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j Portions of the upper part of the landslide n•~ass show evidence of contemporary movement. <br />However, from the configurations of the scarps, their distribution and the absence of any signs <br />] of heave on the lower bench, these features are consistent with a series of separate, relatively <br />small scale landslides which have developed on steeper parts of the old landslide mass. <br />~ Current proposals for longwall mining involve mining beneath the upper or south part of the <br />landslide mass. There is no evidence to suggest that the past history of instability would have <br />any effect on the longwall operation. The drilling results in the area of interest suggest that the <br />ssa;a above the 'B' seam are continuous and without detectable dislo~ion. The supe1'icial, <br />1 neaz-surface instability should not affect the roof conditions or the caving characteristics of the <br />overburden. <br />Mining will probably result in subsidence at the surface. Based on local experience, this <br />subsidence should be less than 10 feet at the surface. Subsidence may be accompanied by <br />surface cracking. This cracking, if it occurs, would be similar to the cracking associated with <br />the existing landslide movements. Given aneast-west orientation of the longwall panels, <br />resulting cracking due to subsidence would be more or less parallel to the ground contours <br />which, again, would make them difficult to distinguish from the natural cracks and scarps. <br />Subsidence is unlikely to aggravate the instability of slopes on Jumbo Mountain. It may actually <br />improve stability by: <br />• back\aazd tilting of the strata, decreasing the slope of potential failure surfaces. <br />• forming factures and steps which would interrupt continuity of the bedding. <br />planes which form potential basal failure surfaces. <br />Subsidence cracks may lead to increased infiltration of snowmelt. However, cracks, scarps and <br />crevices aze already widespread so that infiltration is already high. <br />CONCLUSIONS <br />' From our investigations on Jumbo Mountain, we have concluded: <br />• The large Jumbo Mountain landslide occurred thousands of years ago at a time when <br />the topography was quite different from its current condition. <br />I -13- <br />0:\Off\30\JPT\ARCO-Rpt.lkl <br />