Man-Induced Land Subsidence
How would you feel if your land had sunk 9 m in the past 50 years because of human activity? It happened in the San Joaquin Valley. In fact, land subsidence has been caused by man’s activities in at least 37 of the 50 states of the United States and affects more than 40,000 km2 in this country alone. Data from a few sites where economic impact is documented suggest a total annual cost to the nation of more than $100 million; worldwide, the total economic impact is astounding and growing. These nine papers, dedicated to Joseph Fairfield Poland's life work, constitute a major contribution to measuring and understanding this problem. They are arranged in three categories: (1) fluid withdrawal from porous media; (2) drainage of organic soil; and (3) collapse into man-made and natural cavities.
Coal mine subsidence is the local lowering of the ground surface caused by coal mining. Subsidence processes above underground mines consist of a gradual downwarping of the overburden into coal extraction panels, which causes depressions, or a sudden collapse into individual mine openings, which causes pits. Subsidence type (depressions, pits), amount, areal extent, rate, and duration are controlled by (1) thickness of coal mined, (2) mine geometry and mining methods, and (3) thickness, lithology, structure, and hydrology of the bedrock and surficial material in the mining area. Subsidence in surface mining areas is caused by compaction of rehandled overburden material (spoil), dewatering of aquifers near surface mines, and (or) stress and strain readjustments. Depressions (troughs), cracks, and pits (sinkholes) can occur under either underground or surface mining procedures.
Maximum vertical displacement in depressions above extraction areas large enough to cause maximum subsidence in the western United States ranges from about 45 to 90 percent of the thickness of coal mined. However, pits may be deeper than the mining thickness, where the caved material can move laterally into adjacent mine openings. In surface mining areas, although little detailed information is available in the western United States, subsidence may range from a few percent to perhaps as much as 10 percent or more of the thickness of the reclaimed spoil. The limit angle, which determines the area of subsidence above underground mines, ranges from about 5° to 30° (from a vertical reference) above underground mines in the western United States. In surface mining areas, however, the subsidence area can range from nearly equal to the mining area, where compaction of rehandled spoil occurs, to many times the mining area, where aquifers are dewatered and undergo compaction.
The time or duration between mining and complete subsidence above underground mines commonly ranges from a few months to a few years, where downwarping occurs above extraction panels, to many years or decades, where pillars are not mined. The duration between mining and the occurrence of pits (sinkholes), however, can vary from a few decades to many decades or even centuries. The time necessary for depressions and pits in surface mining areas is not well known but apparently depends on factors such as methods of emplacing and grading and rate of wetting the rehandled material, rate of dewatering of aquifers near the mine, and stress-strain readjustments.