Modelling the effects of soil variability on stability analysis of natural slopes in Durban.

Abstract

Slope failure occurs due to various factors, one of the most significant being that of soil variability in a slope and associated geological threats such as unconsolidated soils, settlement, groundwater seepage and infiltration. The analysis of slope stability should incorporate and analyse the interactions between slope configuration, shear strength resistance, pore-water pressure and water conditions of a slope. This study focuses on the causal effects and slope stability of two natural slopes in Durban, KwaZulu-Natal. Large parts of the study area are underlain to great and varying depths by problem soils, namely the Berea Red Sands. These are dune soils, deposited by ancient wind activity, that are found parallel to the east coast of Durban. The Berea Red “sands” vary greatly in soil type ranging from fine grained sands to silts and clays. Those of looser consistency are known to undergo significant settlement under loading, and also with water interaction. The clay and silt varieties are known to exhibit heave under the same circumstances. In some cases, liquefaction of Berea sands may occur due to the loss of soil structure upon water introduction into the soil mass. The aim of this research is to formulate and compare the stability of the two slopes under different water conditions in the form of Factors of Safety and Probabilities of Failure, using RocScience© software. Site investigations were conducted to classify and collect soils, which were then put through rigorous laboratory testing. The results from testing were applied where possible to the modelling software and a host of important findings were made. The liquefaction potential of poorly graded, uniform Berea sands was observed first-hand on site, in the laboratory and again during slope stability analyses. As anticipated, the slope stability of both sites proved to increase reaching “optimum” conditions due to the positive effects of matric suction. Upon increasing water conditions further or saturating the slope, increasing incidences of failure and instability occurred due to the loss of matric suction and cohesion. This instability can also be attributed to the proven decrease in shear strength properties of the soil, cohesion and internal friction, leading to loss of shear strength in the slope. The positive effects of matric suction were further proven when the slope of Site A that considered matric suction (in the form of an air entry value), exhibited a slightly higher FOS and improved slope stability than the one without. The results and conclusions of this research project prove the importance of investigating a soils variability and the subsequent slope reaction under varying moisture conditions. These are key factors to consider prior to civil construction on problem soils, so as to mitigate major failures and the consequences thereof.