Prevention and Reclamation of Mining-Induced Land Subsidence

Underground mining created voids or mine-out areas that cause changes in the magnitude and orientation of the in-situ stress field and induce earth fractures and ground subsidence. Although some subsidence incidents are anticipated in mining areas, they can strike with little or no warning. Mine subsidence is a geohazard that can result in catastrophic and costly damage and even fatalities. Ground movement is typically swift and sudden for pit subsidence, while trough subsidence, which can develop over mines of any depth, appears as a gentle depression in the ground and can spread over an area as large as several square kilometers. Many factors including mining method, depth of extraction, size and configuration of openings, rate of advance or extraction, coal thickness, topography, lithology, structure, hydrology, in situ stresses, and rock strength and deformational properties affect the initiation and maximum lateral extent of subsidence; the most important ones are extraction width, coal thickness, and mining depth.

Mine subsidence and the factors that affect the subsidence vary significantly in different regions and different mines. Any investigations are to be designed to meet project objectives and tailored to site-specific conceptual site models. The requirements on data resolution typically increase as the project progresses from preliminary phase through planning phase to design and construction phases. During the preliminary and planning phases of a project, the primary objective is to evaluate the mine subsidence risk levels, whereas the primary objective of design and construction phases is to determine the depth, thickness, engineering properties of soil and rock, and hydrogeological conditions under the project site so that any potential of subsidence risk can be mitigated. As more linear projects such as roadways, railways, tunneling, and gas and oil pipelines cross large areas and new detection technologies are developed, selection of investigation techniques is not as distinctive between study scales. Many methods and techniques such as borehole exploration, groundwater monitoring, geophysics, tracer tests, and aquifer tests are applicable to in mine subsidence investigations. The level of effort depends on the complexity of the project site and severity of subsidence. It is a common practice that non-intrusive tasks precede the intrusive ones.

Mining of coal seams in the study area has created single-layer goafs of approximately 21,079 m². The underlying goafs of are in an unstable state, and subsidence or collapses may occur if no engineering measures are taken. The presence of the goafs has become one of the key factors restricting the land use of the area. The goafs are remediated to build a residential community for resettlement of residents. Three types of boreholes are drilled to remediate the potential subsidence area: hydrogeological investigation boreholes, grout curtain boreholes, and grouting boreholes. Two hundred fifty-two boreholes are completed with drilling footage of approximately 12,067 m. Approximately 55,327 m³ of grout were used to address the coal seam goafs.

The surface deformations caused by shallow goafs can result in serious damage to buildings (structures) during mining. The movement duration is typically short, and the underground void rate and residual deformation are relatively small. The collapse pits formed on the surface of shallow unconventional room-and-pillar goaf can cause serious damage to ground buildings. The underground voids and residual deformation left in the goafs may also pose a potential hazard to surface stability. A paste filling technology is successfully used to remediate the shallow goafs. Paste filling materials have the characteristics of high solidity, high strength and good economic benefits. They have great advantages for goafs in coal mines where the roof rock strata are relatively complete and hard, the goaf space is relatively large, and the roof has not collapsed or has not completely collapsed.

Huangsha Mine of north China adopts the multi-level development method with production levels at − 280 level and − 500 level. The average water flow in the mine is approximately 3.5 m³/min. A water inrush incident occurred in 2011 at the bottom of the − 280 level, and a large volume of groundwater flowed into the mine from the Ordovician limestone underlying the goafs. The peak water flow exceeded 400 m³/min, which was greater than the drainage capacity. The working face level was completely flooded as well as the pump house at the − 500 level. In response to the water inrush, the drainage capacity of the mine increased significantly by installing large-capacity submersible pumps to control the rate of the groundwater level rise. A concurrent grouting program was implemented to fill the goafs and cut off pathways from the water sources to the mining area and the water passageways between Huangsha Mine and the neighboring mines. The water inrush and engineering activities associated with the grouting program caused significant groundwater level fluctuations, which are contributors to collapse pits or subsidence depressions on the ground.