Advances in dolomite land risk assessment for safe development within the current legislative framework : the case of Khutsong North in Merafong City, Gauteng Province, South Africa

Abstract

At least 45% of the Merafong City Local Municipality (MCLM) is underlain by dolomite of the Malmani Sub-Group, Chuniespoort Group of the Transvaal Supergroup. The current research area, Khutsong North Townships (KNT’s), is directly underlain by chert-rich dolomite of the Monte Christo Formation and is considered to be 100% dolomite land. Such terrain has a notorious reputation in South Africa due to its susceptibility to the formation of ground instability events, and thousands of people currently reside and work on such land.

In the past, an uncontrolled land use was common in building practice and this led to the development of geologically unsuitable land, i.e. dolomite land. The current South African National Standard (SANS) 1936:2012 – a deemed-to-satisfy standard, requires that geotechnical and/ or dolomite stability investigations be conducted on all dolomite land earmarked for development and that, appropriate precautionary measures be implemented to ensure safe and sustainable land use. However, Khutsong North was established around 1957 and no legislated requirements for conducting geotechnical studies were available at the time.

This research aims to assess scientific advances made over time and influence of dolomite land risk assessment for safe development within the current building legislative framework. Various classification systems/ methods aimed at evaluating the stability of dolomite sites in South Africa have been proposed since the 1970’s. As such, a number of dolomite land risk assessment (geotechnical) studies have been conducted across the research area since 1980.

A total of 219 ground instability (sinkhole & subsidence) events have been recorded in KNT’s between 1975 and year-2020. No lives are reported to have been lost during this period, but several houses are either damaged or have been completely destroyed by this phenomena and wet services network is critically affected. Therefore, the accepted method for dolomite land hazard and risk assessment – “Method of Scenario Supposition” described by Buttrick et al (2001) and existing data (borehole, events and erven size distribution) were used to calculate hazard rating, compile vulnerability and risk map of the KNT’s.

In line with the objectives of this research, historic site zoning (from 1997 & 2013 studies) and scientific advances in determining the Inherent Hazard Class (IHC) were used to investigate possible influence, (1) on the extent of allowable development over time since Buttrick et al (2001) and later promulgation of SANS 1936:2012, and (2) of the current building legislation in ensuring that a tolerable hazard rating is achieved in Khutsong North.

However, compliance with SANS 1936 technical requirements have not been fully enforced in the research area including non-implementation of the Dolomite Risk Management Strategy (DRMS). Furthermore, the relevant authority has not replaced the broken wet services with those that are SANS 1936 compliant.