Year

2013

Degree Name

Doctor of Philosophy

Department

Department of Civil, Mining and Environmental Engineering

Abstract

Vacuum assisted radial consolidation has been increasingly used to stabilise soft clays worldwide. The advantages of the combined vacuum-surcharge pressure over the conventional surcharge alone such as reduction of the embankment height, increasing the rate of consolidation, and increasing the factor of safety of the embankments against failure are appreciated however they are not precisely addressed in the literature. In this research a comprehensive laboratory study was conducted to capture the effects of combined vacuum-surcharge pressure during radial consolidation. The modification of a Rowe cell was initiated at the University of Wollongong to monitor lateral effects of vacuum pressure on excess pore water pressure and associated effective stresses. The new apparatus was then employed to develop a radial consolidation model incorporating the effects of vacuum pressure and non-linear flow relationship via flow velocity-hydraulic gradient relationship during radial consolidation.

Based on the laboratory observation, degree of consolidation and back calculated coefficient of consolidation increase with increasing vacuum pressuretotal surcharge ratio (VSR). Coefficients of consolidation back calculated from excess pore water pressure dissipation showed lower values than those back calculated from settlement data. Empirical formulations were then developed to calculate both these coefficients for varying VSR based on settlement based determined from oedometer testing.

Decrease in excess pore water pressure across the soil due to the application of vacuum pressure inside the vertical drains is not immediate and its non-linear variation depends on the radial distance from the drain and elapse time after the application of vacuum. During laboratory studies in this research, it was shown that the application of a combined vacuum-surcharge pressure results in a higher rate of excess pore pressure dissipation and settlement than the application of conventional surcharge alone. Higher gain in effective stresses and higher overconsolidation ratio were consequently observed after the removal of vacuum. Considering these effects, a design methodology was proposed incorporating VSR to minimise the removal of embankments which in turn reduces both the time and the cost of the projects and plays as a more environmentally friendly technique by reducing the extent of earthworks that is typically associated with surcharge-only embankments.

Using modified Rowe cell, new non-linear flow relationship was proposed during vacuum assisted radial consolidation eliminating the shortcomings of the conventional methods. The relationship was then used to develop a radial consolidation model incorporating the effects of vacuum pressure. It was shown that the proposed model can effectively be used for both laboratory and field conditions and its predictions provided better agreement with the measured data than the existing models.

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