Soft clay foundation improvement via prefabricated vertical drains and vacuum preloading

This thesis includes the numerical modelling of prefabricated vertical drain (PVD) subjected to vacuum loading in a 2-D plane strain finite element model employing the modified Cam-clay theory, and the experimental evaluation of effectiveness of combined vacuum and surcharge preloading using a large scale, radial drainage consolidometer. The original axisymmetric analysis and plane strain analysis of vertical drains including the effect of smear and well resistance have been well documented in the past for surcharge preloading. In this study, the existing axisymmetric and plane strain theories of a unit cell are modified to incorporate the vacuum pressure application. Unsaturation of drain soil boundary owing to the vacuum pressure is also considered in the numerical modeling. Thereafter, a multi-drain, plane strain analysis is conducted to study the performance of the entire embankment stabilised with vertical drains subjected to vacuum preloading, for two case histories taken from Thailand. A laboratory technique of evaluating the effectiveness of combined vacuum and surcharge preloading is elaborated. In this approach, a central vertical drain was installed in soil specimens placed in a large stainless steel cell (450 mm in diameter and 950 mm in height) using a specially designed mandrel, and then the vacuum and surcharge loads were applied using the two different loading systems. The results clearly show the effectiveness of vacuum preloading. Following initial laboratory simulation in the large-scale radial drainage consolidometer, a different approach to conventional analysis is adopted to analyse the vacuum assisted consolidation around vertical drains. It is assumed here that a linear variation of negative pore pressure along the drain length and a constant (maximum) suction head at the ground surface are realistic and sufficient. The observed retardation of pore pressure dissipation is explained through a series of finite element models, which consider the effect of unsaturation at the drain-soil interface. The results indicate that the introduction of an unsaturated soil layer adjacent to a PVD improves the accuracy of numerical predictions. The knowledge gained from the modeling of large-scale consolidometer cell is applied to study the behaviour of two embankments built on soft clay, stabilised with vertical drains subjected to vacuum loading. A multi-drain analysis is conducted and the field measurements are compared with a series of numerical model predictions. The best predictions of settlement, lateral displacements and pore pressures are obtained when the numerical analysis included the time and depth dependent changes in vacuum pressure, in addition to having an unsaturated layer of elements along the external boundary of the PVD. Finally, a comprehensive multi-drain analysis is used to predict the failure height of embankment, considering various parameters such as embankment geometry, construction method, sub soil properties and soil improvement techniques