Analytical and numerical modelling of soft clay foundation improvement via prefabricated vertical drains and vacuum preloading

A system of vertical drains combined with vacuum preloading to accelerate soil consolidation by promoting radial flow is an affective method. In this thesis the analytical modelling of vertical drains incorporating vacuum preloading in an axisymmetric condition is proposed. The distribution of vacuum pressure along the drain and variations in volume compressibility, and permeability of soil based on large scale laboratory observations are included in the proposed solutions. These analytical predictions are compared with the experimental results using a large-scale consolidation chamber and agree with the measured data. In order to conduct a multi-drain, plane strain analysis using numerical modelling, the method of transforming permeability and vacuum pressure between axisymmetric and equivalent plane strain conditions are described through analytical and numerical schemes. After this a multi-drain, plane strain analysis is used to study 4 case histories taken from Thailand and China where the entire embankment was stabilised with vertical drains and subjected to vacuum preloading. This analysis employing the writer’s model indicates an improvement in the predictions in relation to the field observations. The data show that the efficiency of the prefabricated vertical drains depends on the magnitude and distribution of vacuum pressure, and the extent to which air is prevented from leaking. The design procedures for single stage and multi-stage construction proposed are based on the research knowledge learned from laboratory and field nvestigations. The length of vertical drain, anisotropic soil permeability and vacuum pressure are considered, and a reduction in consolidation time through vacuum preloading is compared to other available methods. Design charts are then developed using the equivalent drain diameter as an independent variable to obtain the relevant drain spacing, and eliminate cumbersome iterative procedures. The design methodologies and examples for single stage and multi-staged construction are also provided.