Although it is a relatively neglected research topic, surface structural damage due to underground coal mining is a major concern in many coalfields. This is especially true in southwestern Pennsylvania where several coal seams are being mined extensively. In this area there were 340 homeowners who claimed their houses were damaged by underground coal mining between 1973 and 1978. After the investigations, the Pennsylvania State Subsidence Office only supported 152 claims as mining related cases. The remaining 182 cases were determined to have been caused by non-mining related factors.
The Pennsylvania Bituminous Mine Subsidence and Land Conservation Act stipulates certain underground layouts for protecting surface structures (Fig. 1) (1). If the surface structure such as house is located in a level terrain, the dimension of the supporting area left at the seam level is equal to the tangent of 15° (offset angle) multiplied by the depth of overburden, plus an offset distance of 15 ft as a safety factor. If the structure is located on a terrain with slope larger than 5 percent, additional length for the supporting area equivalent to the product of the slope in percent multiplied by the depth of overburden must be added to the downslope side. A maximum of 50% extraction is allowed within the supporting area. This measure has been proven empirically if the limiting slope of damage is 4 mm/m(2 ,3).
The objective of this paper is to determine, through case analyses, the types and causes of damages to the residential houses as a result of surface subsidence due to underground coal mining employing room and pillar method
METHOD OF STUDY
Among the 152 claims that were supported by the Pennsylvania Subsidence Office, 6 mines including 35 cases were randomly selected for study.
For each case, several visits were made to the mine for collecting the following information: mining height, depth of overburden, extraction ratio, stratigraphic column, a topo map and a mine map which shows the detailed mine layouts including location of the damaged houses. If available, interviews were held with the personnel involved in the mining operation that resulted in the damage of the residential houses. Several visits for each case were also made to inspect the types and location of the damage to the ground and the house. Data such as type and dimension of the house were also collected. Interviews were also conducted with the home owners who would generally discuss the type, sequence and history of the damages occurred in the premise. Room and pillar mining was employed in all cases.
No subsidence measurements were taken. However, the type and location of damages were measured and recorded for each house.
All of those data collected were analyzed to establish the causes for the damages. If the causes were not obvious based on the current knowledge as described below, the 3-D finite element analysis were performed.
TYPES AND CRITERIA FOR DAMAGES
Since the types of damage are closely related to ground movements, a brief description of various types of ground movements, associated with a surface subsidence profile will aid in understanding the causes of various types of damage