A study was made on the consolidation problem of a vertical drain which was installed in two layers of clay having different consolidation properties. Numerical analysis was performed on the problem using finite difference method, and a parametric study was made to examine the effect of variations in Cv and k values between the two clays. The study shows that a small deviation in k value between two clays has a very strong influence on the consolidation rate, but as the deviation becomes larger the degree of influence diminished very rapidly. Also the applicability of numerical analysis was examined by performing a two dimensional model test consisting of two layers of marine and kaolin clays. The comparison between the theoretical prediction and the experiment shows that the use of numerical analysis is useful to understand the interaction of consolidation between the two layer system.
Vertical drains are often used to improve the strength of soft seabed clays including dredged seabed materials which are often encountered during the construction of artificial fills or man-made islands for the offshore development. Barron's consolidation theory is widely used to estimate the rate of settlement of the soft ground with vertical drain. There are, however, many simplified assumptions utilized in the Barron's equation. For example, the theory is one-dimensional in radial direction and therefore only applicable to a consolidation of thick uniform clay layer. The theory also assumes a linear stress-strain relationship of soil with small deformation, therefore no accounts are made to incorporate a highly non-linear and large deformation behavior of soft clays. However we often encounter much different field situation, and the ground often consists of layered soils with different permeability and compressibility. The soil properties of dredged materials are far from those of an ideal soil with linear stress-strain relationship.
