Sinkhole formation due to subsurface erosion above undermined ground

Abstract

In November 2016, 38 sinkholes formed due to subsurface erosion above undermined ground on Donkerhoek farm near Sasolburg. The fact that the sinkholes formed more than 24 years after mining in the area ceased, emphasised the need for a method to predict the likelihood of sinkhole formation due to subsurface erosion. This is especially relevant for mining houses required to quantify their mine closure risks. The purpose of this study was to investigate the mechanisms and soil properties involved in the formation of the 38 sinkholes on Donkerhoek farm through a fieldwork and laboratory testing programme and develop a method to predict the likelihood of sinkhole formation. Three areas were selected for test pit excavation: an area with large sinkholes, one with small sinkholes and a third with no sinkholes but with a history of subsidence crack formation. The same mechanism of sinkhole formation was observed in both the large and small sinkhole areas: at the soil-rock interface, soil was being eroded into a subsidence crack in the rock, resulting in an upwards migrating cavity and the eventual formation of a sinkhole on surface when the cover over the cavity collapsed. This was the first time that this mechanism was observed in the Sasolburg area, having previously only been identified in the Secunda area. The laboratory tests, carried out on samples obtained during the fieldwork, included foundation indicator, dispersivity, XRF and XRD, soil water retention curves, consolidated undrained triaxial and permeability tests. By comparison of the results from the different study areas, it was concluded that the larger sinkholes form in areas with highly dispersive soils, higher clay content, higher swell potential and a higher capacity to retain suctions when wetted. The following soil properties and factors were identified as the main contributors to sinkhole formation: • The strength of the lid, which is highly influenced by the unsaturated behaviour of the lid material. The lid strength determines the sinkhole diameter; • The erodibility of the material which is influenced by dispersivity and the material obtaining a crumb structure due to desiccation; • The activity of the soil, which influences the volume changes during drying and thereby the width and depth of desiccation cracks; and • The thickness of the soil layer overlying the rockhead. With the critical factors identified, the Van der Merwe method to predict sinkhole size was used as basis to develop an index method to predict the likelihood of sinkhole formation due to subsurface erosion above undermined ground. The method considers combinations of the following factor groups to express a high or low likelihood of the formation of large or small sinkholes in a particular area: • The strength of the lid material, mainly influenced by the unsaturated behaviour of the soil; • The capacity for cavity formation (CCF), which combines the dispersivity, erodibility and activity of the soil; and • The influence of the thickness of the soil layer overlying the rock, evaluated by calculating the ratio between crack width in the rock and the soil layer thickness.