Use of Polymer and Bentonite support fluids, and their impact on bored piles

Synthetic polymer fluid is gradually gaining popularity as a replacement for conventional bentonite slurry in bored pile construction. Compared to bentonite, polymer can offer potential benefits including, for example, smaller site set-up footprint, ease of mixing and lower fluid disposal cost. Some field trials have demonstrated that piles installed using polymer have improved load carrying characteristics beyond equivalent piles installed using bentonite. However, the mechanism leading to improvement of the pile’s shaft shear resistance is not fully understood. This paper discusses an experimental investigation carried out on eight, fully instrumented, 4.5 m long bored piles constructed, in the National Buried Infrastructure Facility (NBIF), at the University of Birmingham, with four piles constructed with a polymer and four using bentonite as the support fluid. The installation of optical fibre and strain gauge sensing instrumentation allowed a near full-scale laboratory controlled comparative set of pull-out tests to be undertaken, to enable a direct comparison of the impact of the two support fluids on the frictional resistance of bored piles. The results showed that increased resting time post construction improved the stiffness and strength of the polymer piles. Observation of the exhumed piles and 3D laser scanning revealed the formation of nodules along the length of the piles and the development of surface cracks in the ground (emanating from the position of the pile and propagating outwards in a radial pattern) when polymer was used. This might indicate a stronger concrete/soil bond when the polymer slurry was used, compared to the bentonite slurry, as no similar ground cracks were observed for the bentonite piles when they were removed. The bentonite filter cake layer was thick and increased as the fluid resting time increased. The filter cake layer, associated with polymer piles, was much thinner and independent of the fluid resting time.