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.

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The Phase 2 of the proposed Chennai Metro Rail Project contains underground stations of depths below ground varying from 20m for a 2 level station to 32m for a 4 level station. Diaphragm walls have been chosen for the purpose of earth retention during construction stage and also to be a part of the permanent structure. The centre part of the stations is constructed using top-down methodology and the ends of the station which are used for the purpose of TBM launching and retrieval activities are constructed using either bottom-up or top-down methodologies. The behaviour of the part of the station constructed using top down sequence will be different from the part constructed using bottom up sequence. The current study focusses on the comparison of the influence of the various constitutive models available in the FEM software package PLAXIS 2D such as the basic Mohr-Coulomb model and advanced models such as Hardening soil model and Hardening soil small strain stiffness (HSS) models on the bending moments, shear forces of the diaphragm walls, ground deformations, strut forces etc. This comparative study has been performed on both the top-down and bottom-up sequences and for the different ground conditions along the proposed second phase metro rail alignment in Chennai region of India.

The interactions between soil and a buried circular geosynthetic reinforcement layer placed below a circular load are studied using elastic continuum theory with particular regard for the reinforcement effect in reducing surface settlements. To represent the displacement at the soil-inextensible geosynthetic reinforcement interface, the net radial displacements are equated to zero, and for the soil-extensible geosynthetic reinforcement interface, the net radial displacements are equated to the radial elongations assuming the reinforcement behaves as a thin membrane. A parametric study illustrates the effect of the reinforcement layer radius and depth on reducing surface settlements.

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