Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering


In this research, analytical, experimental and numerical investigations were carried out to pursue a better understanding of the consolidation of soft clay stabilized with prefabricated vertical drains (PVD) subjected to preloading (with and without vacuum pressure application). This investigation was carried out in fourfold. First, an analytical solution based on Cylindrical Cavity Expansion analysis incorporating the Modified Cam Clay theory has been formulated to estimate the extent of the smear zone. Second, the smear zone characteristics were evaluated using a large-scale radial drainage consolidometer and then compared with predicted values. Third, the existing axisymmetric and plane strain theories of a unit cell were modified and incorporated with a linearly distributed (trapezoidal) vacuum pressure for both Darcian and non-Darcian flow. Finally, multi-drain plane strain analysis was conducted on a number of case histories taken from Australia and Thailand to study the performance of the entire embankment stabilised with vertical drains.

A series of large-scale model tests were conducted using a specially designed consolidometer 650 mm diameter by 1040 mm high to study the characteristics of the smear zone. A central vertical drain was installed in soil specimens (previously placed inside a large cell) with a band shaped (rectangular, hollow) steel mandrel, during which any variations in pore water pressure along the radial direction were recorded. Then, the surcharge load was applied in stages up to 200 kPa and the clay then consolidated. At the end of consolidation, soils samples were collected from several locations in order to carry out a number of oedometer and triaxial tests to establish the variations of soil properties. It was observed that drain installation disturbed the soil immediately adjacent to it, creating a "smear zone" in which a change in clay properties was caused by reconsolidation due to dissipation of excess pore pressure, and remoulding due to shear from the periphery of the mandrel. The smear zone was 2-3 times the equivalent radius of the mandrel with horizontal permeability (in the smear zone) varying from 1.09 to 1.64, an average of 1.34 times smaller than that of the undisturbed zone. The estimated extent of smear zone from the large-scale test was very close to the predicted value, based on Cavity Expansion analysis.

The analytical solutions are incorporated in the finite element code (e.g. PLAXIS, ABAQUS) employing the Modified Cam Clay theory. Selected numerical analysis incorporating the proposed solution was carried out to study the behaviour of a number of case histories, in view of various ground improvement schemes applied to stabilize the soft clay foundation and the predictions are compared with available field data. A good agreement between the finite element analysis and field data was found. Finally, numerical analyses incorporating the proposed solution were conducted to study the effect of embankment slope, construction rate, drain spacing, the characteristics of smear zone, multi-stage loading and surface crust on the failure of the soft clay foundation.