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Sanborn Creek Project Page 14 August 21, 1992 percent of the total seam extracted. This would indicate that 1260 feet of interburden should be present for low-cover and 940 feet for high cover. Obviously, it will not be possible to retreat mine panel pillars in either the "C" or "B" Seams and adhere to the USBM guidelines if water is impounded in the Oliver No. 1 and Hawk's Nest Mines. Neate and Whittaker (1979) present a case study on drainage of the partially water filled abandoned upper Main Seam room and pillar workings in the Lynemouth Mine in England during longwalling 230 feet below in the Brass Thill Seam. The interburden thickness between these two seams at the Lynemouth Mine is considerably less than suggested the USBM guidelines. The tensile strain zone ahead of and over the down dip advancing longwall face progressively drained water from the overlying workings in the upper seam. It is interesting that no water entered the mine from the overlying North Sea, 295 feet above the Main Seam and 525 feet above the Brass Thill seam. The 525 feet between the Brass Thill Seam and the North Sea would have to have been approximately 1800 feet to meet the obviously conservative USBM guidelines. Ropski and Lama (1973) describe how the ground in the tensile strain zone above and around the edges of a longwall panel opened up along bedding, "like pages in a book", as the longwall face passed directly below and then closed as the face advanced • further. The tensile strain zone over the main, sub-main and panel ribside barrier pillars will open up under the tensile strain, but will not be recompressed. These zones of increased permeability will remain after retreat mining is complete and mining has proceeded to the next panel. The magnitude of tensile strain decreases with distance above the mining horizon. The decrease in calculated strain upward from the mining horizon is indicated on Table 4. It is possible to conservatively estimate the "D" and "E" Seam trough subsidence effects, maximum vertical subsidence (SST) and maximum horizontal strains (+E tensile and -E compressive), resulting from failure of the retreated, robbed, panel pillars in the ^C" and "B" Seams. Figure 2 provides a schematic of the downward trough subsidence deflection of the ground surface from longwall or pillar robbing room and pillar mining. The horizontal strains are caused by the flexure accompanying differential vertical trough subsidence. Tensile strains that develop over the sides and adjacent to caved panels will increase the permeability of the strata subjected to tensile strains. The method used for prediction of trough subsidence is the conservative Subsidence Engineers Handbook (National Coal Board (British), 1975) for longwall mining as modified by Abel and Lee (1984) for pillar failure in retreat mined room and pillar panels, • Appendix A. Table 4 presents the results of the maximum trough