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-~ i <br />R. J. Irish <br />Consulting Engineering <br />Geologist, Inc. <br />the strength of the concrete that would be used i n the planned dam. <br />GEOLOGIC CONSIDERATIONS FOR DAM DESIGN <br />The planned roller compacted concrete dam conceivabl y could be founded along its anti re length <br />directly on the reletivel y fresh, strong grenodiorite, or could be founded on the olluviol soils with <br />its abutments notched into the rock wells of the gorge. The former option would necessitate the <br />removal of es much as 30 to 40 feet of alluvial sands. Those alluvial sands should be able to <br />support a properly designed concrete gravity dam, but are relatively permeable, thus would need <br />to be treated, probably by grouting, to minimize the volume of underseepage flows. <br />Six engineering properties are significant to the foundation characteristics of the <br />grenodiorite. These include unconfined compressive sirength,angle ofinternelfriction,cohesion, <br />discontinuity surface roughness, Poisson's ratio, modulus of elasticity {i. e., deformation}, and <br />bulk weight. We have had samples of the grenodiorite tested in the laboratory for Uc strength, but <br />rwt for the other parameters. We suggest, however, that the laboratory test results for Uc <br />strength are less reliable, less indicative of actual rock strengths then our estimates. That is to <br />say, the Uc strength of the'finer grained variety of the grenodiorite typically should have a <br />strength of about 20,000 to 25,000 psi; the coarser grained variety should have a Uc strength <br />typically of about 15,000 psi; and the pegmatite should have a Uc strength generally on the order <br />of 6000 to 10,000 psi. <br />Yelues for the other four parameters have been reported for granitic rock comparable to the <br />grnnodiorite atthis site in both published end unpublished (proprietary) reports numerous <br />times. Consequently, for your initial design effort, we believe estimates of those parameters <br />would have considerable utility, although site-specific, rock-specific values should be established <br />by laboratory test results at an appropriate time. <br />The angle of internal friction for the grenodiorite could range from 30 degrees to 45 degrees. <br />We suggest a value of 42 degrees for your initial designs. <br />Cohesion along rock discontinuities islikely to be only slightly lessened by the iron-oxide <br />coatings prevalent on those discontinuities to the depths drilled. A value in the range of S to 10 <br />degrees should be practicable for your preliminary designs. The surface roughness factor (i} is <br />likel y to range from 5~ to 12SB. We suggest en average val ue o(8;8. <br />Poisson's ratio, the relationship between axial and diametral strain, for the grenodiorite <br />should approximate 0.20. The modulus of elasticity could range from 7 to 9 x 10 -6 psi, but we <br />suggest a more conservative value of 5 x 10 -6 psi for your initial design effort. The grenodiorite <br />is estimated to weigh about 1701bs. per cubic foot. <br />5 <br />