C153514 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I I I pinhole dispersion tests conducted on selected samples of typical embankment fill materials indicate that these soils do not display dispersive characteristics. This test result implies that sinkhole development through the embankment materials is probably not initiated or assisted by any characteristics or properties of the fill materials, but rather is probably caused by the slow infiltration of embankment fines into fractured foundation materials, existing voids or the outlet conduit under relatively high hydraulic gradients. The only evidence of possible voids or sinkholes in the embankment encountered duri ng the subsurface expl orati on and geophysical exploration programs was noted in borehole BH-B. A void or a zone of very soft material was noted from 57.5 to 59.0 ft below the top of dam, immediately above the probable bedrock/embankment contact. When tested for permeabil ity using a gravity standpipe permeability test, this zone accepted inflow readily. The causitive mechanism for this void or softened zone is not known with certainty. However, its proximity to the leaky outlet conduit and its location along the right valley wall in the area of historical sinkhole development indicates that uncontrolled seepage through the embankment/foundation and possibly into the outlet conduit. may be responsible. Water levels measured in the standpipe piezometer installed in BH-8 indicate that at this time this zone is apparently not hydraulically connected to the reservoir. This may indicate that: 1) the void or softened zone is a localized product of poor foundation preparation/construction practices rather than piping in response to seepage; or 2) that the sinkhole and tunnel grouting operations have intercepted the majority of the previously available seepage paths to this area; or 3) that the previously available seepage paths have been closed off by progressive collapse, filling or some other mechanism providing no visible manifestation on the embankment or natural ground surface. The third scenario is believed to be relatively unlikely and the first and second scenarios are believed to be equally likely. The remainder of the embankment appears to be in good condition with no visible evidence of sinkhole development. -18-