Experimental investigation of the effect of shearing rate on the capacity of piles in soft silt

Recent improvements in effective stress design methods for the shaft capacity of displacement piles in clay require the inclusion of the interface friction angle at failure. This paper provides results from ring shear tests conducted on samples of Belfast soft sleech, which is a deposit of estuarine silty clay. The sleech has an upper (1-3 m thick) horizon with a high sand content, whilst the lower sleech has a high clay fraction. The stratification of the deposit is shown to be significant, with low residual friction angles being mobilised in the lower clayey sleech. The influence of shearing rate on the interface friction angle is examined using the ring shear apparatus and a positive rate effect is identified. In addition, results from field experiments conducted using the instrumented University College Dublin (UCD) model pile are presented. The pile axial resistance is shown to be a function of the test technique, with the peak capacity exhibiting strong rate dependence. Consideration of the porewater pressure response during loading revealed that during load application, the pore pressures measured at the pile-soil interface reduced temporarily after small pile head displacements. This pore pressure drop resulted in temporary increases in radial effective stress which were associated with the development of peak pile resistance values. This suggests that relatively high magnitude temporary loads (such as wind or wave loading) could besustained for short periods whilst maintaining an adequate factor of safety on a pile.