Rehabilitation of sinkholes and subsidences on Dolomitic Land Ekurhuleni Metropolitan Municipal Area of Jurisdiction Gauteng South Africa

Abstract

Extensive research has been done on the investigation, evaluation, development and management of land underlain by dolomite to prevent the formation of sinkholes and subsidences. Little emphasis is, however, given to the various processes and methodologies with respect to the investigation and rehabilitation of sinkholes or subsidences. In South Africa most sinkholes and subsidences have been recorded on dolomite of the Malmani Subgroup of the Chuniespoort Group on the Far West Rand and in an area south of Pretoria within the City of Tshwane, with numerous studies done on the occurrence of sinkholes and subsidences in these two areas. However, a large number of sinkholes and subsidences have also been recorded on the East Rand in the Ekurhuleni Metropolitan Municipal area, which is the focus of this research. Very little is published on the occurrence of sinkholes and subsidences, the related geological models, investigations and rehabilitation work done in the Ekurhuleni Metropolitan Municipal area. It has been attempted with this thesis to document the various processes and methodologies applicable to the investigation and rehabilitation of dolomite-related sinkholes and subsidences within a non-dewatering environment. This is based on experience gained during the investigation and rehabilitation of various sinkholes and subsidences within the Ekurhuleni Metropolitan Municipal area on the East Rand, over the past ten years. Various methods of investigation, including non-intrusive and intrusive methods have been tried and tested to determine the extent of subsurface erosion within an affected sinkhole or subsidence area. The gravity geophysical method in association with the drilling of percussion boreholes; exposure of the affected area by means of excavation and the use of the Dynamic Probe Super Heavy (DPSH) method, in areas not accessible for a drilling rig where gravel, cobbles and boulders are absent in the subsurface profile; is the most appropriate methods for sinkhole or subsidence investigations on the East Rand. The gravity method is, however, not always successful in delineating narrow grykes in a shallow dolomite environment. The use of a specific method or methods of investigation is prescribed by the accessibility of a site. Accessibility constraints within a build-up area, may lead to the use of an investigation method other than what is generally preferred. The specific method or combination of methods used to rehabilitate a sinkhole or subsidence will depend on the available funds, current and proposed land use, subsurface conditions, accessibility constraints caused by existing infrastructure for equipment and the impact of the rehabilitation procedure on existing infrastructure. The various sinkhole and subsidence rehabilitation methods used on dolomite in South Africa and their applications are discussed, including: the Inverted Filter Method, Dynamic Compaction Method, combination of the Inverted Filter and Dynamic Compaction Methods, Compaction (backfill) Grouting Method, Combination of the Inverted Filter and Compaction Grouting Methods, Combination of the Dynamic Compaction and Compaction Grouting Methods and the use of Self- Compacting Concrete or Soil-cement Mix. Nearly half of the Ekurhuleni Metropolitan Municipal area is classified as dolomite land. A total of 241 ground movement incidents had been recorded, since 2005 to mid-2013. The Ekurhuleni Metropolitan Municipal area of jurisdiction is divided into three regional areas, namely:  Southern Regional area: More than 50% of the region is directly underlain by dolomite and chert of the Monte Christo Formation of the Malmani Subgroup of the Chuniespoort Group. A total of 141 ground movement incidents had been recorded, with all dolomite-related sinkholes and subsidences caused by ingress of water. Approximately 85% were caused by sewer lines, 7% by leaking water lines and valves, 4% by surface water ponding and 3% by concrete stormwater lines.  Northern Regional area: The northern portion of the region is directly underlain by dolomite of the Malmani Subgroup; from west to east by, chert-poor dolomite of the Oaktree Formation, chert-rich dolomite of the Monte Christo Formation, chert-poor dolomite of the Lyttelton Formation and chert-rich dolomite of the Eccles Formation. Dolomite of the Monte Christo Formation is also encountered in the south-eastern portion of the region. A total of 83 ground movement incidents had been recorded. Ground movement incidents are related to both ingress of water and dewatering of dolomite groundwater compartments including the Bapsfontein, Elandsfontein and Sterkfontein-East Dolomite Groundwater Compartments and Sub-Compartment.  Eastern Regional area: More than 50% of the region is regarded as dolomite land, with large portions of dolomite of the Monte Christo Formation and the Oaktree Formation of the Malmani Subgroup covered by the Karoo Supergroup. Dolomite of the Monte Christo Formation also occurs in the south-western portion of the regional area. A total of 17 ground movement incidents had been recorded. None of the recorded ground movement incidents are, however, related to dolomite. Incidents are related to poorely backfilled old wet services, natural erosion of subsurface soils, collapse of shallow coal workings and collapse of mine shafts and ventilation shafts. The sinkhole and subsidence rehabilitation method mostly used in the Ekurhuleni Metropolitan Municipal area is the Inverted Filter Method accounting for 85% of rehabilitation work, 10% for compaction grouting and 5% for the use of the Dynamic Compaction Method. The sinkhole and subsidence rehabilitation method should not be prescriptive, given the vast number of variables involved. A comprehensive understanding of the affected area is essential although for cost effective and practical rehabilitation measures. A site specific set of criteria for the rehabilitation of the features and affected infrastructure must be developed to ensure proper stabilisation and safe future use of the area. Basic principles can however be applied to each sinkhole or subsidence, such as: Ensuring the trigger of the sinkhole or subsidence has been identified and removed; the position and extent of the receptacles have been determined as best as possible and erosion paths sealed; the known eroded area, possible voids and cavities properly backfilled and densified; a proper impervious or engineer designed earth mattress created; ensuring that all affected subsurface wet services are replaced and comply with industry standards; and that proper surface drainage exists.