Mitigation of Negative Skin Friction on Piles in Soft Soil Subgrade Using Prefabricated Vertical Drains (PVD) -an Experimental Study

When piles are installed in soft clay, negative skin friction may develop when surrounding soil settles more than piles. This study examines the effectiveness of prefabricated vertical drains (PVDs) prior to pile installation in reducing the downdrag forces under cyclic loading. Model tests on a coupled pile–PVD system were conducted using a Dynamic Consolidation Apparatus. Cyclic vertical stresses were applied to vary from 25kPa to peak values of 65–95kPa at loading frequencies of 1 and 5Hz. Preconsolidation using PVDs was carried out to achieve degrees of preconsolidation (DOPC) between 40% and 100% under a surcharge load of 65kPa before pile installation. The results demonstrate that preconsolidation to approximately 60-80%DOPC markedly increases the soil stiffness, thereby limiting the relative displacement at the pile–soil interface. Lower loading frequency and higher cyclic stress amplitudes result in greater excess pore-water pressure (EPWP) generation, larger settlements, and stronger downdrag force mobilisation. Increasing the cyclic stress amplitude from 40 to 70kPa amplifies the magnitudes of vertical strain (~30%), EPWP buildup, and corresponding downdrag forces. PVDs could significantly enhance drainage and accelerate EPWP dissipation during cyclic loading, leading to deeper neutral planes. Overall, the findings provide guidance on optimising the timing of pile installation and PVD-based consolidation.