Investigation of Smear Zone Around PVD Mandrel Using Image-Based Analysis

Vertical drains are extensively used along with the preloading technique as an effective ground improvement method for soft compressible soils. The installation of prefabricated vertical drains (PVD) in soft clay soils causes disturbance in the soil around them. This disturbed zone is called a smear zone. The smearing in the soil affects the permeability and consolidation properties. In this study, an attempt is made to determine the extent of smear zone caused by different rates of mandrel installation (100–500 mm/min) through laboratory model tests employing digital image correlation (DIC) technique. This technique includes a camera, lighting, and control system that facilitates image capturing at a fast rate. In addition to measuring the smear zone, excess pore pressures were monitored at different radial distances, and undisturbed soil samples were collected at various distances for oedometer tests. This study revealed that the extent of the smear zone, maximum excess pore pressure, and displacements associated with mandrel installation decrease with an increase in the rate of installation. The radius of the smear zone is also estimated based on the variations in permeability and moisture content of the soil samples collected at different distances from the mandrel. The extent of the smear zone is estimated to be about four to six times the equivalent radius of the mandrel. The radial permeability inside the smear zone is found to be about 1.3 times lower than that in the undisturbed zone.