Effects of Time on Capacity of Pipe Piles in Dense Marine Sand

Investigations into pile behavior in dense marine sand have been performed by the Institut Français du Pétrole and Imperial College at Dunkirk, northern France. In the most recent series of tests, strain-gauged, open-ended pipe piles, driven and statically load tested in 1989, were retested in 1994. An 85% increase in shaft capacity took place between six months and five years after installation. The possible causes are evaluated in relation to previous case histories, laboratory soil tests, pile corrosion, and new effective stress analyses developed using smaller, more intensively instrumented piles. The available evidence suggests that a circumferential arching mechanism develops during pile driving that limits the radial stresses acting on the pile shaft. It is concluded that creep leads to a breakdown of these arching stresses, allowing increases in radial stress and hence gains in shaft capacity. Increased dilation due to sand aging may also contribute. Finally, the measured pile shaft capacities are compared to predictions made using existing design methods.