Research into ground control problems resulting from the mining of seams in close proximity has been carried out using finite element and body-loaded photoelastic modeling methods in conjunction with statistical and empirical analysis of numerous case studies. As a result of this research, an integrated design model has been constructed and formed the basis for a computer program MSEAM which can assist field engineers in dealing with interaction problems caused by multi-seam mining.
Interact/on among work/rigs on adjacent seams is a major cause of ground control problems in multi-seam mining. This is particularly true in the Appalachian region where up to thirteen contiguous mineable seams exist in some places (Haycocks & Karmis 1983). Interaction is accentuated when seams are in close proximity, with vertical separations of less than 120 feet, where both under- and over-mining can result in serious ground control problems. Historically, research has produced a considerable number of useful design criteria dealing with specific aspects of multi-seam mining (Stemple 1956, Peng & Chandra 1980, Haycocks & Karmis 1983). However, a totally integrated approach has been lacking. In an effort to develop such an integrated approach, individual components have been combined in an full design model for over- or under-mining close seam situations. The model can predict the sites and magnitudes of possible interaction problems, and can be used to develop designs for new multi-seam mines and avoid or minimize damage due to interaction.
To formulate an integrated model, the different components that represent the behavior of selected mine structures were developed or refined.
Pillar load transfer: Overburden load which is concentrated through upper seam structures, such as pillars and abutments, will be transmitted some distance through the innerburden, possibly to the detriment of workings in the lower seam. Innerburden depth, through which upper seam stress concentration can affect lower seam structures, can be up to 120 feet, depending on the pattern of load distribution, lithology, and degree of stratigraphic layering (Haycocks & Karmis 1983). Based on the load distribution over pillar or gob lines (Lu 1984), the incremental load distribution under upper seam structures can be classified into three types (Figure 1): load on gob line interfaces, load on wide remnant pfilars (100'<Wp<200'), and load on narrow remnant pillars (Wp<100').
Figure 1. Three Types of Load Distribution on Upper Seam Gob Line Interface or Remnant Pillar(s). (available in full paper)
The magnitude and distribution of stresses under these structures can be calculated using a modified analytical solution based on the theory of elasticity. Initial determination of stresses at point P(x,z) under an upper seam structure can be obtained from the following equations:
(mathematical equations)(available in full paper)
