Time-related increases in the shaft capacities of driven piles in sand

Abstract

Displacement piles can gain capacity with time through pore pressure dissipation and consolidation, not only in clay but also in sand. In recent tests on strain-gauged pipe piles in dense sand at Dunkirk, France, the shaft capacity increased by 85% between six months and five years after installation. This technical note describes the Dunkirk results and similar cases, involving steel, concrete, and timber piles in saturated, unsaturated, and carbonate-free sands. Possible mechanisms to explain the observations are reviewed. The most plausible explanation seems to be a change in the stress regime, created during pile driving. Creep may reduce arching effects around the pile shaft, increasing the radial effective stresses. Ageing may also result in stronger dilation during shearing, and larger increases in radial stress during pile loading. There are several important practical implications. The set-up time allowed in pile testing needs to be standardised, and should be at least 20 days. There could be substantial cost savings, if piles can be installed early and allowed to age before loading. Higher working loads on existing aged piles may be permissible. Capacities, assessed from pile driving, are unlikely to match those available in medium to long term. Some needs for further research are indicated.