I) Principles - rapid subsidence - creation of underground cave removes support for overlying rock, which collapses over cave
(A) Coal mining - involves human-constructed cavity (underground coal mine). For full recovery mining (remove ~all coal), subsidence of overlying ground surface occurs within days or weeks. Subsidence is indicated by circular or elongate depressions (depending on shape of mine workings) + ground fractures. For room + pillar mining (~50% coal removal), subsidence may not occur (if rock is strong). Pillars = unmined areas that support overlying rock. Subsidence is worst for underground mines of low-grade coal (e.g., lignite + bituminous in WY + ND) "soft coal" in weak sedimentary rock. Examples = ND/WY, CO, PA, IL, + western Great Britain, Wales. Critical to balance economic value of coal with aesthetic/property value.
(B) Sinkholes (link #2) - circular depressions (commonly water-filled) usually in limestone (dolomite, less commonly) bedrock + in other soluble bedrock such as marble, gypsum + halite (geologically uncommon). In USA sinkholes are common in FL, AL, GA, TN, MO + PA. Karst topography = landscape dominated by underground caves + surface sinkholes, lakes, + disappearing streams.
Formation of Sinkholes (link #2) - form during periods of fluctuating water table (rainy/dry seasons or light/heavy groundwater (GW) removal).
Step 1 - During high water table level, slightly acidic GW dissolves limestone, creating underground caves. Rain is naturally acidic due to atmospheric CO2 dissolved in rain, which produces carbonic acid (H2CO3). GW derives more acidity from CO2 produced in soils by decomposition (oxidation) of organic matter.
Step 2 - During low water table level, water drains out of cave + weakens rock (removes buoyant support) + collapse can occur quickly (hours to days).
Similar process of fluctuating water table produces sinkholes in gypsum + halite (much more soluble than limestone + don't need acidity to dissolve).
II) Mitigation of rapid subsidence - produce subsidence hazard maps + restrict land use, regulations for GW use or coal mining methods.
III) Case History - Rapid Subsidence (sinkhole)
Winter Park, central Florida in 1981 - sinkhole (scroll to Slide #10, scroll to Fig. 62) quickly grew over 48 hour period, taking several home, 6 Porsches, parts of 2 streets, + part of city swimming pool; ~100 m diameter + ~30 m deep. Damage = ~$2 million. City eventually stabilized + sealed sinkhole, + converted it to urban lake.