The excavation of a longwall panel causes a continuous stress redistribution in the surrounding strata for every face advancement. The understanding of the behavior of rock to high level of stresses is instrumental to the structural design of underground mines. In order to achieve a realistic analysis of the behavior of strata, the three-dimensional finite element method is frequently used. Also the model developed for the analysis has to be designed large enough to represent the influenced strata in the study area and a sufficient number of elements has to be used to provide detailed information on the stresses and displacements. Due to the size of the model and its numerical solution the CRAY X-MP/24 supercomputer has been used for this study.
The mine site selected for the analysis is a coal mine located in the Black Warrior Basin in Alabama. The mine has two longwall sections at a depth of 2,000 feet with various dimensions of chain pillars, including yield pillars. Various research programs at this mine have been carried out by Park et. al. ( 1984 ). Therefore data were already available on the geology, and physical properties of rock and coal. In addition a rock testing program has been carried out to obtain rock mass classification parameters.
The progressive failure technique has been utilized to simulate stress redistributions. The failure criterion after Hoek and Brown ( 1980 ) which is based on rock mass behavior was used for the iterative technique. Simulated stress redistributions are therefore very realistic. In order to automate the process involved in the application of progressive failure technique, a computer program has been developed. The combination of a large scale three-dimensional finite element model and the automation of progressive failure simulation provided a tool that can generate a large amount of information related to the behavior of rock and coal as a response to a longwall mining operation.
2. FINITE ELEMENT MODELING
The three-dimensional finite element model covers an area of 1,160 feet * 1,050 feet which consists of three rows of pillars and two half longwall panels, with a half panel width of 310 feet, situated on both sides of the chain-pillar entry system ( Fig 1. ). The total height of the model is 2,130 feet accounting for an overburden above the coal seam of about 2,000 feet and incorporating two layers below the coal seam. The model is discretized vertically into 14 layers of strata groups. This gives a practical representation of the stratigraphic sequence above the mine. Each layer consists of 531 eight-noded hexahedral elements, resulting in 7434 elements in total. In the area of the panel face the finite element mesh has been refined to allow a detailed analysis of stresses in the pillar corners and at the longwall panel face. Boundary conditions for the model were imposed with single constraints in the x-direction for the two vertical y-z boundary planes and in the z-direction for the two vertical y-x boundary planes. The bottom x-z boundary plane was constrained in the y-direction.