Year

2010

Degree Name

Doctor of Philosophy

Department

Department of Civil Engineering

Abstract

Prefabricated vertical drains (PVDs) combined with preloading have gained in popularity among the most effective ground improvement techniques available to mitigate the unacceptable differential settlements caused by the heterogeneity and high compressibility of soft soil deposits. In this thesis the installation of mandrel-driven PVDs and associated disturbance in cohesive soils were studied by conducting analytical investigations, laboratory experiments, and numerical modelling. The pattern of disturbed regions surrounding the mandrels and the distribution of stresses in soils obtained from the analytical and numerical predictions agreed with the results of the laboratory tests. A number of case histories taken from Malaysia, Australia and Thailand were also analysed to evaluate the associated soil disturbance during installation of prefabricated vertical drains.

An analytical study of mandrel penetration and the resulting disturbance in soft saturated clays was carried out with a new elliptical cavity expansion theory (CET). This research postulated that installing PVDs in the field with commonly used mandrels would create elliptical cavities with a concentric progression in the horizontal plane. An elliptical CET was developed using modified Cam clay parameters for undrained analysis with a formulation based on polar coordinates that accounts for the rate of mandrel penetration and the time for predicting internal pressure in the cavity, corresponding stresses and excess pore pressure in the soil while driving the mandrel. The pattern of distribution calculated for the excess pore pressure was verified using data available in the literature. A more realistic elliptical smear zone based on the elliptical CET was introduced while the disturbed soil surrounding the mandrel was characterised by the plastic shear strain normalised by the rigidity index.

A number of large-scale laboratory tests that incorporated the field conditions and effects of confining pressures were performed. A consolidometer specifically designed for the purpose, and a machine capable of driving mandrels at realistic rates were used in these experiments. The variations of pore water pressure during installation of a mandrel-driven PVD and withdrawal of the mandrel were monitored by fast response pore pressure transducers connected to a digital data logger. The extent of smear zone in the large-scale consolidometer was determined using the results of moisture content tests on samples, which in relation to the installed PVD were cored along different polar axes from various locations. The smear zone was then analysed to establish a relationship between its size and the in-situ effective stresses.

The installation of a mandrel was simulated numerically using a commercial finite element software package, ABAQUS. The finite element models included coupled analyses with a large-strain formulation. Coulomb’s law of friction and the penalty method were incorporated into the numerical technique. It was shown that the soil surrounding the mandrel moved radially and downwards as the mandrel was installed. The variations of pore water pressure at different locations during the installation of a mandrel-driven PVD and withdrawal of the mandrel were illustrated. There was an agreement between the pore pressures measured in the laboratory and the finite element predictions. The extent of smear zone was studied according to a numerical simulation of the mandrel installation.

The analytical formulation incorporating the elliptical CET presented in this thesis was applied to case histories from the Muar clay region in Malaysia and the Sunshine Motorway in Australia. The ratio of plastic shear strain to the rigidity index was found useful for estimating the extent of the smear zone in the field because in practical situations the basic soil parameters may be used without sophisticated large-scale testing.

Moreover, the numerical model of mandrel installation was specifically developed to study a case history from the Second Bangkok International Airport in Thailand. The variations of pore pressure while installing a vertical drain and withdrawing the mandrel were obtained. The plastic shear strain was evaluated to indentify different aspects of disturbance in the soil elements. The results of this analysis indicated that the model developed can be applied to field conditions.

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