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dewatering or drying out activities were necessary in excavation areas. Even though the lower <br /> end of the slope in Zone 3B is topographically capped by the natural draw, which could prevent <br /> the complete drainage of the slope material,the slope material appears to be freely draining. _ <br /> Onsite investigation found water within test holes in 2018. Of the four test holes with water <br /> levels identified, only two were in Zone 313, TH-2 and TH-3. Both of these holes show water at <br /> least 10 feet below the surface in August and September of 2018. TH-3 is below the grade of the <br /> subsurface drainage pipe while TH-2 is within the slide repair area, but at its downhill edge. <br /> During construction of the buttress in Zone 3B no standing water was documented in any areas <br /> above the test holes. Moist soil was noticed on occasion but given the construction time frame's <br /> overlap with snow fall,this is expected. These pieces of information demonstrate that the slope <br /> material drains downhill towards the draw, naturally, following precipitation events. <br /> The installation of surface water retention ponds on the slope is believed to be the primary driver <br /> for the slide. The surface water retention ponds were installed in the early 2000s (Figure 4). Prior <br /> to this point aerial photography shows the tailings fill material in a stable configuration on the <br /> western slope of the quarry. The presence of the ponds interrupted the natural flow process: <br /> surface water that infiltrated the tailings would typically seep back out at the tailings toe, but <br /> now was blocked by the ponds. This blockage reduced the structural strength of the tailings and <br /> colluvium embankment, leading to slides. As such, restoring the slope to a naturally freely <br /> draining condition is as simple as removing water flow blockage points such as the ponds. The <br /> addition of the subgrade drainage pipe system will act as an additional level of protection against <br /> water build up in the slope material. The pipe outlet of the subgrade drainage system flows down <br /> the surface slope on an uninterrupted path. <br /> For slope stability analysis,the Z313 Low Pipe is assumed to be the lowest point at which the <br /> piezometric surface is not found within any material above the shale.No evidence exists to <br /> shows that the shales bears water. It is assumed that any surface water that infiltrates the slope on <br /> the west side of the Ralston Quarry drains down to the shale contact and west, downhill, into the <br /> draw. <br /> Ralston TR 03 10 Greg 1—wki and Associates.PLIA <br />