We recently performed an assessment in an area of Illinois that was extensively mined for coal in the first half of the last century. The neighborhood of the property was situated on top of former mining operations which resulted in land and building movement from mine failures.
The failure of mines is due to the technique of extraction which is known as “room and pillar” in which pillars of coal were left in place to support the “roof” (which is typically limestone) of the mine tunnel. The entrance to the coal seam was provided by vertical shafts driven downward from the ground surface, and from which the room and pillar mining proceeds in a generally horizontal fashion. The shafts were used to bring the coal to the surface and to provide ventilation. The room and pillar method does not typically consist of a regular pattern of rooms and pillars such as those used in underground limestone mines. Instead it consists of long rooms up to 1,000 feet in length with widths typically ranging from 15 to 25 feet. Between these rooms, barrier pillars up to 400 feet in length are left intact, although in some cases, the pillars are removed when the mining is in the final stages as the mining operation retreats to the shaft location.
Over time the “roof” can settle either between pillars or as a result of a failure of the pillar itself. This typically results either as a “pit” (a failure very close to the surface level) or a “trough” (a failure well below the surface which yields surface depression over time). It is similar to an inverted orange traffic cone, with the mine failure at the tip of the cone within the mine tunnel and the circular cone area representing the surface area disturbance. Thus, a building structure can be impacted even if the mine failure point is not directly underneath the property.
Once the subsidence begins, it typically continues until the mine has fully collapsed or reaches a state of equilibrium. The general rule of thumb is that the amount of subsidence at the ground surface is around 1/3 of the thickness of the mined coal seam. So, if the mined coal seam was six feet thick, the movement at the ground surface could be in the order of two feet of vertical settlement.
Imagine the problems that arise when portions of a building experience a 24” vertical displacement of walls. At best, the property has to be vacated while the vertical displacement is structurally corrected to ensure that the roof will not collapse as the roof framing of the building pulls away from walls. Initial signs are frequently seen in door alignment issues, cracks in walls and ceilings, plumbing system failure or damage to exterior finishes.
Typically the analysis of the site consists of three phases. During Phase 1, deep borings are drilled below the coal seam (think 100’ to 200’ deep borings). Laboratory tests are then performed on the soil in the samples, to include Atterberg Limits (determining the nature of the soil), natural moisture content, grain size analysis, unconfined compression tests and consolidation tests. A geologist or geotechnical engineer supervises the drilling and sampling procedure. Phase 2 of the process typically includes the design of underground support for the building and Phase 3 involves the mitigation of the subsidence – such as additional footings, piers and other subsurface supports.