Our Services
 

GeoStructures offers a wide range of professional services in the geotechnical engineering and construction monitoring fields. All of our work is backed by an unwavering commitment to excellence, innovation, and practicality.

 
Concrete Laboratory Testing
Construction Monitoring
Dams Analysis & Rehabilitation
Environmental Assessments
Foundation Investigation & Design
Geotechnical Laboratory Testing
Highway and Bridge Geotechniques
Lime and Cement Stabilization
Mine Subsidence Analysis & Remediation
Pavement Design & Rehabilitation
Percolation/Permeability In-Situ Testing
Pond and Lagoon Liners Design
Research and Special Studies
Sinkhole Evaluation & Remediation
Slope/Retaining Wall Evaluation & Design
Soil Improvement Technologies
Soil-Structure Interaction
Subsurface Investigations

Mine Subsidence: Investigations, Stabilization and Risk to Structures

Deep room-and-pillar mines have been active in Pennsylvania’s coalfields since the late 1700s. Sinking of the ground surface above the mines occurs as long troughs or local circular potholes. Troughs are caused by the crushing failure of coal pillars or the punching of coal pillars into the underclay (fireclay) of a mine floor. Potholes form when a mine roof collapses and the overburden is less than about 50 ft. Similar to dome sinkholes in karst regions, potholes usually occur suddenly and can cause severe structural damage.

Factors controlling mine subsidence include height of the mined-out coal bed, pillar size, width between pillars, overburden thickness, and rock strength. According to the Ohio Division of Geological Survey, the potential area of subsidence extends beyond the extraction area along a 35° angle of projection, called the angle of draw. The deeper the mine, the larger the area potentially affected by mine subsidence at the surface.

Site investigations and risk assessments start with research of historic mine maps kept in the Department of the Interior’s Office of Surface Mining (OSM). This is followed by a program of deep test borings, geologic mapping, and geophysics to further delineate subsurface openings. Examples of geophysical techniques are borehole tomography and cross-hole seismic surveys. Detailed subsurface investigations enable different levels of risk to be assigned based on various controlling parameters, such as the ratio of overburden to void height. An added comfort level can be gained by using FEM to model underground stresses and deformations due to surface loads and site-specific subsurface factors.


Commonly employed methods for stabilizing the ground above old mines involve compaction grouting—a technique similar to that used to repair karst-related sinkholes. Low slump concrete mixes may be initially injected as a barrier in advance of using less expensive, high slump and low strength cement and fly ash as production grout. The grout fills mine openings in the area of the collapse and also builds columns from the floor of the mine to the ground surface.

 

 

 

 
 
 
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