Behaviour of soft soil improved with vertical drain accelerated preloading incorporating visco-plastic deformation

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Creep also known as time dependant viscous behaviour of soil is a significant part of the soft soil settlement, which may cause substantial deformation in the long-term. Post-construction settlement of soft soils can be significant throughout the life time of the structure. Consequently, to minimise the post-construction deformation and improve the bearing capacity and the shear strength of the soft soil deposits, preloading combined with vertical drains is frequently used as a ground improvement technique. Soil disturbance induced by the installation of vertical drains results in reducing the horizontal soil permeability and the shear strength in the disturbed zone. Thus, the soil disturbance contributes to the reduced hydraulic conductivity and overconsolidation ratio (OCR) of the soil in the vicinity of drains, influencing soil deformation. Based on the available literature, there is a lack of understanding with respect to the combined effects of the overconsolidation ratio and the hydraulic conductivity profiles in disturbed zone and the nonlinear visco-plastic behaviour of soft soils. These combined effects influence the creep parameters and the settlement rate and accordingly deformation of soft soils improved using vertical drains assisted preloading. In this research, the elastic visco-plastic model has been incorporated in the consolidation equation to investigate the effects of soil disturbance induced by the installation of vertical drains on the long term performance of soft soil deposits. The elastic visco-plastic model consists of a nonlinear creep function with a creep strain limit. The applied elastic visco-plastic model is based on the framework of the modified Cam-Clay model, capturing the soil creep during the excess pore water pressure dissipation. Finite difference formulations for fully coupled one dimensional axisymmetric consolidation have been adopted to model the time dependent behaviour of the soft soil, combining both vertical and radial drainage. Crank-Nicholson scheme is applied in formulating the finite difference procedure, since this scheme uses two steps in partial differentials of pore water pressure over distance, stabilising the process quicker. An array of laboratory tests were carried out using Oedometer and small and large Rowe cells apparatus to verify the developed numerical code for the axisymmetric solution. The Oedometer tests were conducted to choose the soil mixtures for disturbed and intact zones. Two sets of small Rowe cell tests were carried out on selected soil mixes to obtain the elastic visco-plastic model parameters. A large Rowe cell was used to carry out the vertical drain assisted consolidation tests by installing a vertical drain in the centre of the cell. To simulate the disturbed zone for the area surrounding the vertical drain, a different mix with reduced permeability was used. A compacted sand column covered with flexible porous geotextile was installed in the centre to simulate the vertical drain. The cell is fully instrumented and consists of a vertical displacement gauge at the surface level and nine pore water pressure transducers on the sides and at the base of the cell. Comparison of laboratory measurements and numerical predictions shows that the proposed finite difference procedure incorporating the elastic visco-plastic soil behaviour is appropriate for the consolidation analysis of preloading with vertical drains. Two case studies of vertical drains assisted preloading were numerically simulated to investigate the effects of soil disturbance caused by the installation of vertical drains. Different variations of the overconsolidation ratio and hydraulic conductivity in the disturbed zone in combination with time dependant behaviour of soft soils were considered. Different OCR and initial hydraulic conductivity profiles in the disturbed and transition zones result in various visco-plastic strain rates and creep strain limits. Consequently, the induced changes in visco-plastic strain rate and creep strain limit influence the settlement rate at any given time. Therefore, the selection of OCR and initial hydraulic conductivity profile in the disturbed zone has a significant effect on selecting unloading time and therefore the post construction settlement. It was observed that the creep coefficient and the creep strain limit vary during loading and unloading and also during excess pore water pressure dissipation. The creep coefficient and the creep strain limit are functions of the vertical effective stress and time. The proposed solution can readily be used by practicing engineers considering layered soil deposits, time dependent loading and unloading, while incorporating combined effects of soil disturbance and visco-plastic behaviour.
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