Coal mining frequently results in surface subsidence, either inadvertly as voids caused by remnant workings propagate to the ground surface or intentionally as full or partial extraction techniques cause strata to deform into the excavated portions of a coal seam. The U.S. Bureau of Mines estimates that over 7800 km 2 of land in the United States have been affected by subsidence caused by coal mining (Johnson and Miller, 1975). In the next twenty years, the President's Council on Environmental Quality (Council on Environmental Quality, 1979) forecasts that an additional 6200 to 9800 km 2 will be similarly influenced. Because subsidence is likely to play a vital role in the development of coal reserves, it is important to devise methods for estimating the vertical and horizontal surface movements that accompany subsidence. Methods for predicting movement patterns will provide a basis for assessing risk to structures and for developing remedial and design measures to minimize movement effects.
Mining subsidence is a complex phenomenon that depends on the mining methods, the geometry of the mined-out area, the height of the rock and soil above the mine, and the geologic conditions in which the mining takes place. These combined influences make it difficult to model strata deformation and the resulting patterns of surface movement without strong input from field observations. In the United Kingdom, for example, the widespread use of longwall mining is linked closely with empirical charts developed by the National Coal Board (NCB) from a comprehensive review of many case histories. These charts are used extensively in mining areas for planning the underground workings, developing structural designs for new buildings that minimize subsidence effects, and choosing remedial measures to protect existing structures.
This paper examines the subsidence patterns caused by longwall mining. Conceptual models and empirical relationships for estimating subsidence patterns are reviewed with special concentration on the empirical charts developed for mining conditions in the United Kingdom. The geology of the British Coal Measures and associated mining techniques are discussed and compared with the geologic conditions and mining methods associated with various U.S. coal fields. Relationships developed by the NCB among surface displacement, mine geometry, and mine depth are reviewed and their application to U.S. conditions is evaluated on the basis of comparisons drawn between field measurements at U.S. mines and movements predicted by the NCB charts. Surface settlement and horizontal movements are characterized in terms of curvature and horizontal strain, which may be used directly to assess the potential for damage to various surface structures.
INTERPRETIVE FRAMEWORK FOR SUBSIDENCE
Coal removal generally is concentrated in tabular sections referred to as panels. In the U.S., panels typically are 120 to 150 m wide and 450 to 600 m long. The excavated thickness of a given panel will depend on the thickness of coal and the condition of the rock immediately above the coal.
Longwall mining entails the complete removal of a given thickness of coal. Excavation is performed by mechanical breaking tools that are drawn continuously across the working face so that no pillars are left in the mined-out area, or gob. The roof is allowed to cave behind the face.