Measurement of stresses around closed-ended displacement piles in sand

Calibration chamber experiments are reported that investigate the evolution of stresses around closed-ended, highly instrumented, model displacement piles during simulated driving into a heavily instrumented sand mass. The soil stresses are shown to vary spatially relative to the pile tip location. As well as showing considerable radial variation, the stresses developed at any given depth build sharply as the tip approaches, and reduce rapidly as it passes. Clear differences are evident between the behaviours seen close to the shaft during alternate penetration and pause periods. Load-cycling effects are most significant close to the shaft, where the local stress paths indicate a tendency for constrained 'dilatant' behaviour, with radial stresses increasing, during loading. In contrast, markedly 'contractant' radial stress reductions are evident on unloading.