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I I I I I I I I I I I I I I I I I I I Geotechnical Engineering Investigation IV. Subsurface Conditions A. Soil and Rock The subsurface soils encountered in our exploratory borings along the downstream face of the embankment, B-2, B-3, B-3A and B-4, consisted of gravelly medium to coarse sand with little or no fines. This gravelly sandy embankment shell was medium dense to dense. The consistency of the soil increased with depth. The standard penetration test, N, values were consistent with mechanically compacted fill material. Stratification in the embankment shell, an idea that was inferred by Hydro-Triad's 1987 Feasibility Report does not appear to be valid. No distinct silt or clay layers were identified during the exploration program. In addition, grain size analysis on all retrieved soil samples indicate that the silt and / or clay content is between 2 to 10 %. About 40 to 70 % of the shell material is fine to coarse sand with the remainder being gravel. Cobbles and boulders were also encountered at varying depths during drilling. Boring B-1, which was drilled at a 45 degree inclination from the dam crest into the abutment, encountered silt and sand layers to an inclined depth of32 feet, corresponding to a vertical depth of23 feet below the embankment's crest. Below this depth higWy fractured rhyolite breccia rock, RQD of zero, was encountered to 35 feet (25 feet vertical corresponding to EI. 10,265 feet). The quality of the rock increased with depth beyond the 35 feet, RQD between 83 and 100, and the fractures became tighter and at larger spacing. This infonnation confinns that the depth of weathered and fractured rock in the abutment extends to approximately 35 feet below the crest and is higWy penneable. Detailed descriptions of the soil encountered in the exploratory borings are presented on the boring logs in Appendix A. The passage oftime could result in changes in the subsurface conditions due to environmental changes, i.e.: weathering of the soil and rock and stress relief joints opening near the surface of steep slopes. The attached logs and related infonnation depict location-specific subsurface conditions, encountered during our field investigation. The approximate locations of the exploratory borings were detennined by taping from the existing piezometers (Figure 3) and should be considered accurate only to the degree implied by the method used. Continental Dam Page 9