LDFE study of bottom boundary effect in foundation model tests

Abstract

Centrifuge model tests of deeply penetrating foundations have been widely used to assess the vertical bearing response, particularly in relation to the installation of spudcan foundations that support offshore drilling rigs. The potential influence of boundary effects owing to the proximity of these large foundations to the rigid base of the model container has not been previously addressed. In this study, large deformation finite-element (LDFE) analyses were conducted to assess the extent of the bottom boundary influence zone. Various foundation diameters were considered, with soil samples of sand overlying clay and uniform clay. The sand plug developed beneath the foundation is a major contributary factor to the boundary effect problem. The boundary effect is increased for sand over clay conditions, where a sand plug is entrapped beneath the foundation. The LDFE results were utilised to predict the thickness of the entrapped sand plug for different geometry and soil strength conditions. The results are distilled into a simple relationship that can be used to ascertain the bottom boundary influence zone when planning physical model tests and reinterpreting previous studies. The boundary influence zone predicted by the LDFE analysis agreed well with a corresponding centrifuge test.