Gypsum (CaSO4.2H20, the raw material for plaster) is a rock which dissolves so rapidly that, wherever it occurs, it has a detrimental effect on development, construction and water supply. The hazardous characteristics of the rock are underestimated in the developed world and ignored in the developing world. Its presence, and the associated cave systems within it cause natural catastrophic subsidence that affects many urban and pen-urban areas and their infrastructures throughout the world. Gypsum encountered in damsites, tunnels and canals has resulted in expensive failures and difficult site conditions. Groundwater contaminated with dissolved gypsum is common, and gypsum karst aquifers are very sensitive to pollution. These aquifers are also very sensitive to water table drawdown and the associated generation of an enhanced subsidence risk. There is a need for improved understanding and recognition of gypsum as a geohazard and to introduce geological information into the planning and development process in gypsiferous areas.
This report summarises a collaborative study undertaken in China and Lithuania looking at gypsum karst geohazard problems. It was carried out under the ODA/BGS Technology Development and Research Programme (Research Contract R6490) as part of British Government recognition and aid to developing countries. The aim of the project was to help with the avoidance of gypsum geohazards in national planning and to develop generic guidelines to deal with gypsum geohazards.
A framework for incorporating gypsum geohazards in National, Local and Site Specific Planning is presented with examples from China, Lithuania and the UK. The most cost-effective way of dealing with gypsum geohazards is to avoid them rather than try to engineer around them. For National Planning, geological hazard maps, incorporating gypsum as a geohazard, are the most practical starting point for planning. On a local basis, detailed maps and planning zonation, with local regulation of groundwater abstraction, can mitigate the impact of the problem. On a site-specific basis, special forms of construction, including strengthened and extended foundations, can help to allow development to proceed; these measures are expensive and avoidance of the worst areas is the best practical solution. Measures to control surface water runoff and the avoidance of soak-aways is another practical way of reducing the subsidence caused by this hazard.
The avoidance of gypsum geohazards has an economic benefit. A framework for undertaking a Cost Benefit Analysis (CBA) is presented with an example of how it might be implemented to assess the impact of hazard avoidance versus non-avoidance.
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