The Continuous Helical Displacement (CHD) pile is an auger displacement or screw-type pile, recently developed by Roger Bullivant. A steel hollow stem boring head with helical flights or 'bullet', as shown in Figure 1, is screwed into the ground while displacing the soil laterally. When the founding depth is reached the direction of rotation is reversed and the auger is withdrawn.
During withdrawal concrete is pumped at a high pressure through the hollow stem and tip, forming a pile with flanges that follow the path created by the helical flight. An example of extruded piles is shown in Figure 2. This method of pile construction involves no ground vibration and no soil arisings, making the pile particularly useful in contaminated sites. Soil is displaced laterally and not transported up the auger flights thus increasing the strength of the soil and thus enhancing pile capacity, while requiring less concrete and manpower.
Roger Bullivant has carried out a large number of full-scale pile tests to investigate the performance of CHD piles in various ground conditions and to validate its design method. This work was supplemented by research work funded by Roger Bullivant and carried out at Cambridge University in collaboration with the Geotechnical Consulting Group which was involved in the analysis of the full-scale tests. The experimental work was carried out using laboratory-scale model augers and the results were compared with the results from the full-scale tests. This work has concentrated on the following aspects:
investigating the load-settlement and time-settlement behaviour during compression and tension load tests
optimising the design of the bullet and of construction parameters to ease penetration and to improve the resulting pile shape
determining the effect of pile construction on the surrounding soil
identifying the principal displacement shear surface
determining the appropriate parameters for design.
Three different model augers, one-tenth scale, were machined out of aluminium alloy. Two augers are shown in Figure 3. The third is the same as the auger on the right but with the bottom flange removed. The model construction process was set up to resemble the full-scale rig. A high quality E-grade Kaolin clay (PL=30%, LL=51%, PI=21%) was used to produce compacted and consolidated samples. Some constructed piles were extruded without load testing to examine the shape of the formed piles and the effects of the construction process on the formation of shear surfaces and the surrounding soil. Load tests were performed under undrained conditions and piles were loaded in compression and tension by maintained load increments. First-time tension tests were compared to tension loading following failure in compression. A total of 25 piles were constructed, most of which were loaded to failure. Layered soil and use of dyes gave information on the location of shear surfaces.
The shaft capacity was assessed as the difference between the total compressive failure load and the base capacity estimated using conventional calculations, or as the tension failure load. Simplified models of the actual dimensions of model pile shear surfaces were used. The slight taper of the pile shaft and the pointed tip were taken into account.
The research produced very useful results on the behaviour of the CHD piles in terms of:
a) different bullet head designs;
b) effects of pile installation technique;
c) integrity of the flanges during construction and loading and with depth;
d) effect of different clay stiffness;
e) formation of shear surfaces during construction and loading;
f) performance in fill overlying a hard stratum;
g) behaviour in compacted versus consolidated clays;
h) correlation between laboratory-scale and full-scale performance;
i) effect of the pile installation on the moisture content of the surrounding soil;
j) the range and variation of a values under the different conditions tested.
The work has shown how simple 1g tests can be used to investigate construction method and pile loading behaviour in different soil conditions. Laboratory model tests provide a fast, cheap and effective technique for studying the physical processes governing pile capacity and performing parametric studies under controlled conditions. They have complemented and augmented the understanding gained from full-scale pile tests for this novel and environmentally- friendly pile construction technique.
Dr Abir Al-Tabbaa,
Cambridge University Engineering Department,
Cambridge CB2 1PZ, UK
Tel: +44 (0) 1223 332715
Fax: +44 (0) 1223 339713
PO Box 60180,
Tel: +357 9466577
Fax: +357 6234355
Tel: +44 (0) 1283 511115
Fax: +44 (0) 1283 512233
e-mail: simon.bullivant@ roger-bullivant.co.uk.
Dr Fiona Chow,
Geotechnical Consulting Group
Tel: 0207 581 8348
Fax: 0207 584 0157