A pplic pplicatio tion of of Gr G r ound I mpro provement: J et Gr Grouting CONTENTS
1 I ntro ntroduction duction
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2 A pplica ppli cati tion on of Ground ound I mprov prove ement
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2.1 J et Grouting Typ Typica ical Use Uses Tec Techniqu ique Risk Health and Safety I ssues Design Consi onsiderati rations V alidation
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Applica pplicati tio on of of Gr G round I mpro provement: J et Gro Gr outing
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I ntroduction ntroduction
Groun Ground d im improvement is is becom becoming an an increa increasingly singly common techni technique quefor solvi solving ng a number of tem temporary and and perm perma anent proble problems in in the field of civi civill engineering. ri ng. The The aim of this document is to provide ide a basic understanding ing to the engine ineer such that :i) ii)
The There is a recognitio ition n when and how ground improvement can be applied lied* The There is equally a knowled ledge of the limits its and risk isks inv involve lved
The The success of ground imp improvement is due to good design ign, appropriat iate techniqu iques and most importantly experience and skill on site. 2
A pplica ppli cati tion on of of Gr G r ound ound Im I mprov prove ement
The The successful ful applica lication ion of ground improvement require ires both a correct design ign concept and also correct correct executi execution. on. I t can therefor therefore e be be rega regarded rded as a desi design gn and construct method. The use of the systems describe scri bed d below is i s inte intende nded to be taken taken as as a guide guide,, so it it is is im important to recogni recognise se that some ground conditi conditions ons may becompl complex ex leading to di difficult cult sol solutions. Consul onsulta tati tion on with with engi ngineers expe experie rienced in in the the design sign and execution xecution of jetgrouti etgrouting ng iis s al always recomm recommende ended. d. 2.1
J et Grou Grouting ting
Je J et grouting ing differs fro from permeation ion grouting ing in that the int intent with ith the lat latter is not to disturb the fabric/structure of the soil whereas with the former this has to be the case for successful application. Typ Typica ical Use Uses Because the product of jet grouti grouting ng tends to bemore structural structural in nature, it i t has has a great great range of application. When insta installed bene beneath buildi buil dings ngs it it can exten extend d foun founda dations tions through through poor ground or support support them them whil while excavations, xcavations, often often unsupported, nsupported, are carried carri ed out next next to them. I t can provi provide support underground underground where where openi openings ngs are necessary such as for for TB T BM break out or break in in or cross passage ssages s or equa equally to support support tunnel faces. Because it does does not rely rely on passa passage ge through the ground its its product usually is is more predictabl predictable e and signif signifiicantly tly les l ess s depe dependent on ground stren strength gth or composi compositi tion. on. I t can provi provide de ground water control at the the ba base of of excavati xcavation on and and prop prop retai retaining ning wal walls at thesam same tim time. I t can can truly truly be a multi ulti--appli plicati cation tool tool dealing with with supp support, groundwater control and increasing efficiency of site usage simultaneously.
Technique Jet grouting consists of drilling down with a small diameter rod system, typically 90130mm in diameter and then injecting a high pressure fluid while rotating and withdrawing the rod. There are three basic systems available:(a) Single System (b) Double system (c) Triple System The single system as the name implies, is the injection of grout only at high pressure. This was the first system to be used and gives limited column diameter and the borehole has a tendency to become blocked often resulting in ground heave. Column sizes are small usually up to 1m in diameter. The double system is effectively the single system with the addition of an air shroud to the nozzle. The addition of theair shroud increases jetting efficiently dramatically giving typically 30% or more increase in diameter for the same jetting energy. This systemis very common currently and a number of specialists operate similar systems of variable power. Typically column diameters being designed and used with the double system are now approaching 3mdue to the advent of more powerful high pressure pumps. There are other company specific systems. These are very versatile and powerful systems with column diameter capability of up to 4-5m in most soils. The triple system differs from the single and double systems in that the erosion of the ground is carried out by a high pressure water jet shrouded with air with an additional low pressure grout line. The column diameters achieved with this system are greater than those of the single systembut theenergy is relatively low compared to thedouble systems. Columns are usually only up to 1.7m to 2m (in exceptional ground) in diameter. Some companies have recently increased the power of the triple systemto match that of the double systemgiving diameter capabilities of 2-3m. The Xjet system is unique in that it consists of a pair of intersecting air-shrouded water jets with separate grout jets and is designed to cut a nominal 2mdiameter column in any ground. The figure below shows the concept.
J et Grout
X-J et
It is most applicable in variable weak ground such as soft clays and peat where overcutting of the design diameter can be a problem. The ranges of applications for the systems are similar and the selection is a site specific requirement. The single systemis rarely used unless there are concerns about the air usage and loss of strength so thechoice normally rests between the use of the double or triple system. Generally the triple system can produce more spoil but is it usually less viscous and hence offers less risk for blockage and potential structural or ground movement. Risk Jet grouting historically has been considered as a risky ground improvement process due to thehigh pressures involved and the spoil generation. With good experience, site operatives, site control and proper design thesystemcan be no risky than other processes. The main risks associated with the process are the use of high pressures and the potential generation of movement. When jet grouting is carried out beneath buildings care must betaken to ensure that heave of the walls or floors is maintained within agreed limits. It is absolutely critical that the construction of the floors and walls can withstand the potential effect of jet grouting. Where floor construction is poor and non sealed then it is advisable to maintain a visual inspection and potentially monitoring during jet grouting. Wall movement during jet grouting can be controlled in the long term by precise levelling and in the short term by the use of rotating laser targets fixed to the wall. Sequence is extremely important during underpinning operations as there is the possibility of loss of support if too large an area is jetted at thesame time. Where structural foundation condition is poor i.e. poorly cemented masonry or brick then a pregrouting exercise using lancing or end of casing can be considered with the intent to reinforce the available bonding and reduce the risk of partial wall collapse into a freshly completed column. For underpinning work it is usual to restrict the jetting of adjacent columns to intervals of at least 12 hours. For isolated foundations, the sequence must be designed to prevent loss of support and in extremecases temporary load transfer should be provided during the jet grouting operation. There is usually excellent bonding between the foundation and the jet grout columns as subsequent adjacent columns always treat this zone and ensure perfect contact. Careful attention to the location of any service or underground structure is needed as grout can end up in services, especially old sewers where the lining is not competent. It is essential to identify such services prior to grouting and consider the effects of the scheme on the service. In particular for sensitive services (i.e. gas or chemical pipelines) additional monitoring and control will be required. For grouting adjacent to tunnels, due consideration to theeffects of the injection pressure needs to be taken especially in the case of segmental construction.
When jet grouting, care must be taken to control the spoil return as any blockages are detrimentaland can cause ground movements if uncontrolled. Health and Safety Issues Apart from theH&S issues arising out of adopting this solution in situations where failure could impact on safety, the technique requires grout to be pumped at high pressure. Potential failure of grout hoses in this situation needs to be addressed as escaping grout can impact on a wide areaand cause serious injuries through the high pressure fluid or whiplash of hoses. Grouting close to underground structures needs careful consideration and any grouting carried out within 3-5m of any underground structure will require a specific risk assessment. Wherever possible, consultation with the original designer is advisable to understand the specifics of the ground improvement. The use of jet grouting for remedial ground improvement for dams or other water retaining structures needs careful consideration due to the additional risk of spoil blockage and subsequent ground fracture creating leakage paths which could ultimately cause serious problems with the safety of the structure. Design Considerations Because of the ability of jet grouting to bond soil particles and create relatively strong bodies (UCS range 2-10Mpa), thedesign process is similar to the design of brick or masonry. Strength of treated ground is usually assessed on the basis of unconfined compressive strength tests of samples obtained by insitu wet sampling or coring. The histograms shown in the Figure below demonstrate experimental unconfined compressive strengths in sandy and cohesive soils. The Japan Jet Grouting Association has adopted these distribution charts, defining the unconfined compressive strength used for design to be the minimum safe values which range between 1 to 5 percent from the least values in the whole group.
50 r e b 40 m u 30 N
N(Number) = 542 X (Average) =12.7
N(Number) =133 50
20
20
10
10 2 4 6 8 10 121416182022242628 (MN/m2) Unconfined compressive strength in sandy soil
X (Average) =2.8
r e b 40 m u 30 N
1
2
3 4 5 6 2 (MN/m) Unconfined compressive strength in cohesive soil
This definition gives the standard unconfined compressive strengths as follows (where the Water/Cement ratio of the grout is typically 1)
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Table 1. Standard strengths in designing Soil Type
Qu: Unconfined C : Cohesive f : Bond σ t : Bending Compressive strength strength tensile strength strength (MN/m2) (MN/m2) (MN/m2) 2 (MN/m ) Cohesive 1 0.3 0.1 0.2 Granular 1-3 0.2-0.5* 1/3C 2/3C * depending on grout typeaccording to the JGA Strength is usually only an issue for base slabs and shaft break out or in. In these cases the strength can influence the design as the jet grouting is required to span across openings and hence has to be designed with a minimum structural integrity. Jet grout column layout is designed based upon the two factors:(a) Hole Deviation (b) Column Diameter When drilling holes for most operations, hole verticality tolerance is unlikely to be better than 1 in 100 and is typically specified at 1 in 75. For shallow holes this does not create an issue but as depth increases it becomes a more important influence on the design of the jet grout layout. When creating a jet grout base slab, the design is normally based on a triangular grid. No Deviation
With Deviation
This figure shows the effect when a single column deviates from the design position. The solution is to reduce the column spacing. This has a significant influence on cost as column diameter and spacing determine the quantity of columns. Validation Validation is as described above. Becausejet grouting generally is structurally more sound, coring can bemore successful if care is taken. In addition the process lends itself to trial exposure whereby a number of initial columns are jetted on site and then exposed after 24-48 hours to determine size and uniformity. With jet grouting, the column diameter tends to bethe more important parameter for validation as this is
fundamental in the construction of gravity or underpinning blocks. In granular soils, strength is not usually an issue. New techniques being used by a number of jetgrouting specialists include theuse of microphones installed in boreholes close to the columns to assess whether the diameter is achieved by detecting thenoise as the grout/water jet strikes the microphone tube. Also in-column calliper systems have been used to measure diameter directly by extending calliper arms till they touch the sides of the columns Interpretation of the results can be problematic but it shows promise in providing a usable system to demonstrate the column diameter. REFERENCES 1. Xanthakos, Abramson and Bruce, Ground Control and Improvement, Chapter 8, 1994, published by Wiley (An excellent guide to jet grouting covering both design and execution) 2. RD Essler, Shibazaki, J et grouting Chapter, Ground Improvement 2nd Edition 2005 published Taylor & Francis (Covers design and case histories)
Application of Ground Improvement: Deep Mixing
CONTENTS
1 Introduction
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2 Application of Ground Improvement
2.1 Soil Mixing Typical uses Technique Quality control and quality assurance Design Considerations
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3 References and literature on deep mixing techniques
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Application of Ground Improvement: Deep Mixing
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Introduction
Ground improvement is becoming an increasingly common technique for solving a number of temporary and permanent problems in the field of civil engineering. The success of ground improvement is due to good design, appropriate techniques and most importantly experience and skill on site. Deep mixing is an in-situ soil stabilization technique using cement and/or lime as a stabilizing agent, was developed in Japan and in the Scandinavian countries independently in 1970s. The Various methods in the Deep Mixing Methods are classified in Fig. 1.1. Soil mixing methods Deep mixing Uniaxial rotation
WET
Single/multiple shaft & single/multiple blade
Multiple non-interlocking auger
DRY
Multiple interlocking auger
Single shaft, single or multiple blades
Multiple shaft, multiple blades
Hybrid mixing methods
Uniaxial rotation + hydraulic
Uniaxial rotation + linear translation
In-plane rotation +linear translation
WET
Single shaft, blade & HP jet
Linear translation only
DRY
Bladed chain/cam or wheel(s)
Single shaft & blade. Mass stabilisation
Ploughs
Figure 1.1. Classification of Deep Mixing Method. 2 Typical uses Deep mixing has been frequently applied to many structures: foundation of structures, retaining wall, embankments, slopes, excavations, soil tank and shield tunnel. The purpose of these applications is varied, including reducing settlement, increasing bearing capacity of the ground, increasing stability and ground liquefaction strength, reducing active earth pressure, cutting off ground water, and increasing pile’s bearing capacity against lateral forces, etc. The Deep Mixing Method (DMM) has usually
been applied to the improvement of soft clays and organic soils for various purposes and in various ground conditions. Depending on the purposes and ground conditions, several configurations have been conceived as shown in Fig. 2.1; a) block-type improvement, wall-type improvement. lattice-type improvement (grid-type) and group column-type improvement. Table 2.1 shows a comparison of the characteristics of the above mentioned improvements. It is concluded that the block-type improvement achieves the most stable improvement, but it is expensive. The wall-type improvement and the latticetype improvement also achieve stable improvement, and are more economical, but both require high quality continuous overlapping work.
Figure 2.1. Typical improvement patterns of treated soil mass.
Table 2.1. Characteristics of improvement types. Stability
Cost
Installation
Design Consideration
Large solid block resists external forces. Highly stable.
Volume of improvement is greater than other types. High cost.
Takes longer time because all columns are overlapped.
Design of size of block is in the same way as the gravity structures.
Requires precise operation of overlapping of long and short units.
Requires consideration of unimproved soil between walls. Improvement size affected by internal stability.
Installation sequences are complicated because lattice shape must be formed.
Requires design on threedimensional internal forces.
Installation Where lateral requires short Overlapping Group forces are period, and operation is not Colum small, high volume of required. n Type stability is improvement is obtained. small. Low cost.
Requires design on overall stability and on internal force of piles.
Block Type
Wall Type
Where all improved walls are linked firmly, high stability is obtained.
Volume of improvement is smaller than block type. Lower cost.
Cost range is Lattice Type Highly stable between (Grid next to Block Block type and Type. Wall type. Type)
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Execution
(1) DRY MIXING Dry mixing is normally carried out in accordance with some general principles, summarised in Fig. 3.1. As can be seen in the flow chart, the binder is fed into the soil in dry form with the aid of compressed air. Two major techniques for dry mixing exist at present: the Nordic and the Japanese techniques. Ai r compressor
Tank lorry
Control house
Ai r dryer
Air tank
Binder
Binder silo
Power supply
Binder
Column installation
Figure 3.1. Flow chart for the execution of dry mixing
Figure 3.2. The installation is carried out according to the following procedure, from left to right: 1) mixing tool is correctly positioned; 2) mixing shaft penetrates to the desired depth of treatment with simultaneous disaggregation of the soil by the mixing tool; 3) after reaching the desired depth, the shaft is withdrawn and at the same time, the binder in granular or powder form is injected into the soil; 4) mixing tool rotates in the horizontal plane and mixes the soil and the binder; 5) completion of the treated column.
Nordic technique
Equipments used in the Nordic countries are able to install columns to a depth of 25 m with a column diameter of normally 0,6 m to 1,0 m. The columns can be inclined up to about 70° in relation to the vertical. The machines have one mixing shaft with the injection outlet positioned at the mixing tool. Mixing energy and amount of binder are monitored and in some cases automatically controlled to achieve sufficiently uniform treated soil. The mixing tool is drilled down to the final depth and a predetermined amount of binder is added through an inner tube with an opening at the mixing tool (during the retrieval phase). During the retrieval phase, the soil and binder are mixed by continued turning of the mixing tool, eventually changing the direction several times (homogenisation phase). Both phases can be repeated for the same location, if required. The rotation speed of the mixing tool and the speed with which it is withdrawn is adjusted to produce uniform mixing, sufficient for the purpose. The amount of mixing work involved in producing a dry-mixed column depends on the type of binder, quantity of binder and type of soil. When using cement as binder compared to lime only, a higher amount of mixing energy is required. Bellows equipment has been developed to contain air and dust. J apanese technique
There are several variations of execution machines, which have either one or two mixing shafts. Each mixing shaft of these machines have several blades with 0,8 m 1,3 m diameter and are able to install columns to a depth of 33 m. The binder, usually cement powder, is brought to the mixing machine by compressed air. A bellows covering the mixing shaft has the function to avoid scatter of the air that comes up from the ground. The mixing tool is composed of several stacks of mixing blades to achieve uniformity of the treated column. The injection outlets are positioned above and below the mixing blades at the mixing shaft. A steel bar fixes the distance between two mixing shafts. The bar, and sometimes additional freely rotating (undriven or counteracting) mixing blades, also function to prevent rotation of soil adhering to the driven mixing blades and shaft. Air pressure and amount of binder are automatically controlled to achieve homogeneity of the treated column. The binder is injected during the penetration stage or both during the penetration and retrieval stages.
(2) WET MIXING Wet mixing is carried out in accordance with some general principles, which can be summarised as shown in Figure 3.3. Water
Binder
Additives
Mixing
Agitator (storage) Pressure pump Depth and rotation control
Column installation
Fl ow control
Figure 3.3. Flow chart for the execution of wet mixing In wet mixing the binder is usually cement slurry. Filler (sand and additives) may be added to the slurry when necessary. The specific quantity of slurry added can vary with depth. For machines with the outlet below the mixing tool the slurry must not be added during the retrieval phase. Whereas a continuous flight auger may be sufficient for predominantly granular soils, increasing fineness and stiffness requires more complicated mixing tools provided with mixing and cutting blades of different shapes and arrangements The rotary drives, turning the shaft, must have enough power to destroy the matrix of the soil for intimate mixture with the slurry. Depending on the type of soil and slurry, a mortar-like mixture is created which hardens during the hydration process. Strength and permeability depend strongly on the composition and characteristics of the soil (fines content, organic content, type of clay, shape of the grains, grain size distribution, grain hardness), the amount and type of binder and the mixing procedure. Wet mixing is common in Central and Southern Europe, North America and Japan. A column of treated soil is usually manufactured by the procedure as shown in Fig. 2.5. During the penetration of the mixing shaft to the desired depth of improvement, mixing blades at the bottom end of the mixing shafts cut and disturb the soft soil to reduce the strength of the original soil. At the same time, the stabilizing agent is forced into the soft soil at a constant flow rate. In the withdrawal stage of the machine, the mixing blades rotating reversibly in the horizontal plane mix the soft soil and the stabilizer again.
Figure 3.4. Procedure of the DM method for on land work. European technique
In Europe the installation of wet-mixed columns is either carried out by means of flight auger(s) (continuous or sectional, single or multiple) or by means of blades, depending on ground conditions and applications. In reinforced soil wall structures, steel bars, steel cages or steel beams can be installed into the fresh mixed-in-place columns or elements. The aid of a vibrator may be required for the installation process. J apanese technique
In Japan, the wet mixing technique has been used frequently for both on-land constructions and marine constructions (CDIT, 2002). In on-land construction, machines with one, two or four mixing shafts have been used (Figs. 3.5, 3.6). The mixing tool is composed of several stacks of mixing blades to achieve uniformity of the treated column. The mixing blade has a diameter of about 1 m, which can make a column with a cross-sectional area ranging from 0.8 m2 to 1.5 m2. The maximum depth of treatment reaches down to 40 m. The two shaft-type mixing machine usually has a bracing plate to keep the distance of the two mixing shafts (see Fig. 3.6). The plate is also expected to function to increase the mixing degree by preventing the “entrained rotation phenomenon”, a condition in which disturbed soft soil adheres to and rotates with the mixing blade without efficient mixing of the cement slurry and soil. For the single blade type mixing head, a "free blade", an extra blade about 10 cm longer than the mixing blades (see Fig. 3.6), is usually installed close to the mixing blades to prevent the "entrained rotation phenomenon".
Figure 3.5. Typical examples of DM machine for on land work.
Figure 3.6. Mixing blades of DM machine for on land work. In marine constructions, large execution vessels are usually used for rapid treatment of considerable soil volumes, Fig. 3.7. On the vessel, a mixing machine, a batching plant, storage tanks and a control room are installed. The machines for marine works usually have more than two mixing shafts. The deep-mixing machines currently available in Japan are capable of constructing columns with a cross-sectional area of 1,5 m2 to 6,9 m2 to a maximum depth from the sea water level of up to 70 m.
Figure 3.7. J apanese vessel for execution of marine wet mixing. 4 Quality control and quality assurance The design, construction and quality control procedure for the Deep Mixing method (DMM) is shown in Fig. 4.1. To ensure the sufficient quality of the stabilized column, quality control and quality assurance is required before, during and after construction. For this purpose, quality control for the Deep Mixing Method mainly consists of i) laboratory mixing tests, ii) quality control during construction and iii) post construction quality verification through check boring and column head inspection.
Figure 4.1. Example of a Flow chart for quality control and quality assurance for wet method..
5 Design Considerations (1) GROUP COLUMN-TY PE IMPROVEMENT Group column-type DMM-improved ground has frequently been applied to embankments or retaining walls in order to prevent sliding failure, to reduce deformation and, to increase bearing capacity. In the current design in Japan, group column-type improved ground is considered to be a sort of composite ground with an average strength of DMM columns and surrounding untreated soil. The slip circle analysis is carried out to determine the width of the improved ground. In the examination, the composite ground consisting of treated and untreated soils is assumed to have the average strength of the improved ground. The sliding failure of the improved ground is then examined, in which the improved ground is assumed to behave as a whole. In the examination, the stability is calculated based on the unbalanced earth pressure acting on both sides of the improved ground. The amount of consolidation settlement of improved ground overlying a stiff layer can be obtained by taking into account the stress concentration effect. (2) BLOCK-TYPE IMPROVEMENT As the treated soil strength is much higher than the surrounding soft soil strength, the block type treated soil is not considered to be a part of the ground, but rather to be a rigid structural member buried in the ground to transfer the external loads to a reliable stratum. The Japanese current design procedure was established by the Ministry of Transport, in which the design concept is, for the sake of simplicity, derived by analogy with the design procedure for a gravity-type structure such as a concrete retaining structure. In the design procedure, the external and internal stabilities are evaluated. In the external stability analysis, three failure modes are examined for the assumed cross section of the treated soil mass: sliding, overturning and bearing capacity failures. The sliding and overturning stabilities are calculated by the equilibrium of horizontal and moment forces. In the bearing capacity analysis, the induced pressure distribution on the bottom of the improved ground is calculated based on the simple force and moment equilibrium of external forces, and is confirmed to be lower than the allowable bearing capacity. In the internal stability analysis, the induced stresses in the improved ground are calculated based on elastic theory. The shape and size of the improved ground are determined so that the induced stresses become lower than the allowable strengths of the treated soil. 6
References and literature on deep mixing techniques
AFNOR: Execution of special geotechnical works - Deep mixing. prEN 14679: CEN TC 288, AFNOR, Apporved standard, 50 pp., 2005. Al-Tabbaa, A.: Soil mixing in the UK 1991- 2001: State of practice report. Journal of Ground Improvement, ThomasTelford, Vol. 7, No. 3, pp 117-126, 2003. Bredenberg, H., Holm, G. and Broms, B.B. edit. Proc. Int. Conf. on Dry Mix Methods for Deep Soil Stabilization, Stockholm, 358p., 1999. Broms, B.B.: Design of lime, lime/cement and cement columns. Keynote Lecture. Proc. of the Int. Conf. on Dry Methods for Deep Soil Stabilization, Stockholm, pp. 125-153, 1999. Bruce, D.A. : An introduction to the Deep Mixing methods as used in geotechnical applications. Federal Highway Administration, US Department of Transportation, Publication No FHWA-RD-99-138, Washington, D.C-., 135 pp., 2000. Coastal Development Institute of Technology. The Deep Mixing Method - Principle, Design and Construction-, A.A. Balkema Publishers, 2002.
Kitazume, M. and Maruyama, K . 2006. External Stability of Group Column Type Deep Mixing Improved Ground under Embankment Loading, SOILS AND FOUNDATIONS, Vol. 46, No. 3, pp.323-340. Kivelö, M.: Stabilization of embankments on soft soil with lime/cement columns. Doctoral Thesis, Royal Institute of Technology, Stockholm, 170 pp., 1998. Rathmayer, H. edit. Proc. 4th Int. Conf. on Ground Improvement Geosystems, Helsinki, 570p., 2000. Rydell, B. and Westberg, G. edit. Proc. Int. Conf. on Deep Mixing Best Practice and Recent Advances, Stockholm, 2005. Yonekura, R., Terashi, M. and Shibazaki, M. edit. Proc. 2nd Int. Conf. on Ground Improvement Geosystems, Tokyo, 1996. Åhnberg, H.: Strength of stabilised soils. A laboratory study on clays and organic soils stabilised with different types of binder. PhD-thesis LUTVDG/TVSM-06/1020, Lund University. Department of Construction Sciences. Soil Mechanics. 80 +/85/ p. 2006.
TC - 17 WORK ING GROUP D – Draft 3 APPLI CATI ON OF GROUND IM PROVEMENT : VIBRO REPLACEM ENT
1.
Introduction Ground improvement techniques are widely adopted in many European countries and are increasingly being used in North America and world-wide for a vast variety of projects. As with all systems there is the requirement for a basic understanding of the geotechnical problemthat requires to be solved. The comments provided below should be considered as an introductory guide. Discussions on the design requirements and practical issues are strongly recommended with Consultants and Specialist Contractors who are experienced in their performance.
2.
Vibro Replacement Vibro Replacement is the construction of vibrated stone columns where the added granular material is compacted using depth vibrators. These relatively rigid columns of stone then form a composite structure with the surrounding soils to achieve increased strength and reduced settlements under applied load. The columns are constructed and compacted using depth vibrators that contain oscillating weights which cause horizontal vibration. The in-situ densification of loose granular soils using similar equipment but without the addition of stone is named Deep Vibratory Compaction and is described in BS EN 14731:2005. Methods which use impact, top vibratory driven casing or ramming techniques are not included in this Standard. This note refers only to the Vibro Replacement technique, sometimes named Vibro Stone Columns, and is applicable to a wide range of soils. Typical Uses The technique is widely used to support new housing, industrial and commercial developments on poor ground. It has also been used beneath road/rail embankments, bridges, projects requiring protection against potential liquefaction caused by earthquakes, slope stability and coastal reclamation works. Stone columns can be made stiffer by either the addition of cement to the granular backfill, by the use of concrete or other binder, or in combination of concrete through particularly soft layers with stone within more firm layers. Such approach is however relatively rare in comparison to the numbers of projects that simply utilise stone backfill material.
Technique Vibro Replacement consists of large depth vibrators, typically about 300 to 600mm in diameter, that penetrate the ground to suitable depth and then constructing continuous stone columns up to the ground surface. There are three basic systems available: a) b) c)
Dry top-feed system Wet top-feed system Dry bottom-feed system
All three systems may be performed using similar types of depth vibrator, normally electrically or hydraulically driven. The eccentric weight assembly is housed in the base of a heavy tubular steel casing with tapered nose to assist penetration and vertical fines to prevent rotation as shown in the figure below. Steel follower tubes are then added to extend the length of the vibrator. The vibrators are either suspended from crawler cranes or attached to customdesigned Vibro rig units.
Cross-sections through (a) normal and (b) bottom-feed vibrators
The typical depth of treatment is within the range of 3 to 10m, although this has been extended to depths of up to 56m. Stone columns can also be constructed from barges below sea and river beds. The custom-designed Vibro rig units provide detailed computer-generated records of the construction of each stone column. The dry top-feed system is generally used where the bore is formed using airflush assisted penetration of the vibrator and would remain stable after withdrawal of the vibrator. A charge of suitable granular material is then fed from the ground surface into the base of the bore. The vibrator is re-inserted into the bore to compact the stone and again withdrawn. This sequence is then repeated to compact the column in stages up to the surface to form a continuous column of stone that interlocks tightly with the surrounding soils. The dry top-feed system is normally used when improving soils above the water table of greater than about Cu =30kPa soil strength. Typical gradings of the granular material are normally about 40 to 75mm. Stone column diameters of about 400 to 700mm will depend upon the strength of the soils and size/power of the vibrator. The wet top-feed system is the original construction method which has been largely replaced by the first and third systems. However, the wet top-feed system can be used for the improvement of soft soils below the water table where larger column diameters are required. The penetration of the vibrator is assisted by water flushing. On attaining the required depth the vibrator can be partially withdrawn to then surge and flush out to further increase the column diameter. Following formation of the open hole the water pressure is reduced and vibrator kept in the ground. Clean inert granular material is then placed around the top of the bore at ground level. This then falls under gravity against the slight upward flow of water in the annulus between sides of vibrator and soil down to the base of the bore. The vibrator is then slightly withdrawn and pushed back into the stone at the base of the bore to construct the stone columns in a series of lifts in similar manner to the top-feed system. It is however important that the water flow be maintained to ensure stability of the soils and to avoid potential contamination of the stone column. Typical grading of the granular material is about 20 to 75mm with stone column diameters of about 600 to 1200mm being constructed. The wet construction system requires consideration of provision of water supply, drainage ditches, settlement lagoons, disposal of effluent and contamination of the site surface. The dry bottom-feed system combines the benefit of improving unstable soils below the water table with dry air-flush construction technique. The bottomfeed vibrator has a permanent heavy duty supply tube attached to the vibrator to form an integral vibrator/granular material supply to the base of the vibrator. Granular material is fed into the delivery tube via a chamber on the top of the
vibrator by a hopper system that is either suspended from a crawler crane, attached to special rig unit or by water pumping. The construction sequence is the same as for the wet top-feed system, but without the ability to surge for larger column diameter, with vibrator remaining in the ground to maintain the stability of thebore. Typical material grading is about 10 to 50mm for stone column diameters of about 500 to 1000mm. The first and third systems can also be preceded by pre-boring using piling rigs to ensure that the vibrator can penetrate more competent upper layers to attain the required treatment depth. Risk Vibro Replacement is considered a safe ground improvement technique but, as with all geotechnical works, depends on experience, good site operatives, suitable controls and design for successful completion. The main risks are using the stone column systems in unsuitable soils and where there is wide variation in soil beneath the structure, for example a building that straddles a backfilled quarry edge. The stone columns develop their capacity from the confining action of the soils in which the columns are constructed. Where the soils provide little lateral support, for example layers of very soft possibly organic soil of greater thickness than the column diameter, the stone columns would potentially bulge or even shear under load to not provide the desired settlement performance. Many UK ground improvement projects involve the treatment of man-made fills. These carry the risk of variable constituents, density and strength. The main risk is however the possible localised presence of matter liable to longterm decay that would progressively reduce the column confinement with time. Some soils and fills are also susceptible to self-weight, collapse, and inundation settlements. The main risks during construction are the transmission of vibration. The proximity of operations to sensitive utilities/services, retaining walls and structures will depend upon the power characteristics of the vibrator and condition of the adjacent feature. Vibrations can be reduced but not wholly overcome by the use of cut-off trenches and pre-boring thecolumns. There is an increasing trend for the use of reclaimed materials as granular backfill. These are often not as hard as natural gravel or rock with lower angle of friction for design. The British Standard Ten per cent Fines Value, BS 812: Part 111:1990 is a useful measure of suitability of stone for use whilst the ASTM C131-89 Los Angeles test resistance should be less than 40 to avoid excessive particle breakdown during construction of the stone columns. Health and Safety Issues
The height of equipment and proximity to adjacent roads, overhead power wires and railway lines will require specific risk assessment as will performing treatment on sloping sites and near to retaining walls. Design Considerations The basic design approach is that Vibro Replacement stone columns are relatively rigid springs, the rigidity of which depends on the confining action of the soil. When the soils are of a clean granular nature, they will be densified by the horizontal vibrations to provide even greater column rigidity.
The applied loads are then supported by an array of generally stronger stone column springs surrounded by weaker springs representing the soil. The density of column construction grid centres to residual volume of soil will then dictate the degree of improvement that can be achieved. The main bearing capacity design procedures in use are: a) b) c)
DIN 4017 (adapted by Priebe) Hughes and Withers (1974) Brauns (1978)
Reduced settlements are then assessed by application of the Priebe (1995) formulae and design charts to standard classical methods of settlement prediction for the untreated soils Further design can then be applied using the stone columns’ ability to act as drains, to effectively accelerate the rate at which the reduced settlements occur, using with care vertical drain theory. Similarly, stone columns are inclusions of material with high angle of friction that can be used to enhance the stability of slopes, particularly for road embankments. The basic approach is to simply modify soil parameters pro-rata to theArea Ratio for both total and effective stress analyses. Arching theory can also be applied with care for higher loading on the columns to further increase their shear resistance. The majority of Vibro Replacement schemes are carried out on the basis of treating the full depth of the weak soils, terminating after a short penetration of better strata, with columns specifically located beneath foundation layouts. The numbers of columns and grid centres depend upon the soils profile, imposed loadings and required settlement tolerances. However, there is increasing trend to provide treatment to such depth that would ensure the majority of the stresses from the imposed loads would be accommodated within the treatment zone where stresses on the untreated soils beneath would be of a low order and unlikely to contribute significantly to total and differential settlement. Clearly, if this design approach is adopted it is
important that sufficient site investigation be performed to be able to properly assess such settlements. The age and manner of placement of deep fills are important factors in this assessment. Validation The majority of Vibro Replacement projects involve the support of structures and hence validation is normally by load test. These can range between: a) b) c)
Short-term plate load test Zone load test Embankment load test
The plate load tests are normally of less than two hours duration with plate diameter comparable to the design column diameter, normally standardised at either 600 or 900mm. Loading is quickly applied to the test column to at least twice thedesign pressure over the plate areausing craneor Vibro rig as reaction. The main aim of these tests, which are inexpensive, is quality control. The zone load tests are normally of similar size to the project bases and are performed over a group of columns by either direct loading or against a system of grillage and kentledge for a period of up to one week. These tests are far more meaningful than plate tests since they apply load to the composite structure of both stonecolumns and soil, but are far more expensive Embankment loading tests can impose loads over larger areas for longer period of time. These are particularly useful for road embankments on soft ground where time-dependent performance may be important. As with untreated soft soils, the rate of loading should be carefully controlled. In-situ tests are not normally performed to Vibro Replacement projects unless the aim is also the in-situ densification of granular soils. References 1.
DI N 4017: Shear Failure Calculations for Shallow Foundations with Vertical and Central Loading, 1979.
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Baez, J .I. and Martin, G.R.: Liquefaction observations during installation of stone columns using thevibro replacement technique, Geotechnical News, p 4144, 1992.
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Brauns, J .: Die Anfangstraglast von Schottersaulen in bindigemUntergrund, Bautechnik 55, N8, p 263-271, 1978.
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Degen, W.S.: 56mdeep vibro compaction at German lignite mining area, Proc. of 3rd Int. Conf. on Ground Improvement Geosystems, Thomas Telford, London, p 127-133, 1997.
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Engelhardt, K. and Golding, H.C.: Field testing to evaluate stone column performance in a seismic area, Geotechnique, p 61-69, 1975.
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16. Greenwood, D.A. and Kirsch, K.: Specialist ground treatment by vibratory and dynamic methods, Proc. Piling and Ground Treatment for Foundations, London, p 17-45, Thomas Telford, London, 1984. 17. Hu, W.: Physical modelling of group behaviour of stone column foundations, PhD Thesis, University of Glasgow, 1995. 18. Hughes, J .M.O. and Withers, N.J .: Reinforcing of soft cohesive soils with stone columns, Ground Engineering, Vol. 7, No 3, p 42-49, 1974. 19. K irsch, K . and Chambosse, G.: Deep vibratory compaction provides foundations for two major overseas projects, Ground Engineering, Vol. 14, No 8, 1981.
20. K irsch, K. and Sondermann, W.: Ground Improvement, Geotechnical Engineering Handbook, Vol. 2, Ed. V. Smoltczyk, Ernst & Sohn, 2003. 21. McKelvey, D., Sivakumar, V., Bell, A. and Graham, J .: Modelling vibrated stone columns in soft clay, Proc. Inst. Civ. Engrs. Geotechnical Engineering, Vol. 157, p 137-149, 2004. 22. Mitchell, J .K. and Huber, T.R.: Stone columns foundation for a wastewater treatment plant – a case history, Geotechnical Engineering, Vol. 14, p 165-185, 1983. 23. Muir Wood, D., Hu, W. and Nash, D.F.T.: Group effects in stone column foundations – model tests, Geotechnique, Vol. 50, No 6, p 689-698, 2000. 24. Munfakh, G.A., Sarkar, S.K . and Castelli, R.J .: Performance of a test embankment founded on stone columns, Proc. Piling and Ground Treatment for Foundations, Thomas Telford, London, p 259-265, 1984. 25. Priebe, H.J .: Abschatzung des Setzungsverhaltens eines durch Stopfverdichtung verbesserten Baugrundes, Bautechnik 53, H5, p160-162, 1976. 26. Priebe, H.J .: The design of Vibro Replacement, Ground Engineering, p31-37, Dec. 1995. 27. Priebe, H.J .: Vibro replacement to prevent earthquake induced liquefaction, Ground Engineering, p 30-33, Sept. 1998. 28. Raison, C.A., Slocombe, B.C., Bell, A.L. & Baez, J .I.: North Morecambe terminal, Barrow, Ground Stabilisation and Pile Foundations, Proc. of 3rd Int. Conf. on Recent Advances in Geotechnical Earthquake and Soil Dynamics, St Louis, Missouri, 1, paper 3.02, p 187-192, 1995. 29. Raju, V.R.: Vibro replacement for high earth embankments and bridge abutment slopes in Putrajaya, Malaysia, Int. Conf. on Ground Improvement Techniques, Malaysia, p 607-614, 2002. 30.
Raju, V.R., Hari K rishna, R. and Wegner, R.: Ground Improvement in Asia 1994 to 2004 – a 10 year review, Proc. Conf. on Ground Improvement Techniques, Kuala Lumpur, Malaysia, 2004.
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Raju, V.R. and Hoffmann, G.: Treatment of tin mine tailings in Kuala Lumpur using vibro replacement, Proc. 12th SEAGC, 1996.
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Van Impe, W.F. and Madhav, M.R.: Analysis and settlement of dilating stone column reinforced soil, OIAZ 3, 1992.
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