Evaluation of the soil disturbance due to the installation of prefabricated vertical drains: An analytical approach applied to case histories

Prefabricated vertical drains (PVDs) are among the most effective methods to control the water table and excess pore water pressure developed as a result of seepage or seismic event. Preservation of historic sites demands techniques that promptly reduce the excess pore pressure within the soil at the site and increase the shear strength. Installation of mandrel-driven PVDs and the disturbance created in soft cohesive soils were investigated in this research with a new elliptical cavity expansion theory (CET). The elliptical CET formulated is based on modified Cam clay model for undrained analysis, and accounts for the rate of mandrel penetration. Time dependent internal pressure in the cavity developed, corresponding stresses and excess pore pressure in the soil were evaluated during driving the mandrel. The pattern of distribution calculated for the excess pore pressure was verified using data obtained from large-scale laboratory tests. A more realistic elliptical smear zone around mandrels that are commonly used in the field was introduced while the disturbed soil surrounding the mandrel was characterized by the plastic shear strain normalized by the rigidity index. This analytical formulation was subsequently applied to practical situations. Case histories from the Muar clay region in Malaysia, and the Sunshine Motorway in Australia were studied to characterize the smear zone associated with the installation of PVDs. The ratio of plastic shear strain to the rigidity index was found to be useful for estimating the extent of the smear zone in the field because the basic soil parameters can be readily obtained without sophisticated large-scale testing.