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of voids in the dam of about 50 percent. Silt potential faihtre modes were <br />considered for the concrete dam and are described below: <br />a. Sliding -Sliding potential for the concrete dam is driven by the upstream <br />hydrostatic reservoir forces and resisted by the shear friction beneath the dam. The <br />concrete dam foundation was described as five feet wide. It was estimated that the <br />upstream 3 feet of the dam foundation was hand cleaned to sofid rock while the <br />remaining downstream 2 feet consisted of remnants of a previous bentonite dam. <br />For a high plasticity clay such as bentonite, the extreme strength value is <br />represented by the condition identified as residual strength which develops as the <br />reau& of lazge, slowly Occurring strains. It was conservatively assumed that the <br />portion of the foundation on the bentonite would produce strength represented by <br />the residual valve which was estimated to include an internal friction angle of 12 <br />degrees with no cohesion. Some additional sliding resistance could be provided by <br />the gunite Layer extending from the downstream base of the dam further <br />downstream by about 6 feet to beneath the soil phrg material. 'There was <br />inauScient information to determine whether this gunite feature possesses <br />sufficient bucking capacity W provide significant sliding resistance. Therefore, <br />sliding safety factors were calculated both with and without crnrsideradon of this <br />downstream gunite. Since the garotte would posses liWe tensile strength and the <br />type and location of reinforcement is not certain, the upstream gunite layer was not <br />considered to provide sliding resistance. Uplift pressures along the sliding surface <br />were asstuned to vary linearly from the upstream toe of the concrete dam to the <br />downstream toe of the slope. This was based on the assumption that the upstream <br />and downstream garotte layers would crack and not provide significantly <br />imperrtteable seepage resistance. The attached calculation sheets show safety <br />factors against sliding based on the strength characteristics previously described. It <br />is seen that, without consideration of the effects of the downstream gumte layer, the <br />sliding safety factor at full reservoir level (water level at the top of the concrete <br />dam) is 0,61 which indicates definite sliding potential. At a water depth of about <br />3.4 feet, the indicated safety factor is approximately 1 (incipient sliding} and, in <br />order to achieve rite desired safety factor of 3.0, the maximum water level would be <br />2.3 feet. However, with the maximum potential effects of the downstream gunite <br />layer, the sliding safety factors are siguificantly increased. At full reservoir level, <br />the sliding safety factor ig practically 3 (actually 2.9). The downstream gunite layer <br />almost certainly provides some sliding resistance hut, as greviously discussed, the <br />maximum amount of sliding resistance that the gunite is capable of contributing ig <br />partially dependetrt on its buckling strength which cannot be determined with the <br />existing information. As wilt the soil plug sliding evaluation discussed above, it <br />should be noted that the safely factor catcutafions are somewhat conservative since <br />they don't include the effects of friction along the sides of the concrete dam. <br />b. Erosion Along the Base of the Dam -'lire most erodible material along the <br />base of the dam is the bentotrite }aver. T1te mechanisms for erosion to occur are <br />similar to those previously discussed for internal erosion of the soil plug and the <br />potential for such erosion can be estimated using bane's Weighted Creep Ratio. <br />