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SEP-ld-1°97 1243 HfaRDING LAWSON 861 363 44 r6 P.64 Page Three Mr. Norm Every Mountain Coal Company Harding Lawson.45SOClatse drained, as would appeaz to be the present case, than during times of high groundwater pressure tirom snow melt and infiltration, this layer would also act to collect and distribute groundwater and high pore pressures to a large portion of the slide failure plane. Lying above the stream bed deposits azc tow density silts and clays that comprise the majority of the existing slide mass. Recent inclinometer irtstallations have confirmed the location of the principal failure plane at the interface between these silts and clays and the stream bed deposits (i.e., at the very base of the existing slide mays). These inclinometers have also disclosed the existence of shear zones at higher levels within the slide mass that aze believed to represent the movement of discreet independent blocks within the slide mass that can be correlated with surface scarps and the segmrntation of the larger slide mass. The silt and clay materials that comprise the main slide mass are believed to have been accumulated though successive mud flow/debris flow events deposited on the surface of the older stream bed deposits. In fact these events may well have been responsible for the displacement of the existing North Fork of the Gunnison River northwazd to its rumor position. Due to the nature of the depositional mechanics, the clay soil deposits are loose normally cotuolidatcd materials with very high void ratios and low sheaz strength. Yct due to the high clay content they should also exhibit low permeabilities (assumed to be in the range of 10-ti cm/s to lU-8 cm/s). The back analysis of the slide mass, assuming the Failure to exist at the basal interface, indicates a coefficient of friction of 9.6 degrees and cohesion of 320 psf. This is consistent with the weals low density nature of the clay mud flow/debris flow materials. It is our understanding that the existence of a landslide mass has been known from the I inception of the mine in the early 1980's and that local surficial slides neaz the portal area have plagued this area from the start. However, Facilities located on the larger landslide mass north of the portal facilities did not experience any significant distress that detrimentally affected operations until approximately 1992-93. Monitoring of movements in this lazger landslide mass did not begin until 1993. It is also our understanding that pillaz robbing of a portion of the F-seam panels known as the NW Submains was terminated in 1991 and sealed. R~centinspection of the seals around these abandoned workings have disclosed seepage in the rack around the seals indicating that presence of a significant quantity of water and pressure in the old workmgs at this location. Therefore, it is likely that these old workings may represent a source of groundwater and pore pressure contributing to the reactivating and acceleration ot'the slide mass since 1992. If in fact these old workings aze collecting end accumulating water during significant mow melt and infiltration evenu, then it would be expected to have two principal effects on the behavior of the slide mass: