ABSTRACT
Wild brining is uncontrolled pumping from the brine streams; it greatly accelerates formation of the linear subsidences, which may then form in tens of years. Deep solution mining (controlled brining) and modern deep mines in dry salt are both stable: no subsidence. Pumping brine from old shallow mines (bastard brining) causes serious collapses; now illegal in Britain. Cheshire has the worst salt subsidence in Britain; houses and structures in Northwich all have timber or steel frames or concrete rafts that can be jacked up. Now that wild brining has almost ended, subsidence due to natural solution is very slow – but does continue. Surface movements are small and slow; engineering precautions are as for longwall mining (section 31). GYPSUM SOLUTION Gypsum may be dissolved and removed naturally. Solution is slower than of salt, faster than of limestone – rock can dissolve within the lifetime of a built structure. Rockhead pinnacles may be dissolved by groundwater, so may not be safe for foundations in the long term. Caves are smaller and less common than in the strong limestones, but may create a significant hazard where weak roof rock collapses easily to create sinkholes. Plugging or filling cavities in gypsum requires care, as diverted groundwater may rapidly create new caves.
