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• STEFFEN~ ERTSON AND~KIRSTEN Alana Scott 'i September 16, 1994 Page 3 ' We do not believe that a localized slope failure has occurred. Permeability of the fill has been measured at 1 x 1l7' to 5 x 10° cm/sec and is too low for saturation of the fill at depth to Gave occurred in the shoot time frame involved. Furthermore, cracking of the upstream face slope of the embankaent and consequent movement of the fill was noticeably absent. Both of these physical manifestatons are normally associated with a slough type of failure. Local low spots in the safety berm allowed overtopping and limited erosion of the upstream embanlmlent face. 1Le most noticeable erosion feature on the embanlmhent face was the galley described earlier. Run-on concentrations were also responsible for the liner cover erosion experienced azound the impoundment perimeter. 'Ihe two erosion areas on the north perimeter appeaz to coincide with !ow spots in the access mad where rainwater accumulated and overtopped the roadside berm. ' We understand that the westwazd draining south diversion ditch seldom flows. 'Ihe nadhre of the ditch design is such that poadiag of up to 12 ft is required before dischazge through the culvert will occur. It appears that flow will prefer to exit the west cad of the ditch at the culvert prior to overtopping into ' the impoundment. Sediment accumulation in the solution collection ditch below the toe of the embankment occurs during rainfall events that produce runoff. 'ILe source of tris sediment appears to be largely thb native soil and liner Dover material adjacent to the downstream embankment slope/abutment contact. 4.0 RECObIl1iIIVDATIONS '[he immediate concern is to insure the stability of the Phase 2 raise and to minimi~a potential impacts to the permitting, design and construction of the Phase 3 raise. 'IThe measures already taken to eliminate pondiag on the crest should be effective towards helping to achieve this result. In addition, BMRI should ' survey the crest and regrade, if necessary, to produce a relatively level, free-draining sur~'ace that slopes toward the impoundment. If possible, the pipeline should be used as the inside safetylbarrier, and if possible, an earthen berm should be installed only on the outside (downstream) edge. 'ILe cracked fill should probably be repaired by excavaton and re-placement in compacted lifts. 'Ihe ' excavation should be executed in a manner that completely removes material within the craack, within the confines of the crestines, and allows placement and compaction of the material back idto place. 'IThe rationale for this excavation and re-placement of material is to eliminate the crack feattre in order w I prevent a preferential seepage path which may re-saturate the fill Locally and produce a similar sloughing. Also, as winter is approaching, ice accumulation within the crack due W alternate freete-thaw cycles could induce movement of the wedge. Figure 2 illustrates the proposed guidelines for excavation of material from this area and re-placement procedures.