Doctor of Philosophy
Department of Civil, Mining and Environmental Engineering
Redana, I. Wayan, Effectiveness of vertical drains in soft clay with special reference to smear effect, Doctor of Philosophy thesis, Department of Civil, Mining and Environmental Engineering, University of Wollongong, 1999. http://ro.uow.edu.au/theses/1285
This thesis includes mathematical simulation of smear effects in a 2-D plane strain finite element model employing the modified Cam-clay theory, and the experimental evaluation of smear zone propagation around vertical drains using a large scale, radial drainage consolidometer. The original axisymmetric analysis of vertical drains including the effect of smear and well resistance has been well documented in the past. In contrast, in this study, the analysis of a unit cell is conducted by converting the axisymmetric (radial) permeability into an equivalent plane strain model Thereafter, a multi-drain, plane strain analysis is conducted to study the performance of the entire embankment stabilised with vertical drains, for number of case histories taken from Southeast Asia.
laboratory technique of evaluating the extent of smear around vertical drains is elaborated. In this approach, a central vertical drain was installed in soil specimens placed in a large steel cell (450 mm in diameter and 950 mm in height), using a specially designed mandrel. Once the drain was installed, smaller specimens were recovered from various radial distances from the drain, and the horizontal and vertical permeabilities of these specimens were determined by standard oedometer tests. The extent of the smear zone was established as a function of the horizontal and vertical permeability ratio (kh/kv). It was found that the ktJh ratio decreased significantly towards the central drain. It is also revealed that the krfkv ratio approaches unity soil drain interface, and is about 1.3 at the undisturbed region outside the smear zone. Axial and radial permeabilities were converted to 2-D plane strain parameters based on the mathematical model developed in this study. The mathematical model was used in conjunction with finite element analysis (CRISP) to determine the settlements, lateral movements and pore pressures.
selected case histories include a number of embankments stabilised with vertical drains in Thailand, Malaysia and Indonesia. The embankments were well instrumented with settlement gauges, piezometers and inclinometers. The settlements, excess pore pressures and lateral displacements were predicted using the plane strain, multi-drain model, and the results were compared with the fields measurements. The effect of smear was found to be significant in controlling the performance of vertical drains, while the role of well resistance was found to be insignificant. The model described in this thesis provides an excellent plane strain formulation capable of predicting the behaviour of embankments constructed on soft clay foundations stabilised with vertical drains.