GENERAL46842 - Laserfiche WebLink

Home Browse Search

5 rock or regolith slips along a concave upwazd-curved surface while at the same time rotating backward. The toe of the rockslide exhibits characteristics of an "earthflow;' which is a sluggish, erratic flow of clayey or silty regolith down relatively gentle slopes. Although water may have lubricated or hydrostatically opened some critical bedding planes, the failure material was not fluidized or saturated, and instead consisted of large, intact sequences of strata. Observation of structure contour maps provided by the mine indicates that the dip of strata becomes locally steeper beneath the rockslide area (figure 9). The maximum dip calculated from structure contours on the L Seam is 11.7 degrees, compared to values of 9-10 degrees outside the slide area. The strike of tension gashes documented at the head of the rockslide, in the slump azea, is parallel to the strike of structure contours on the underlying coal seams. This suggests that the rockslide moved from the southwest, which is the structurally and topographically highest point, down-slope toward the northeast, which is not only the direction of maximum dip but also the point of lowest structural and topographic elevation. Additionally, the transverse shear ridge that bounds the western side of the failure is approximately perpendicular to the strike, or in other words parallel to the maximum dip direction. The inclinometer data shared by the mine indicate that slip has been detected in or near all six of the mined coal seams, although the greatest displacement has historically been recorded proximal to the L Seam. Movement near the L Seam has reportedly pinched off water monitoring wells, and pinched off inclination monitoring holes. Despite the reported presence of a tonstein (altered volcanic ash layer) being associated with the L Seam, the exposure visited in the field was not characterized by water-sensitive clay, and the irregular bedding nature of the layer did not suggest that it represented a major, laterally extensive bedding-pazallel weakness even if it can serve as a useful marker horizon. Mine personnel further indicated that slip is recorded at a vaziety of stratigraphic levels even near the same seam, and is not localized to a consistent level below, within, or above any seam. It is therefore likely that the bounding fireclay and roof mudshales, which may host water-sensitive clay, represent general layers of weakness. The generally low permeability of fireclay and mudshale associated with coal commonly defines an aquiclude, which can allow ponding and saturation on top of the shale, as well as an increase in hydrostatic pressure along the bottom of the shale where dipping strata define a confined aquifer with hydrostatic head. That this scenario exists at the Trapper Mine is supported by mine personnel observations that some exploration holes yielded a substantial amount of water under pressure. It is difficult to attribute the initiation of the rockslide solely to the additional 2° of dip beneath the failure area. Although the event occurred in a relatively short time frame (45-60 minutes), failure may have been initiated up to two yeazs ago, following the development of the G-5 strike pit that undercut the toe of the slope. This interpretation is supported by the review of ground movement data conducted in the May 2, 2005,