Degree Name

Doctor of Philosophy (PhD)


School of Civil, Mining and Environmental Engineering - Faculty of Engineering


Vertical drains increase the rate of soil consolidation by providing a short horizontal drainage path for pore water flow, and are used worldwide in many soft soil improvement projects. This thesis develops three new contributions to the solution of consolidation problems: (i) a more realistic representation of the smear zone where soil properties vary gradually with radial distance from the vertical drain; (ii) a nonlinear radial consolidation model incorporating void ratio dependant soil properties and non-Darcian flow; and (iii) a solution to multi-layered consolidation problems with vertical and horizontal drainage using the spectral method. Each model is verified against existing analytical solutions and laboratory experiments conducted at the University of Wollongong, NSW Australia. The nonlinear radial consolidation model and the spectral method are verified against two trial embankments involving surcharge and vacuum loading at the Second Bangkok International Airport, Thailand. The versatility of the spectral method model is further demonstrated by analysing ground subsidence associated with ground water pumping in the Saga Plain, Japan. New expressions for the smear zone parameter, based on a linear and parabolic variation of soil properties in the radial direction, give a more realistic representation of the extent of smear and suggest that smear zones may overlap. Overlapping linear smear zones provide some explanation for the phenomena of a minimum drain spacing, below which no increase in the rate of consolidation is achieved. It appears this minimum influence radius is 0.6 times the size of the linear smear zone. The new smear zone parameters may be used with consolidation models (ii) and (iii), as mentioned above. The analytical solution to nonlinear radial consolidation is valid for both Darcian and non-Darcian flow and can capture the behaviour of both overconsolidated and normally consolidated soils. For nonlinear material properties, consolidation may be faster or slower when compared to the cases with constant material properties. The difference depends on the compressibility/permeability ratios (Cc/Ck and Cr/Ck), the preconsolidation pressure and the stress increase. If Cc/Ck ≤ss than 1 or Cr/Ck ≤ss than 1 then the coefficient of consolidation increases as excess pore pressures dissipate and consolidation is faster. The multi-layered consolidation model includes both vertical and radial drainage where permeability, compressibility and vertical drain parameters vary linearly with depth. The ability to include surcharge and vacuum loads that vary with depth and time allows for a large variety of consolidation problems to be analysed. The powerful model can also predict consolidation behaviour before and after vertical drains are installed and has potential for nonlinear consolidation analysis.

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