Where evaporite bedrock is exposed at the surface or underlies thin surficial soils, there is some risk that subsidence could occur. The associated risk rises where the frequency of sinkholes increases. Dangerous and life-threatening, spontaneous collapse and opening of subsurface voids are rare, but do occur. More commonly, differential strains and settlement from localized ground subsidence or piping (removal of fine soils into subsurface voids) will damage facilities that are unknowingly constructed over a sinkhole or subsidence trough. Where subsidence features are exposed at the surface and properly identified, they should be avoided if possible.
Many older sinkholes have been covered with recent soil infilling and are completely concealed at the surface. While some concealed sinkhole locations can be seen in aerial photography by vegetation changes, only subsurface inspections, either by investigative trenching, a series of investigative borings, geophysical means, and/or observations made during overlot grading or utility installation, would ascertain whether sinkholes exist within a development area. If such hidden sinkholes are located, an experienced geotechnical firm should be retained to evaluate the hazard and potential for future subsidence on the development.
There are both ground-modification and structural solutions that can help mitigate the threat of subsidence if avoidance is not an option. Drainage issues and proper water management are as important as they are for collapsible soils. Additional increases of fresh water may accelerate dissolution, destabilize certain subsidence areas, re-open or establish new soil pipes to bedrock voids, and rejuvenate older sinkholes.