Numerical modelling of a geosynthetically reinforced sand mass undermined by a subgrade void.

Abstract

The South African railway network is highly developed and the vast span of existing rail lines require maintenance and upkeep to ensure smooth running of locomotives. One of the problems faced by the rail industry is instability in the formation level due to the development of sinkholes. Sinkholes and voids commonly form in karstic topography as a result of dolomite related subsidence but may also result from man-made processes such as mining. Roads and railway lines extend for hundreds of kilometres, occasionally over terrain prone to sinkhole formation, making construction over such terrain unavoidable. In order to prevent catastrophic failure in a railway line, the formation must be reinforced over sinkhole prone areas. Geosynthetic reinforcement is identified as a potential means of spanning developing subgrade voids by providing a temporary means of support in the event of a sinkhole. A laboratory model using single and multiple layers of geosynthetic reinforcement was thus developed at the UKZN geotechnical laboratory. Building physical models however is often expensive and time consuming. The research contained in this dissertation explored the use of numerical modelling in simulating the behaviour of a geosynthetically reinforced sand mass when undermined by a subgrade void. Finite element analysis software was used to provide a numerical representation of the physical laboratory model. Exploration into how best FEM can be used in predicting the behaviour of a reinforced fill layer undermined by a subgrade void is thus required. The FEM simulation should provide results within a reasonable degree of accuracy when compared to the experimental model. Through the course of the investigation the parameters governing the behaviour of the finite element model were identified and modified/varied in order to improve the accuracy of the numerical results. In addition to the numerical model, an analytical investigation in which design methods for reinforcement are used to predict the behaviour of a reinforced fill layer took place.