When a longwall mine cavity is created underground, a change of stress occurs in the surrounding strata. The stress change results in deformation and collapse of the overlying rock layers into the longwall cavity. The thickness of collapsed rock strata is usually three to six times excavated coal thickness. The collapsed rock debris provides support to the overlying rock strata, which tend to sag rather than collapse. Rock deformation reduces towards the surface and a trough shape depression forms at the surface. The depression at the ground surface is usually larger than the footprint of a longwall panel and is called trough or sag subsidence. Tensile and compressive stress-strain fields, and vertical and horizontal deformations typically occur at the surface due to undermining.

Subsidence deformations are a function of mining geometry, overburden geology, and location of prediction points at the surface. The input to a subsidence model includes the following physical parameters:

  • Extraction thickness
  • Overburden depth
  • Seam elevation
  • Subsidence factor
  • Gate extraction ratio
  • Average offset distance
  • Percent of hard rock
  • Strain factor (=0.35 for Appalachian coal fields)
  • Angle of influence

D’Appolonia typically calculates mine subsidence effects using the SPDS software developed at Virginia Tech. We have also conducted research relative to measuring actual subsidence through extensive instrumentation and comparing the results to prediction models.

Study performed by D'Appolonia to assess subsidence versus the progress of longwall mining.

Some descriptions of D'Appolonia projects related to prediction, observation, and mitigation of subsidence effects are provided in the following:


Research on Subsidence Resulting from Longwall Mining

D’Appolonia was retained by the U.S. Department of Energy (DOE) and the U.S. Bureau of Mines to study ground subsidence caused by longwall mining. The three-year study involved monitoring ground subsidence and assessing correlations with mining activities for a longwall mine in the Central Appalachian region.

Mine Subsidence Potential Evaluation for Cogeneration Plant Site

The owner/developer of an 85-MW coal-waste-fired electric cogeneration plant wanted to construct the new facility immediately adjacent to the coal-waste source in order to minimize handling costs. Two coal seams beneath the proposed plant site had been recently mined by the longwall method, and there was a potential for continued ground movement at the surface. D'Appolonia selected a location for the generating facility and recommended positions and orientations for facility structures to coincide with areas of minimal potential for movement.

Remote Sensing of Forestland above Longwall Mining Areas

D’Appolonia was engaged by the Pennsylvania Department of Environmental Protection (PADEP) Bureau of Mines and Reclamation to conduct remote (aerial-platform and satellite) sensing of forestland above longwall mining areas in Washington and Greene Counties, Pennsylvania. The scope of work included analysis of subsidence utilizing state-of-the-practice software and a comparison of predicted subsidence with actual observed canopy and vegetation distress.

Stabilization of Undermined Building Site at College

D’Appolonia was retained to perform foundation investigations for expansion of a recreation building complex that was undermined. D'Appolonia's project responsibilities included determination of the extent of undermining; developing remedial measures; preparing design drawings, construction specifications and bid documents; screening potential construction contractors; selecting grout materials compatible with mine water chemistry; and providing construction management for all phases of a mine filling program.