By Winston Muleba JR
The term sinkhole is most commonly used in the international literature when dealing with engineering and environmental issues and some scholars regard it as a large dissolution cavity that is open to the Earth's surface.
Some sinkholes form when the roofs of caves collapse, others can form at the surface by dissolving the rock downward. In other studies, a sinkhole is a catastrophe. Despite the fact that its impact is usually local and sometimes isolated, the unpredicted nature of sinkholes and the fear that the ground may disappear below the feet, make sinkholes a threat not to be under estimated.1. CAUSES
In 2022 areas like Minsundu in Ndola district where sinkholes are evident shows that gravitational deformation of the ground during sinkhole development may cause severe damage to buildings and other man-made structures including roads, railways, dams and/or ditches. Last year, a sinkhole developed along the Ndola-Sakania railway line in Ndola district and it was inspected by Minister of Transport and Logistics, Hon Frank Museba Tayali; this sinkhole was reported to measure 20metres in diameter and 15metres in depth according to the engineering team from Zambia Railways Limited. Another unpredictable development in Chisankane Ward of Kafue district also suggests that when sinkholes collapse to expose the water table at the surface, the sinkhole could get filled with water forming small circular lakes such as Lake Kashiba in Lufwanyama Kashiba Ward. Few years after independence around 1968, Mufulira Copper Mine recorded a geotechnical issue which shows that sinkholes may also form by slow enlargement of caverns by continued dissolution of the limestone; this may no matter what the level of the water table. Further, damage to buildings caused by subsidence and the loss of lives as a result of the formation of sinkholes, are just some of the risks that could result from dolomitic compartments.
Other research studies indicate that there are several important practical aspects regarding the formation of collapse sinkholes. Of major concern is that they may form in a catastrophic way without showing any previous noticeable warning signs. After formation they may grow in size because their scarped sides tend to degrade by mass wasting and erosion processes as they evolve from a cylindrical to a cone then a bowl-shaped geometry, an evolution, which may be very rapid in cover collapse sinkholes. The volume of the collapse sinkholes at the time of formation provides a minimum estimate of the volume of the subsurface cavities since voids may remain unfilled and the collapse material may bulk and undergo a reduction in density.
2. IMPACTS
Individual sinkhole events may have a large financial impact. For example, in Chalala, the direct economic losses caused by single collapse events that affected buildings worth colossal sums of money. Sinkholes may also cause the loss of human lives when they occur in a catastrophic way. The area in Chalala where the sinkhole developed was called “kuma boreholes” because it had disused boreholes; they were used to dump after being killed by murderers stated Dickson Jere – a Journalist turned prominent Lawyer, on his facebook page. Essentially, areas such as New Kasama, Mahopo, Shantumbu, Apex, Shaft 5 and Chalala are largely built on a Limestone and dolomite belt. Buumba Malambo, Kafue district Mayor indicated in a social media thread that houses in affected areas were built dating as far as 2012. Scholars have further documented that other sinkhole problems are related to hydrogeology and hydraulic structures. Sinkholes can act as water-inlets connected to high-transmissivity karstic aquifers and cave systems making the impoundment of water in reservoirs difficult. They can facilitate the rapid pollution of the groundwater. Moreover, these topographic depressions are frequently prone to flooding either by the concentration of surface runoff or by groundwater flooding when the water table rising above their ground level.
3. MITIGATION
When sinkhole-prone areas are occupied by people, vulnerable buildings and infrastructure, the risk should be mitigated by reducing the activity and severity of the processes (hazard), the vulnerability, or both. Since the control of the subsurface dissolution and subsidence processes involved in the generation of sinkholes may be very difficult, safe mitigation commonly requires careful planning and the application of subsidence protected engineering designs. As such, the safest mitigation strategy is the avoidance of the subsidence features and the areas most susceptible to sinkholes. This preventive measure may be applied prohibiting or limiting development in the most hazardous areas through land use planning and regulations. The preventive planning is commonly most effective when developed by local administrations.
About the author:
Winston Muleba is a Zambian and professionally gravitates towards disaster risk reduction, project management, environmental sustainability and agriculture matters as he is skilled at juxtaposing the latest research and expert opinion with the everyday lives and struggles of people on the ground. He holds a Master’s degree in Project Planning Management and Bachelor of Science in Natural Resources.
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