The geology of an area south of Pretoria with specific reference to dolomite stability

Abstract

An estimated 2.5 million people live on dolomite and in excess of 1.2 billion Rands of property damage has been observed to date and in excess of 800 sinkholes have occurred in the Southern Tshwane area alone. Research on dolomitic terrain is hence crucial in the quest for releasing land for development that is deemed safe from a dolomite risk perspective. This thesis attempts to present a better understanding of the geology and stability of a carefully selected type area east of Irene town in Pretoria, Gauteng, South Africa, and to interrogate the current method of dolomite stability analysis. The study area is located partly on the Lyttelton and Eccles Formations, Malmani Subgroup, Chuniespoort Group, of the Transvaal Supergroup. The Transvaal Supergroup rocks were subjected to complex faulting and folding along the northeastern rim of the Johannesburg Dome. Displacement and duplication of the Transvaal rocks by faulting is common to the east and south east of Pretoria with the karst topography being well developed along these water exploited structural features. The karst development, in particular on the Eccles Formation, has lead to a highly variable dolomite and chert bedrock topography. Cavernous conditions can be expected both within the bedrock and the overburden. A summary is given of how instability occurs. Given sufficient time and the correct triggering mechanisms, instability may occur naturally but is expedited, by many orders of magnitude, by man’s activities. Various authors have over the years attempted to classify dolomitic land. The “method of scenario supposition for stability evaluation of sites on dolomitic land in South Africa”, which has been applied widely by the industry, certainly since 1995, was applied to the study area in 1996/7. The method was successful in focussing the attention of investigators of dolomitic land on the various factors that contribute to instability. However, with time it became evident that modification and further clarification of various concepts was necessary. The modified method was named “method for dolomite land hazard and risk assessment in South Africa”. This method was applied and comparison drawn between the two assessments. The latest investigative and evaluative methodology is explained. The gravity method as applied to dolomite studies is explained and its results interrogated. Shallow dolomite and its associated risks are analyzed. The karst types identified by Waltham and Fookes is considered and compared to the karst identified in the study area and in so doing placed in a South African context. The results of the new assessment are placed in context with current development type and density recommendations. The functions and requirements of the National Home Builders Registration Council are explained. It is concluded that the study area can be divided into 3 broad risk zones. Zone A represents the shallow dolomite areas and largely reflects a high risk of small to medium-size sinkholes and dolines with localized sub-areas reflecting a high risk of large sinkholes (i.e. inherent risk classes 5,3, 6 (7)). Zone B represents areas potentially reflecting a low to medium risk of up to large sinkholes and dolines (i.e. inherent risk classes 1 and 4). Here bedrock is relatively deep (40 m) and mantled by relatively thick stable material. Zone C represents transitional areas between shallow and deep bedrock where bedrock topography is often highly undulating and thick sequences of low density insoluble weathering product and cavernous conditions occur. Here the risk of all size sinkholes and dolines is high (inherent risk classes 5, 6, 7 and 8). The function of water precautionary measures and founding solutions is discussed. It is pointed out that these measures and solutions cannot change the inherent risk classification but rather change the development risk. Under certain circumstances an acceptable development risk may be established, however a stumbling block remains quality of workmanship, practicality of implementation and costs, the latter referring to the costs of some founding solutions, which render many developments unfeasible. Significant financial losses due to dolomite stability are recorded annually. The development present in this study area is 9 years old and already severe damage to structures have been observed. Despite the mounting costs associated with dolomite instability, local authorities and developers continue to develop dolomitic land. Sites such us these will continue to be targeted for development and investigators are under increasing pressure to come up with engineering solutions to the problem. It is essential to continue to better understand the sites earmarked for development from a geological perspective, so as not to leave future generations with large tracts of sterilized land and a community having to deal with injury to life and limb and fearful of dolomite. Many a geologist may continue to discover various generations of weathering products in this study area in the future and continue to grapple with the links between dolomite stability or rather lack thereof, the geotechnical properties of the various weathering products and the effects of geological structures. Copyright