Permeability and stress-strain response of speswhite kaolin

Abir Al-Tabbaa, Cambridge University, Geotechnical Engineering Group


This dissertation is concerned with an investigation into some aspects of the permeability and stress-strain response of reconstituted saturated speswhite kaolin clay.

Experiments were performed to investigate the soil's permeability characteristics during anisotropic consolidation and these indicated that there exist unique relationships between the vertical and horizontal permeabilities and void ratio which are linear on double logarithmic scales. These relationships were found to be independent of the overconsolidation ratio. A ratio of horizontal to vertical permeabilities due to anisotropic vertical consolidation of up to 3 was observed.

Both Terzaghi's and Biot's consolidation theories were used to develop closed form solutions for the axisymmetric problem of consolidation with radial drainage to inner and outer fixed boundaries. Finite element analyses were also carried out using both elastic and elasto-plastic soil models. The elasto-plastic analysis shows a number of effects which are not exhibited by the elastic analysis. The permeability values given by both analyses are, however, similar and agree well with experimental falling head permeability determinations.

A detailed experimental investigation of the stress-strain behaviour of kaolin was carried out using a computer controlled triaxial apparatus supplemented by tests in an oedometer. Experimental results showed that the modified Cam-clay model predicts the soil's behaviour well in normally consolidated states. The behaviour in overconsolidated states were found to be hysteretic, non-linear and showed accumulation of permanent strains during closed stress cycles. This behaviour was characterised as being elasto-plastic and led to the formulation of an elasto-plastic "two-surface" strain hardening/softening model within the framework of critical state soil mechanics. This model, which was developed directly from experimental observations, used the modified Cam-clay isotropically hardening yield surface as a "bounding surface" within which a kinematically hardening inner "yield surface" is introduced. A translation rule for the yield surface is assumed which ensures that the yield and bounding surfaces never intersect. A hardening rule is assumed to predict the plastic strains on the yield surface. The model requires six parameters for the complete description of the incremental stress-strain behaviour.

The predictions of the two-surface model compare well with the experimental results and suggest that it is a definite improvement on the modified Cam-clay model.