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BORED DISPLACEMENT PILES IN STIFF CLAY

TECHNICAL NOTE

Introduction

Bored displacement piling has emerged as an economical pile construction method, particularly for dealing with the problems associated with contaminated land and brownfield redevelopment. The risk as well as the cost of handling, and disposing of, contaminated material is greatly reduced. Noise and vibration are also kept to a minimum.

The development of bored displacement piles can be seen to have followed three stages. These are described by Van Impe (2001) and classifi ed by Baxter et al. (2006).

This technical note is concerned with the latest stage of development, a high displacement, very low spoil, cylindrical bored displacement pile.

These piles are ideally suited to either loose to medium dense coarse grained soils or soft to firm fi ne grained soils and have been to date largely used in these soil types.

A case study of their application in stiff to very stiff London Clay is presented and discussed.The case study forms part of ongoing research being undertaken jointly by piling contractor Rock and Alluvium and the Centre for Innovative and Collaborative Engineering (CICE) at Loughborough University.

Bored displacement piles

A bored displacement pile is formed using a shaped auger to cause lateral displacement of the earth to form a void, which is then fi lled with reinforced concrete to form the pile.

It is useful to consider the construction process as three stages, described below. These are the insertion and passing of the auger, stress relief following the passing of the auger and the withdrawal and concreting phase.

During the first phase, flight design on the leading part of the auger pulls the tool into the ground while the tapered stem displaces the soil laterally to the maximum diameter.

The main displacement is horizontal with static compaction and minimal disturbance to the soil structure (Hollingsworth and ImboBurg, 1992). In fine-grained soils, subsequent behaviour will be further influenced by the generation and then dissipation of excess pore water pressures.

Once the auger head has passed, the narrower stem allows the soil to relax back towards the hole. This is necessary to reduce the friction acting on the tool but complicates the stress history of the soil.

When the design depth has been reached the withdrawal and concreting phase can begin. The auger can be removed with or without rotation. When withdrawal is conducted with rotation, an upper, left handed fl ight causes the displacement of any soil which has fallen in or relaxed towards the hole during construction. This is particularly useful for loose materials. During withdrawal, concrete is pumped through the hollow stem of the auger at a pressure suffi cient to maintain the bore. Figure 1 shows the three stages of the construction process.

High displacement, very low spoil, cylindrical bored displacement piles offer higher structural load capacity than the heliform variety. The full diameter can be employed to carry the imposed stress within the pile structure and there are no concerns over the integrity of helical fl anges.

Case study

Traditionally, bored displacement piles have been used where the ground conditions permit displacement to occur relatively easily.

However, the development of more effi cient displacement augers and of purpose built bored displacement piling rigs have allowed the use of bored displacement piles to be extended to stiffer and denser soils.

At the site selected for this case study, Rock and Alluvium was able to install displacement piles into stiff to very stiff London Clay using a customised Soilmec R625 bored displacement piling rig fi tted with the firm's displacement auger.

The R625 is a 55t piling rig adapted to suit bored displacement piling.

Crucially, it can provide 320kN of pull down force and 250kNm of torque to the displacement tool.

Ground conditions

The site is in Wimbledon, south west London. The soil typically consists of about 1.5m of fill material overlying a further 1.5m of alluvial deposits, largely reported as silty sandy clay. Below 3m the site is London Clay to unproven depth. The clay is weathered to about 5m.

Seven boreholes were put down at the site. Insitu SPT tests were carried out and undisturbed samples taken for quick undrained triaxial testing. The SPT results were correlated to values for cohesion using the relationship proposed by Stroud and Butler (1975). Figure 2 shows the cohesion of the soil with depth.

For design and analysis purposes it has been assumed that the cohesion increases linearly with depth.

A linear regression was therefore performed on the site investigation data to extract the mean cohesion with depth from the underlying variation. The mean regression line is shown in Figure 2. At the location of the test piles the alluvial deposits are indistinguishable from the London Clay. The linear strength profi le is assumed from the surface.

Design and construction

The works comprised about 1900 piles of 400mm nominal diameter.

Working loads on the piles vary widely from 200kN to over 1000kN with pile depths of over 18m being required.

This meant installing displacement piles into very stiff clay. Preliminary test piles were constructed to demonstrate the constructability of such a high displacement pile in this material and to confi rm design parameters.

Production of the working piles was quick and efficient comparing well with installation of CFA piles in similar conditions. Typical production was about 20 piles (350 linear metres) per day.

Test piles

Two test piles were installed and tested to ascertain their ultimate capacity. The piles were constructed as 400mm diameter piles with depths of 15.2m (Test Pile 1) and 16m (Test Pile 2). The tests were of the maintained load (ML) type and were carried out in general accordance with the ICE Specification for piling and embedded retaining walls (ICE, 1996). The load settlement behaviour is shown in Figure 3.

To facilitate design of the working piles, a back analysis of the test pile results has been carried out to ascertain the empirical shaft adhesion factor, alpha. Numerous pile failure criteria exist. In this instance, inspection of the pile load settlement curve reveals that the behaviour is tending towards a clearly visible maximum.

From Figure 3 the capacity can be interpreted as approximately 1700kN for Test Pile 1 and 1800kN for Test Pile 2.

The piles have not been instrumented and hence it is not possible to obtain separate values for the load carried by the shaft and by the base of each pile. The end bearing capacity, Q b, can be calculated from the undrained shear strength and the pile geometry using the formula:

Where C ubase is the cohesion at the base of the pile, A b is the area of the base of the pile and Nc is the bearing capacity factor. Skempton (1959) demonstrated that, for bored piles in London Clay, a value of Nc=9 is sufficiently accurate. End bearing capacity for each test pile is calculated from the mean value of cohesion at the depth of the base.

The shaft capacity of the pile can therefore be deduced as shown in Table 1. The shaft capacity of the pile, Q s, is given by the formula:

Where c u is the average cohesion of the soil over the length of the pile, a is the adhesion factor, and As is the area of the shaft.

By considering the mean shear strength acting over the length of the pile, it is possible to calculate the empirical correlation factor, a, by rearranging Equation 2.

Table 1 shows the measured and calculated values for the capacities of the pile and the deduced adhesion factor. The investigation yielded values for the adhesion factor of 0.70 and 0.69 for the test piles. The a factor represents the adhesion mobilised. It is regularly used to model a collection of complex processes which occur during pile construction and loading. It is even adjusted to mitigate risk of uncertainty (LDSA, 2001), which gives rise to confusion over the performance of piles and the consideration of risk which would be better contained explicitly in a separate calculation of safety factors.

It is important to remember therefore that the values found in this investigation are specifi c to the particular pile type, its method of construction, and the site. A further series of investigations are planned by Loughborough University in collaboration with Rock and Alluvium to extend the research to other sites, soil types and to explore the complex processes undergone by the soil during bored displacement pile construction.

Conclusions

Application of high displacement, very low spoil, cylindrical bored displacement piles has been described for a site where soils consisted of stiff to very stiff clay and demonstrated to be successful. Expendable test piles have been used to demonstrate the capacity of piles constructed in this manner and ascertain values for the adhesion factor for this type of construction in these materials.

David Baxter is a research engineer at the Centre for Innovative and Collaborative Engineering (CICE) at Loughborough University working in conjunction with Rock and Alluvium. Steve Hadley is construction manager at Rock and Alluvium.

References

Baxter DJ, Dixon N, Fleming PR and Hadley SH (2006). Bored displacement piles - A United Kingdom perspective, Proceedings of the 10th International Conference on piling and deep foundations, Amsterdam, May/June 2006 pp.210-218.

Hollingsworth JR and Imbo-Burg RM (1992). The Atlas Screw Pile - construction, design and performance, MJ Sands, ed. In: Piling: European practice and worldwide trends, 7-9 April, Thomas Telford pp.

170-176.

Institution of Civil Engineers (1996). Specification for piling and embedded retaining walls. London: Thomas Telford.

London District Surveyors Association (2000).

Guidance notes for the design of straight shafted bored piles in London Clay. 1. Bromley, Kent: LDSA Publications.

Skempton AW (1959). Cast insitu bored piles in London Clay. Geotechnique, 9(4), pp.153-173.

Stroud MA and Butler FG (1975). The standard penetration test and the engineering properties of glacial materials, Symposium on engineering properties of glacial materials, 1975, Midland Geotechnical Society pp.117-128.

Van Impe WF (2001). Considerations on the influence of screw pile installation parameters on the overall pile behaviour, AE Holeyman, ed. In: Screw piles - installation and design in stiff clay. Proceedings of the symposium on screw piles, 15 March, Balkema pp.127-149.

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