a load settlement plot showing a continuous curvature. In these cases, settlement of the pile top equal to 10% of the pile base diameter should be adopted as the “failure” criterion. 4/27/2015

8

7.6.2.3 ULS from ground test results (insitu tests) Two calculation methods: ► “Model Pile” procedure [clause 7.6.2.3(5)P] ► “Alternative” procedure [clause 7.6.2.3(8)]

4/27/2015

9

“Model Pile” procedure [clause 7.6.2.3(5)P] “Model Pile” method – the values of the ground test results at each individual tested profile are used to calculate the compressive resistance of a model pile at the same location.

The procedure is, in fact, similar to that used with the results of static load tests, e.g. it involves applying a correlation factor  to the calculated resistance to account for the variability of the pile resistance and obtain the characteristic compressive resistance.

4/27/2015

10

“Model Pile” procedure [clause 7.6.2.3(5)P] Design compressive resistance, Rc;d = Rb’d + Rs;d Rb;d = Rb;k/b Rs;d = Rs;k/s The characteristic value Rb;k and Rs;k shall either be determined by:  Rc ;cal mean Rc ;cal min  Rc;k  Rb;k  Rs ;k     Min  ;      3 4   where 3 and 4 are correlation factors depend on the number of profile of tests, n, and are applied respectively to: ─ (Rc;cal)mean = (Rb;cal + Rs;cal)mean = (Rb;cal)mean + (Rs;cal)mean ─ (Rc;cal)min = (Rb;cal + Rs;cal)min Rb;cal  Rs ;cal

Rc;cal

Correlation factors  for n ground test results (Singapore NA Table A.NA.10)  For n =

1

2

3

4

5

7

10

3

1.55

1.47

1.42

1.38

1.36

1.33

1.30

4

1.55

1.39

1.33

1.29

1.26

1.20

1.15

4/27/2015

11

“Alternative” procedure [clause 7.6.2.8(8)] “Alternative” method – the ground test results (shear strength, cone resistance, etc) of all tested locations are brought together before evaluating the characteristic values of base resistance and shaft resistance in the various strata based on a cautious assessment of the test results and without applying the  factors.

4/27/2015

12

“Alternative” procedure [clause 7.6.2.3(8)] (8) The characteristic values may be obtained by calculating:

Rb;k = Ab qb;k and Rs;k =  As;i qs;i;k where qb;k and qs;i;k are characteristic value of base resistance and shaft friction in the various strata, obtained from the values of ground parameters. NOTE If this alternative procedure is applied, the values of the partial factors b and s recommended in Annex A may need to be corrected by a model factor larger than 1.0 (1.4 or 1.2). The value of the model factor may be set by the National annex.

This is the most common method for pile design in UK (Singapore) 4/27/2015

13

“Alternative” procedure [clause 7.6.2.3(8)] SS EN 1997-1:2010 Singapore National Annex to Eurocode 7 A model factor is introduced to account for uncertainty of the calculation results. Model factor = R;d The value of the model factor should be 1.4, except that it may be reduced to 1.2 if the resistance is verified by a maintained load test taken to the calculated , unfactored ultimate resistance.

4/27/2015

14

Outline  Pile Design using EC7  Problems with BS 8004, EC7, and CP4 on dragload  Design example of dragload using EC7  Unified pile design concept


4/27/2015

15

BS 8004 (CP4) on Dragload 1.2 Definitions 1.2.33 Downdrag (negative skin friction) A downwards frictional force applied to the shaft of a pile caused by the consolidation of compressible strata, e.g. under recently placed fill NOTE. Downdrag has the effect of adding load to the pile and reducing the factor of safety 4.5.6 Effect of settling ground and downdrag forces On sites underlain by recent or lightly over-consolidated clays… The drag force should be added to the net additional vertical load applied to the base of the deep foundation in the assessment of allowable bearing pressure caused by downdrag in the bearing capacity of the foundation. Donwdrag can also occur where the groundwater level is substantially lowered or where backfill is placed around the foundation …

4/27/2015

16

BS 8004 (CP4) on Dragload 7.3.6 Negative skin friction …The downdrag drag on the pile may throw enough additional load on the pile point or base to make the total settlement excessive… When piles are driven through sensitive clays the resulting remoulding may initiate local consolidation. The negative friction force due to this consolidation may be estimated as the cohesion of the remoulded clay multiplied by the surface area of the pile shaft. Where it is expected that the soil around the shafts of end bearing piles will consolidate, the skin friction exerted by the downdrag moving soil should be estimated in accordance with the properties of materials. The downward force will need to be taken into account when the allowable load on the pile is calculated... 4/27/2015

17

BS 8004 (CP4) on Dragload 7.5.3 Calculation from soil tests …. Q = f As + q Ab The special case of negative skin friction or downdrag has been mentioned in 7.3.6. Soil strata imposing negative friction forces will introduce negative components into the fAs term. If all the strata above the level of the pile base are liable to settlement, the term fAs will be negative. It should then be treated as part of the design load and not be divided by the factor of safety.

4/27/2015

18

EC7 Geotechnical Design Part 1: General Rules Section 1 Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Section 8 Section 9 Section 10 Section 11 Section 12 Annex A - J 4/27/2015

General Basic of geotechnical design Geotechnical data Supervision of construction, monitoring and maintenance Fill, dewatering, ground improvement and reinforcement Spread Foundations Pile Foundations Anchorages Retaining Structures Hydraulic failure Overall stability Embankments

19

EC7 on Downdrag (actually allow flexibility for correct analysis of NSF as settlement action) 7.3.2.2 Downdrag (negative skin friction) (1)P If ultimate limit state design calculations are carried out with the downdrag load as an action (called the dragload), its value shall be maximum, which could be generated by the downward movement of the ground relative to the pile (2) Calculation of maximum downdrag loads should take account of the shear resistance at the interface between the soil and the pile shaft and downward movement of the ground due to self-weight compression and any surface load around the pile. (3) An upper bound to the downdrag load on a group of piles may be calculated from the weight of the surcharge causing the movement and taking into account any changes in ground-water pressure due to ground-water lowering, consolidation or pile driving. (4) Where settlement of the ground after pile installation is expected to be small, an economic design may be obtained by treating the settlement of the ground as the action and carrying out an interaction analysis.** 4/27/2015

20

EC7 on Downdrag (actually allow flexibility for correct analysis of NSF as settlement action) 7.3.2.2 Downdrag (negative skin friction) (5)P The design value of settlement of the ground shall be derived taking account of material weight densities and compressibility in accordance with 2.4.3. (i.e. use appropriate characteristic values of soil layers to give good estimates of settlements) (6) Interaction calculations should take account of the displacement of the pile relative to the surrounding moving ground, the shear resistance of the soil along the shaft of the pile, the weight of the soil and the expected surface loads around each pile, which are the cause of the downdrag. (7) Normally, downdrag and transient loading need not be considred simultaneously in load combinations.

4/27/2015

21

EC7 on Dragload 7.6.2.2 Ultimate compressive resistance from static load tests …

(5)P In the case of a pile foundation subjected to downdrag, the pile resistance at failure, or at a displacement that equals the criterion for the verification of the ultimate limit state determined from the load test results, shall be corrected. The correction shall be achieved by subtracting the measured, or the most unfavourable, positive shaft resistance in the compressible stratum and in the strata above, where negative skin friction develops, from the loads measured at the pile head. (6) During the load test of a pile subject to downdrag, positive shaft friction will develop along the total length of the pile and should be considered in accordance with 7.3.2.2(6). (The maximum test load applied to the working pile should be in excess of the sum of the design external load plus twice the downdrag force.)

4/27/2015

22

CP4 on Dragload 7.3.6 Negative skin friction … The allowable geotechnical capacity of a pile subject to negative skin friction in the long term (Qal) is given by the following general equation:

Q al  where

4/27/2015

Qb  Qsp Fs

 Pc   Qsn

Qb is the ultimate end bearing resistance Qsp is the ultimate positive shaft resistance below the neutral plane Fs is the geotechnical factor of safety Pc is the dead load plus sustained load to be carried by each pile Qsn is the negative skin friction load  is the degree of mobilization typically 0.67, although 1.0 may be used in specific cases

23

Concluding remarks from BS and EC7 • BS 8004, CP4 and EC7 treat dragload as an unfavourable design load that diminishes pile geotechnical capacity • The pile design can appear to have inadequate safety factor, or, worst, negative capacity • Piled foundation cost will increase significantly and unnecessary • This is grossly incorrect. The codes do not address the issue of dragload holistically. Sounds unconvincing… Well… the following notes will, hopefully, convince you that draglod is not a capacity problem but a downdrag (settlement) issue 4/27/2015

24

Outline  Pile Design using EC7  Problems with BS 8004, EC7, and CP4 on dragload  Design example using EC7  Unified pile design concept

 FE simulation of piled foundation subjected to dragload  Summary

4/27/2015

25

First, Let’s look at example for pile subject to dragload based on EC7 Example 1

(modified from Simpson & Driscoll, 1998)

4/27/2015

Pile type

Bored pile

Pile diameter

300 mm

Soft clay unit NSF, qD;k characteristic value

20 kPa

Stiff clay unit shaft resistance, qs;k characteristic value

50 kPa

Permanent vertical load, Gk

300 kN

(Frank et al., 2005)

26

Pile subject to dragload based on EC7 Example 1

Characteristic and design value of loads ► Permanent load, Gk = 300 kN ► Total drag load, FD;k =  x 0.3 x 5 x 20 = 94.2 kN ► Positive shaft resistance, Rs;k =  x 0.3 x LR x 50 = 47.1LR kN Total design load, Fc;d = GGk + FD;k Design resistance, Rc;d = Rs;k/s + Rb;k/b DA1 Combination 1: A1 + M1 + R1 Total design load, Fc’d = 1.35 x 300 + 1.35 x 94.2 = 532.2 kN Design resistance, Rc;d = 47.1LR/1.0 = 47.1LR kN Condition Fc;d  Rc;d leads to LR  532.2/47.1 = 11.30 m

Note: the correlation factor,  is ignored

DA1 Combination 2: A1 + (M1 or M2) + R4 Total design load, Fc’d = 1.0 x 300 + 1.25 x 94.2 = 417.8 kN Design resistance, Rc;d = 47.1LR/1.3 = 36.2LR kN Condition Fc;d  Rc;d leads to LR  417.8/36.2 = 11.54 m 4/27/2015

27

Pile subject to dragload based on EC7 Example 1

DA2: A1 + M1 + R2 Total design load, Fc’d = 1.35 x 300 + 1.35 x 94.2 = 532.2 kN Design resistance, Rc;d = 47.1LR/1.1 = 42.8LR kN Condition Fc;d  Rc;d leads to LR  532.2/42.8 = 12.43 m DA3: (A1 or A2) + M2 + R3 Total design load, Fc’d = 1.35 x 300 + 1.25 x 94.2 = 522.8 kN Design resistance, Rc;d = 47.1LR/1.25 = 37.7LR kN Condition Fc;d  Rc;d leads to LR  417.8/37.7 = 13.87 m Conclusion DA-3 requires the longest pile length of the three Design Approaches: LR = 13.87 m, compard with LR = 11.54m for DA-1 and LR = 12.43m for DA-2. This is due to the fact that for DA-3 the values of the three partial factors are equal to 1.25 or 1.35. It can also be argued that the application of the correlation factor  to the estimated values of shaft friction qs in DA-1 and DA-2 (see clauses 7.6.2.2(8)P and 7.6.2.3(5)P) would have led to lower values for qs;k than in DA-3 (for which they are not used).

4/27/2015

28

Now… Let’s consider that dragload does not reduce capacity Example 1

DA1 Combination 1: A1 + M1 + R1 Total design load, Fc’d = 1.35 x 300 = 405 kN Design resistance, Rc;d = (94.2 + 47.1LR)/1.0 = 94.2 + 47.1LR kN Condition Fc;d  Rc;d leads to LR  310.8/47.1 = 6.70 m (cf. 11.30m) DA1 Combination 2: A1 + (M1 or M2) + R4 Total design load, Fc’d = 1.0 x 300 = 300 kN Design resistance, Rc;d = (94.2 + 47.1LR)/1.3 = 72.5 + 36.2LR kN Condition Fc;d  Rc;d leads to LR  227.5/36.2 = 6.28 m (cf. 11.54m) DA2: A1 + M1 + R2 Total design load, Fc’d = 1.35 x 300 = 405 kN Design resistance, Rc;d = (94.2 + 47.1LR)/1.1 = 85.6 + 42.8LR kN Condition Fc;d  Rc;d leads to LR  319.4/42.8 = 7.46 m (cf. 12.43m) DA3: (A1 or A2) + M2 + R3 Total design load, Fc’d = 1.35 x 300 = 405 kN Design resistance, Rc;d = (94.2 + 47.1LR)/1.25 = 75.4 + 37.7LR kN Condition Fc;d  Rc;d leads to LR  329.6/37.7 = 8.74 m (cf. 13.87m) 4/27/2015

29

Now… Let’s consider that dragload does not reduce capacity Example 1

Imagine that the project requires 1000 piles, the cost saving will be 1000 x 5.13 m = 5,130 m pile length! Assuming that the thickness of the soft clay layer is now 15m instead of 5m (in Singapore, typical Marine clay thickness is 10-30 m). The dragload force becomes 282.7 kN. Using 13.87m embedded pile length in stiff clay (from DA-3), the pile design will have a negative capacity (Fc;d = 1.35 x 300 + 1.25 x 282.7 = 758.4 kN cf. Rc;d = 37.7LR = 522.9 kN). Therefore, in order to satisfy the total design load based on EC7, the embedment length in stiff clay need to be LR = 758.4/37.7 = 20m. In order words, to sustain 300 kN permanent load, the total pile length required is 25 + 20 = 45m.

4/27/2015

30



4/27/2015

31

The Unified Pile Design method The following slides are extracted from pile design courses given by Dr. Fellenius. For more detail information, please refer to Fellenius B.H. (2012). Basics of Foundation Design. Available freely from www.fellenius.net

4/27/2015

32

dragload is treated as an unfavourable action Drag load must neither be subtracted from the pile capacity nor from the allowable load ALLOWABLE LOAD - (Fs = 2.5)

500

1,000

1,500

2,000

ALLOWABLE LOAD minus DRAGLOAD*1.0

CAPACITY

2,500

0

0

0

5

5

DEPTH (m)

DEPTH (m)

0

LOAD (KN)

Effect of subtracting the drag load from the allowable load -- only!

10

1,000

1,500

2,000

CAPACITY

2,500

10

15

15 DRAG LOAD

20

500

LOAD (KN)

DRAG LOAD

20

INCREASE ! If the pile capacity had first been reduced with the amount of the drag load, there would have been no room left for the working load! 4/27/2015

33

Similarly for the EC7 and LRFD:

Do not include the drag load when determining the factored resistance!

Drag load not subtracted from the factored resistance FACTORED RESISTANCE

LOAD (KN) CAPACITY 500

1,000

1,500

2,000

FACTORED RESISTANCE minus FACTORED DRAGLOAD Factors = 0.6 and 1.5, respectively

2,500

0

0

0

5

5

10 DRAG LOAD

15

DEPTH (m)

DEPTH (m)

0

Drag load factored and subtracted!

500

FACTORED RESISTANCE

LOAD (KN) 1,000

1,500

2,000

CAPACITY

2,500

10

15 DRAG LOAD

20

4/27/2015

20

34

SETTLEMENT Load placed on a pile causes downward movements of the pile head due to: 1. 'Elastic' compression of the pile.

2. Load transfer movement -- the movement response of the soil at the pile toe.. 3. Settlement below the pile toe due to the increase of stress in the soil. This is only of importance for large pile groups, and where the soil layers below the piles are compressible. A drag load will only directly cause movement due to Point 1, the 'elastic' compression. While it could be argued that Point 2 also is at play, because the stiffness of the soil at the pile toe is an important factor here, it is mostly the downdrag that governs (a) the pile toe movement, (b) the pile toe load, and (c) the location of the neutral plane in an interactive — "unified" — process. The drag load cannot cause settlement due to Point 3, because there has been no stress change in the soil below the pile toe.

4/27/2015

35

• Therefore, negative-skin-friction/dragload does not diminish geotechnical capacity.

• Drag load (and dead load) is a matter for the pile structural strength, and • The main question is "will settlement occur around the pile(s) that can cause downdrag. • The approach is expressed in “The Unified Design

Method”, which is a method based on the interaction between forces and movements.

4/27/2015

36

The Unified Design Method is a three-step approach 1.

The dead plus live load must be smaller than the pile capacity divided by an appropriate factor of safety. The drag load is not included when designing against the bearing capacity.

2.

The dead load plus the drag load must be smaller than the structural strength divided with a appropriate factor of safety. The live load is not included because live load and drag load cannot coexist.

3.

The settlement of the pile (pile group) must be smaller than a limiting value. The live load and drag load are not included in this analysis. (The load from the structure does not normally cause much settlement, but the settlement due to other causes can be large. The latter is called downdrag). 4/27/2015

37

"

Construting the Neutral Plane and Determining the Allowable Load 4/27/2015

38

The distribution of load at the pile cap is governed by the load-transfer behavior of the piles. The “design pile” can be said to be the average pile. However, the loads can differ considerably between the piles depending on toe resistance, length of piles.

The location of the neutral plane is the result of Nature’s iterations to find the force equilibrium. If the end result — by design or by mistake — is that the neutral plane lies in or above a compressible soil layer, the pile group will settle even if the total factor of safety appears to be acceptable. 4/27/2015

39

The principles of the mechanism are illustrated in the following three diagrams

The mobilized toe resistance, Rt, is a function of the Net Pile Toe Movement 4/27/2015

40

Pile toe response for where the settlement is small (1) and where it is large (2) LOAD and RESISTANCE 0

SETTLEMENT

1,500

0

0

200

0

Utimate Resistance

1

2

DEPTH

NEUTRAL PLANE 1 NEUTRAL PLANE 2

1 2 Toe Penetrations = Movement into the soil

Note, the magnitude of settlement affects not only the magnitude of toe resistance but also the length of the Transition Zone 4/27/2015

41

Pile toe response for where the settlement is small (1) and where it is large (2), showing toe penetration LOAD and RESISTANCE -500

SETTLEMENT

1,000

0

0

A

B

200

0

Utimate Resistance

1

DEPTH

2

NEUTRAL PLANE 1 NEUTRAL PLANE 2

TOE PENETRATION 0

3

Toe Resistances

a

b

c

Toe Penetrations

TOE RESISTANCE

12

C

0

a

b

c

1

2 3

Note, the magnitude of settlement affects not only the magnitude of toe resistance but also the length of the Transition Zone: 4/27/2015 42



4/27/2015

43

FE simulation of piled foundation subjected to dragload (Single Pile Interaction Analysis) • Verification of unified design pile concept using FE

• Hypothetical site with three soil layers: fill, soft clay and dense sand • Simulation of short-term pile load test (undrained situation) • Ground settlement due to surcharge loading at various magnitude (10, 20 and 40 kPa) • Pile load transfer due to dragload at different working load (2000, 4000 and 6000 kN) • Consolidation analysis to simulate the development of dragload as the soils settle with time • Effect of bitumen coating • Results comparison 4/27/2015

44

Hypothetical site Head load (P) 2, 4 and 6 MN

Surcharge loading (10, 20 and 40 kPa)

Fill (3m thick) s = 20 kN/m3; E50;ref = 10MPa; c’ = 0; ’ = 30o

Soft clay (12m thick) s = 16 kN/m3; Cc = 1.0; Cr = 0.1; eo = 2.0, c’ = 0; ’ = 20o k = 1 x 10-9 m/s

Axi-symmetric model Pile diameter, D = 1.128 m Pile length, L = 20 m (pile cross-sectional area = 1 m2)

25 m

Pile concrete properties: Concrete modulus = 30GPa Rinterface = 1.0 Rinterface = 0.10 (with bitumen coating at fill and soft clay layer)

Dense sand (10m thick) s = 20 kN/m3; E50;ref = 30MPa; c’ = 0; ’ = 40o

Dummy plate pile with EA 1E6 times smaller than real pile Soil constitutive model: Hardening Soil (HS)

4/27/2015

25 m

45

Load – movement curve (short term) 0

2000

4000

Load (kN) 6000

8000

10000

12000

0 Head load - Head Mvmnt

40

Toe load - Toe Mvmnt

Movement (mm)

Elastic comprs

80 120

160 200

4/27/2015

Head Load [kN] 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Toe Load [kN] 0 344 595 931 1269 1714 2228 2779 3445 4053 4752

Shaft Load [kN] 0 656 1405 2069 2731 3286 3772 4221 4555 4947 5248

Head mvmnt [mm] 0 3.785 9.914 20.273 32.066 48.3 68.757 93.178 121.227 150 182.28

Toe mvmnt [mm] 0 3.135 8.708 18.519 29.752 45.411 65.266 89.074 116.491 144.647 176.289

Elastic compr [mm] 0 0.65 1.206 1.754 2.314 2.889 3.491 4.104 4.736 5.353 46 5.991

Typical results at 40 kPa surcharge with WL = 4MN Load (kN) 0

2000

4000

Settlement (mm) 6000

0

200

400

600

Unit resistance (kPa) 800

-100

0

100

200

300

0

(1) Initial load distribution

Subjected to dragload Working load condition

10 (2) DRAG LOAD

Neutral plane

15

20

(1) + (2) Long-term load transfer

25

Drag load does not reduce pile geotechnical capacity 4/27/2015

Net toe penetration

0 Toe movement (mm)

Depth (m)

5

soil settlement

Pile

Toe load (kN) 2000 4000

6000

0 50 100 150

200

Interdependence between soil settlement, pile load-movement and 47 pile load transfer

Pile responses due to various surcharge load (WL=4000kN) Load (kN) 0

2000

Unit resistance (kPa)

Settlement (mm)

4000

6000

0

200

400

600

800

-100

0

100

200

300

0

5

Working load condition

Depth (m)

Initial load distribution

10 Large transition zone

Small transition zone

15

20

soil settlement at 10, 20 and 40 kPa Unit resistance at 10,20 and 40kPa pile settlement at 10, 20 and 40 kPa

0

Long-term load transfer for 10,20 and 40 kPa

Settlement (mm) 100 200

300

8

25

For the same head load, larger soil settlement results in deeper NP, larger drag load and larger mobilized toe resistance. 4/27/2015

Depth (m)

10 12 14 16

48

Pile responses at various level of WL (surcharge 40kPa) Load (kN) 0

2000

4000

Settlement (mm)

6000

8000

0

200

400

600


-100

0

100

200

300

0 2MN

4MN

6MN soil settlement

10 NP: 3436KN

Unit resistance at WL=2, 4 and 6MN

NP: NP: 7224KN 5324KN

15

pile settlement at WL=2, 4 and 6 MN

20 WL=2000kN

Net toe penetration

WL=4000kN 25

WL=6000kN

For the same soil settlement, larger pile head load results in shallower NP, smaller drag load and larger mobilized toe resistance. 4/27/2015

0 Toe movement (mm)

Depth (m)

5

Toe load (kN) 2000 4000

6000

0 50 100 150 200

49

Consolidation analysis at WL=4MN and surcharge 40kPa Load (kN) 0

2000

Settlement (mm)

4000

6000

0

200

400

600


-100

0

100

200

300

0 1 yr 5 yr

5

15 yr

Working load condition

Depth (m)

final soil settlement

10 Neutral Plane

Unit resistance at 1, 5, 15 yr consolidation and fully drained

15

20

25

4/27/2015

Initial 1 year 5 year 15 year Fully Drained

50

Effect of bitumen coating (R=0.1 for fill and soft clay layers) Load (kN) 0

2000

4000

Settlement (mm) 6000

0

200

400

600


-100

0

100

200

300

0 Short-term load transfer without bitumen

5

10

Depth (m)

with bitumen Working load condition without bitumen

without bitumen soil settlement

with bitumen NP=4285 kN

with bitumen

Neutral Plane without bitumen

15

without bitumen N=5324 kN

20

25

30

Bitumen coating of pile shaft reduces drag load significantly and smaller settlement. However, at the same time, pile capacity also reduces. 4/27/2015

51

Importance of toe load – toe penetration curve Variable working load cases Head Load Toe Load Toe Penetration [kN] [kN] [mm] 2000 1180 27.99 4000 2241 65.27 6000 3431 111.58

0

1000

Variable surcharge load cases Surcharge Toe Load Toe Penetration [kPa] [kN] [mm] 10 kPa 1704 44.38 20 kPa 1951 53.42 40 kPa 2241 65.27

Toe load (kN) 2000 3000 4000

5000

6000

0

Toe movement (mm)

40

Toe load - Toe Mvmnt variable working load cases (with dragload) variable surcharge load cases (with dragload)

80 120 160

4/27/2015

200

52

Force and settlement (downdrag) interactive design. The unified pile design for capacity, drag load, settlement, and downdrag Qd 0

2,000

SETTLEMENT (mm) LOAD (KN) 4,000

Pile Cap Settlement

0

6,000 Silt Sand

200

5

Clay

DEPTH (m)

DEPTH (m)

150

10

10 15

100

0

0 5

50

Soil Settlement

15 20

20 O-cell

25

25

Till

30

30

Pile toe load in the load distribution diagram must match the toe load induced by the toe movement (penetration), which match is achieved by a trialand-error procedure.

TOE LOAD (KN)

0 1,000

q-z relation 2,000 3,000 4,000 0

50

100

PILE TOE PENETRATION (mm)

The final solution is based on three "knowns": The shaft resistance distribution, the toe load-movement response, and the overall settlement distribution. Which all comes from basic site and project knowledge. 4/27/2015 53

Simulated Load Tests Results Pile movement [mm]

10

30

50

2000

4/27/2015

4000

6000

LOAD [kN]

SUMMARY RESULTS •NSF do not affect Ultimate Pile Resistance (about 6500 kN in above cases) • Soil settlements (So) produce drag-loads (NSF) on piles • Larger So showed softer pile response; and larger pile settlements

54

Results of load tests on bitumen coated piles Pile movement [mm]

10

30

Uncoated Pile Bitumen coated piles 50

2000

4000

6000

LOAD [kN]

• Bitumen Coating reduces total resistance (geotechnical capacity) of pile from 6500 kN to 5300 kN • But the external ground settlements influence on pile movement is almost insignificant compared to uncoated pile 4/27/2015

55

FE analysis of groups of piles Interaction Analysis Hypothetical cases (1, 2, 4, 9 and 36 piles) as per Fellenius (2012)

3x3m (9 piles) 6x6m (36 piles)

4/27/2015

2x2m (4 piles)

2x1m (2 piles)

1x1m (1 pile)

► Driven concrete pile, D = 318 mm (circumference area, A = 1 m2/m), pile length, L = 20 m ► Each pile in the group has a 1.0m2 portion of the total group area. ► c/c spacing = 1.0 m, i.e., 3.14D ► Soil layers are similar to that of previous slides ► No working load applied, 40 kPa surcharge ► Pile cap thickness= 1m, except for 36 piles (2m thick) 56

FE analysis of groups of piles 6x6m (36 piles) Surcharge 40 kPa

Pile cap thickness = 2m



corner side

Dense sand (15m thick) s = 20 kN/m3; E50;ref = 30MPa; c’ = 0; ’ = 40o

interior centre 4/27/2015

57

FE analysis of groups of piles 2x1m (2 piles)

Surcharge 40 kPa

1x1m (1 pile)

30 m



2x2m (4 piles)

3x3m (9 piles)

Pile cap thickness = 1m for all groups side

corner Dense sand (15m thick) s = 20 kN/m3; E50;ref = 30MPa; c’ = 0; ’ = 40o

4/27/2015

30 m

single

centre

58

Group of 36 piles results 0

200

Load (kN)

400

600

Settlement (mm) 200 400 600

0

0

800

0 soil settlement

5

5 single

Pile settlement

centre

Interior

15

20

25

Single pile corner Side Interior Centre

10

15

Neutral Plane 0

Settlement (mm) 5 10 15

20

12

20

single

25

Group of piles is beneficiary in reducing drag load. The innermost piles see smaller drag load. For group of piles to settle uniformly, the group must have the same neutral plane location. 4/27/2015

Depth (m)

10

Depth (m)

Depth (m)

corner side

piles in group

14

16

soil

59 18

Group of 2, 4, and 9 piles results 0

200

Load (kN)

400

600

0

0

600

5

single

10

15

20

25

400

0

Depth (m)

Depth (m)

5

200

Load (kN)

10

single

centre

side corner

15

20 Single pile 2 piles group 4 piles group

25

Group of piles is beneficiary in reducing drag load. Therefore, designing piled foundation using single pile case is quite conservative 4/27/2015

Single pile 9 piles group (corner) 9 piles group (centre) 9 piles group (side)

60

Measured group response Okabe Field Experiments (1973)

61

Centrifuge Experiments

62



4/27/2015

63

Summary ► Pile design according to EC7 design approaches has been presented ► EC7, BS8004 and CP4 do not address the dragload (or NSF) correctly. ► It has been shown here using FE analysis of single pile and groups of piles that dragload does not reduce pile geotechnical capacity. The key point in pile design is settlement not capacity. ► FE analysis can easily predict the location of NP with no iterations required.

► Group of piles connecting to a rigid pile cap has a beneficiary effect in reducing the dragload. ► Pile design subjected to dragload using single pile scenario is quite conservative. 4/27/2015

64

EC7 Provision for NSF Design • In essence EC7 do allow us to do specialized FEM analysis to design for NSF • This will enable us to take advantage of the actual expected NSF force over the period of design life • It will also allow us to include pile group effects where much reduced NSF will be observed in the inner piles of large pile groups or piled-raft foundations

4/27/2015

65

EC7 ALLOWS FOR INNOVATIVE DESIGN FOR NSF BY USING GOOD FEM PILE-SOIL INTERACTION ANALYSIS TO ACCOUNT FOR CORRECT CONSOLIDATION SETTLEMENTS (TREATED AS ACTION)

4/27/2015

66

04-NSF Design and EC7 (Prof Harry Tan)

Recommend Documents