ENFORCE35126 - Laserfiche WebLink

Home Browse Search

locations which are referred to as the north, middle and south underdrains. The middle underdrain has had measurable discharge only once since its completion. Both the north and south under- drains have discharged water during each year since their instal- lation with the south underdrain discharging the largest volume. The fact that they are both still discharging water is a positive indication that the underdrain system continues to function properly. 1988 REPAIR The 1988 slump is believed to have occurred because the cover material became saturated during the spring melt. The saturated cover material, which became unstable on the slope, slid down the slope until some of the moisture in the material was freed and the effective slope was reduced enough for the cover material to become stable. To repair the slump, Blue Ribbon decided to install three french drains across the slump area. It was anticipated that the french drains would stop the cover material from becoming completely saturated. One of the Prench drains was installed above the area, another across the center of the area and one across the lower portion of the area. The french drains were installed ap- proximately at the interface between the cover material and the coal waste material. The drains were constructed of four inch slotted pipe wrapped in a geotextile fabric. The slump area was then regraded to blend in with the surrounding slope. The Prench drains did not discharge water during 1988. The two lower drains did discharge water during March 1989. The slump area repaired in 1988 was stable in 1989 probably because the un- derdrains provided a path for the water to escape from the cover material. 1989 INVESTIGATION The 1989 slump appears to have occurred because of the same fac- tors that contributed to the 1988 slump. The cover material ap- pears to have become saturated during the spring melt to a great enough extent to cause the cover material to become unstable and slide down the slope. There is no evidence to support the theory that inadequate drainage from the top of the bench allowed water to pond on the top of the bench which saturated the area and contributed to the slump. The top of the bench was basically dry when the slump was first noted on March 22, 1989. Piezometer P-4 has not shown a significant increase in water level since it was installed in November 1985. On April 12, 1989, two test holes were dug 24- inches deep in th top of the bench directly above the slump area. Both test holes were dry and hard and showed no indication of 2