STUDY OF SPOIL SLOPE STABILITY OF THE OPEN PIT LIGNITE COAL MINE IN CHOCTAW COUNTY, MS

Instability of spoil piles is a common problem in open pit coal mining. This problem increases mining costs and threatens the safety of the mine. Red Hill Lignite Mine extracts lignite primarily from the Nanafalia Formation of the Wilcox Group in Choctaw County, Mississippi. The strip mining is done along one open pit where six seams of lignite are mined. For the mining, a dragline is used which moves along a spoil side bench. The spoil bench has a history of failing. Attempts to stabilize it began with dewatering ahead of mine lines, but they did not completely stabilize the system and the failures continued to occur. A review of the geology indicates the presence of a sand channel below the lowest lignite seam. A preliminary geotechnical evaluation has been done on the spoil, concluding that the instability issues are most likely associated to a rapid increase of the water pore pressure. It is believed that this increase of the water pore pressure is associated with the sand layers located below the last mined lignite seam. Most recently, three lines of earthquake drains have been placed in the pit floor and covered by spoil. Review of wire line data together with flow rates indicate that the drains have reduced pore pressure and increased spoil stability. Detailed geological studies of the Wilcox sediments indicate that the spoil stability is dependent on the water pore pressures. Solutions of the spoil instability are specific to geologic conditions and the need of the continuation of the dewatering program ahead of mine is analyzed.