Numerical analysis of cavity propagation in deep trapdoor experiments.

Large areas of the economically important Gauteng Province of South Africa are underlain by extensively weathered dolomite bedrock in which sinkholes are a common occurrence. Deep trapdoor experi-ments were conducted in the geotechnical centrifuge to study cavity propagation towards the surface to obtain a better understanding of the sinkhole development process and the factors affecting it. The experimental pro-gramme is accompanied by a numerical modelling programme in order to gain better understanding on the be-haviour of cavity propagation above trapdoors. A series of finite element simulations were carried out using the Coupled Eulerian–Lagrangian (CEL) method. It was found that in deep trapdoors, shear bands initiate from the edges of the trapdoor, propagating upwards. The bands curve inwards under the influence of the mobilised dilation angle which depends on the stress level and relative density of the placed soil. With large trapdoor movement, near vertical shear zones develop. These results imply that the guidelines for the assessment of sinkhole size in South Africa are conservative and there is room for improving the current guidelines by adopt-ing realistic mechanisms of sinkhole development.