Finite Element Analysis of Volume Change in Expansive Clays

The mechanisms of volume change in expansive clay are complex and cannot be described accurately by simple models. Volume change is a 3-dimensional problem in which simultaneous changes in stress, displacement and water content or soil suction throughout the region including the boundaries, must be considered. The mechanisms can therefore be described only as interactive physical processes, each of which are difficult to describe individually, particularly because of the highly non-linear behaviour of expansive soils. One practical method of simulating these individual non-linear processes is the finite element method. When the programs describing the relevant processes are coupled in a computer with data transferred through disc storage, then the resultant volume change behaviour of soils can be modelled in a more mechanistic and less empirical manner. This paper describes one such approach in which the various processes have been included in two finite element programs which can be coupled together in a micro-computer. One program analyses the load-deformation behaviour and the other the changes of soil suction due to water flow as a result of changes of load, displacements and soil suction throughout the region. Modelling of the non-linear behaviour of the material properties for both water flow and deformation is achieved by expressing the tangential values of the properties as functions of stress and soil suction and using them in incremental analyses. These programs have been used to back analyse suction controlled consolidation tests on Australian clays. This has enabled material properties to be determined, which give good agreement between the analytical and experimental results. The subsequent use of these programs in field problems with material properties determined in this manner, has been encouraging.