Journal of the South African Institution of Civil Engineering
On-line version ISSN 2309-8775
KIRSTEN, H A D; HEATH, G J; VENTER, I S and OOSTHUIZEN, A C. The issue of personal safety on dolomite: A probability-based evaluation with respect to transient passage in a city centre. J. S. Afr. Inst. Civ. Eng. [online]. 2014, vol.56, n.2, pp. 68-77. ISSN 2309-8775.
ABSTRACT For the past fifty years empirical knowledge guided the development of rules regarding population density on dolomite land. The insatiable demand for land, the improvement in transportation infrastructure and the associated need for improving the functionality of towns require that these rules on the risk of personal injury and damage to assets are revisited from a more scientific perspective. Probability theory provides a basis for decision-making in this regard. SANS 1936:2012 defines development densities for different types of land usage, including non-residential improvement, high- and low-rise buildings and single-storey dwelling houses. The paper is devoted to public safety along the roads, thoroughfares and open spaces outside buildings in a heavily populated city centre as a study in "transient density" on dolomite land. People are transported through the city in a range of vehicles. Some people walk through the city and some appear in particular locations as dispersed groups. The densities at which people appear differ during peak hours, other times of the day, and at night. The overall probability of fatal injury is determined by the mutually dependent probabilities of sinkhole occurrence, appearance of the sinkhole in a particular location, appearance of the sinkhole at a particular time, coincidence with the vehicle, people being unaware of the sinkhole, people falling into the sinkhole, people not being protected by the vehicle and the relative number of fatal injuries Sinkholes are invariably caused by water-bearing services that tend to leak at isolated locations, as a result of which only one sinkhole occurs at a time in a particular stretch of land. In developed land the leaky service and the sinkhole are generally repaired soon after the sinkhole has occurred, which precludes the recurrence of sinkholes in that area for a very long period of time. The probability of sinkhole occurrence can therefore be evaluated on the basis of the binomial distribution. The infiltration regime that determines the sinkhole return period for this purpose is based on the water and wastewater reticulation infrastructure, stormwater control measures, landscaping and irrigation provisions, occurrence of impermeable pavements and dewatering protocols characteristic of a business district in a city centre. It is shown that the probability of potential fatal injury during peak time is larger than an internationally prescribed threshold value for Inherent Hazard Classes 6, 7 and 8 for minibus taxis, buses and pedestrians at road intersections for sinkholes 10-20 m in diameter. These unacceptable cases may be resolved by marginally changing the values for some of the input probabilities that may be somewhat conservative. Alternatively, the adopted threshold level for tolerable risk could be relaxed from "As Low As Reasonably Practical" to "Slight", which may more accurately represent the fait accompli sense of risk in the brownfields situation in Centurion City. A further way to view the unacceptable cases is that they are largely compatible with the prescribed land usages in SANS 1936:2012, in that precautionary measures corresponding to area designations D3 + FP1, D3 + DL1 or D4 are required for all but Inherent Hazard Class 1. These requirements are fully justified for Inherent Hazard Classes 6, 7 and 8, may be somewhat conservative for Inherent Hazard Classes 4 and 5, and are quite likely too conservative for Inherent Hazard Classes 2 and 3 in the open spaces in a city centre environment. A fourth way of dealing with the unacceptable cases in a greenfields situation is to implement engineering designs to pavement structures that would mitigate the hazard.
Keywords : sinkhole; coincidence; personal safety; risk; hazard; probability; city centre; transient passage.