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March 1998 Slrtmp Investigation and Mitigation Report ~ Page 11 6.2.2 Subsurface Water The piezometers described in Section 4.3, were installed to approximately 13'k co 18'k feet bgs, well below the sliding surface of the slump, which is estimated to be shallower than 10 to 12 feet bgs. The upper limit of the screen and sand pack sections were generally 5 to 8 feet bgs, which is believed to be at or above the sliding surface. This configuration allows the piezometers to register any water that has percolated along the sliding surface of the slump. As listed in Table 1, water was observed in 5 of the piezometers on October 30, 1997, which indicated the presence of water within and around the slump mass. Between the time of drilling in September 1997 and [he field measurements in October 1997, the site received one to two feet of snow. Subsequent to the snow fall, air temperatures increased, which melted much of the snow. Therefore, the water observed in the piezometers probably reflected infiltration of snow melt into the slump mass and not water table conditions. These data suggested that water lubricated the slump plane as a result of infiltration downward into the general soils and [hen migrated along the surface of the weathered bedrock. The water measured in the piezometers was probably a result of the boreholes intercepting small flows as the water traveled along the surface of the weathered bedrock, near the upper end of the screened intervals of the piezometers. The water apparently entered the boreholes at such a slow rate that it either accumulated very slowly in [he bottom of the piezometer or it seeped out almost as fast as it entered. As such, there may have been an insufficient volume of percolating water to fill the piezometers to the point of water entry into the piezometers. 6.2.3 Existing French Drain The existing French drain was constructed to a depth of approximately 20 feet, with the lower four feet backfilled with drain rock. The upper 16 feet of the trench was backfilled with native materials, consisting of shale, sandstone, and siltstone, which are characterized as extremely weathered and friable. The native backfill material could have zones with locally low permeabilities. Groundwater data from the upper and east end of the slump, as measured in piezometers PZ-1 and PZ-2, indicated that the existing French drain could have intercepted some of the subsurface seepage along that portion of the French drain. However, as indicated by the water levels in PZ-5, the French drain may not have been effective along it's entire length, either due to preferential water pathway directions or pathway elevations. The water pathways may have had a slightly eastern orientation, similar to the slump path, and may have bypassed the western end of the existing French drain. The presence of water within the slump mass, as measured in piezometers PZ-3 and PZ-4, indicated that water was still accumulating at those loco[ions either from subsurface seepage or from surface infiltration. In either case, this water likely contributed to the driving force displacing the soil mass and reduced sliding resistance. 6.2.4 Slump Mitigation Issues Based on review of available site information, the slump appears to be less than ten feet in depth and movement is along an intermittently saturated zone within the shallow surficial soils (reclaimed spoils} or shallow weathered siltstone. The driving forces are the weight of the soils and water within the system, and the resisting forces are primarily friction and cohesion along Montgomery Watson Americas "P.O. Box 7740/8 "Steamboat Springs, Colorado "80477 f97JJ 879-6160 ¢i \bA6lvmp MmRUmn\Slvmp Fryri wvvn e.