The role of physicochemical processes in ageing of shaft friction of driven steel piles in sand

Several studies have reported substantial increases in the shaft capacity of driven steel piles in the months following installation. This paper investigates factors influencing this time dependence of shaft capacity by conducting a series of field tests on piles and a parallel series of interface shear tests using a newly developed apparatus. The piles and interfaces used in the experiments employ mild steel, stainless steel and galvanised steel, while the ageing periods allowed in the laboratory and field were 1 and 3 years respectively. Chemical analyses of the crusts that developed at the sand-steel interfaces are reported. It is shown that the ageing characteristic of sand-steel friction depends on the relative contributions of interlocking and dilation but is controlled by dilation at the crust-sand interface adjacent to the shaft of a driven pile. There is no gain in shaft friction with time in dry sand or for piles with non-reactive steel. The operational friction angle for mild steel piles in moist or saturated sand is the soil-soil friction angle.