Some observations of the effects of time on the capacity of piles driven in sand

Piles driven in sand can show remarkable increases in their axial shaft capacities in the months that follow installation. Many practical benefits follow if service capacities can be relied upon to exceed the levels proven in site tests, which are usually performed within a few days of driving. This paper reports findings from a programme of mainly tension tests on steel pipe piles performed in dense sand at Dunkirk, northern France. The tests demonstrated more marked shaft capacity growth with time than expected. The aged piles also showed surprisingly brittle failure modes; prior testing to failure both degraded capacity and modified the ageing processes, leading to non-monotonic shaft capacity-time traces that fall far below the intact ageing characteristic (IAC) defined by tests on fresh, previously unfailed, piles. Capacity-time trends inferred from repeatedly tested piles can give misleading results. The new findings allow the ageing effects to be characterised more clearly, and permit a re-evaluation of existing databases involving piles of various types driven in a range of silts, sands and gravels. Important practical conclusions are drawn regarding pile design and the interpretation of field pile load tests.