Side-wall stability and side-shear resistance in bored piles constructed with high-molecular-weight polymer slurry

Full-scare, large-diameter `technique' shafts were excavated using high-molecular-weight polymer slurry at two different sites, one consisting mostly of stiff silty clays and one consisting of medium dense sand. The side-wall profiles of the technique shafts were routinely monitored to assess the effectiveness of the polymer slurry in maintaining stable open shafts. Two test piles, 0.915 m in diameter and 21.5 m deep, were installed, one at each site, using the polymer slurry. The test piles were equipped with internal Osterberg Cells at approximately mid-height and loaded by thrusting the two segments apart. Embedment strain transducers were mounted at different levels on the reinforcing cage to measure the transfer of load to the soil. Results from the technique shafts showed that the subject polymer slurry, when introduced with the correct dosage and prior to the borehole reaching the piezometric level, maintained the stability of open boreholes in stiff silty clay and in the medium dense sand for over 18 hours. Conversely, when the polymer slurry was introduced after the borehole passed below the piezometric surface, instability eventually ensued. The side-shear resistance design factors that were back-calculated from shear strength data at the sites and the load-test results on the polymer piles exceeded those recommended for bentonite shafts in USA practice in both the cohesive and cohesionless soils.