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For the concrete dam, the weighted creep ratio at the maximum water level is about <br />0.37 compared to a minimum recommended creep ratio for soft clay of 3.0. This <br />indicates a potential for erosion of the bentonite material at water depths Beater <br />than about 0.5 foot. This is obviously conservative because it discounts the ability <br />of both the upstream and downstream gunite layers to impede either water flow of <br />migration of sail particles. Even if the bentonite material was to be completely <br />removed by erosion, it is unlikely that the result would be failure of the concrete <br />dam. It is possible that the dam possesses sufficient strength to cantilever over the <br />resetting void. It seems [lust the worst case would be that a diagonal crack would <br />develop in the downstream section of the dam and a portion of that section of the <br />dam would drop into the void, but it also seems unlikely that psbceas would cause <br />immediate faihue of the darn. Amore detar7ed evahuation would be required to <br />determine if such a scenario would have related effects that would decrease the <br />safety factor for other potential failure modes. <br />c. Overturrrirttg - It is very unlikely that overturning of a concrete dam would <br />actually occur by rotation about its downstream toe. Typically failure stresses wr71 <br />be reached in either the dam or foundation material before overturning conditions <br />can be achieved For example, maximum crushing or shear stresses in the dam or <br />foundation might be exceeded at the downstream toe of the dam. Alternatively, <br />maximum tensile or shear stresses might be exceeded near the upstream toe. <br />However, the stress analyses necessary to identify critical conditions was beyond <br />the scope of this evaluation. Therefore, overturning was evahrated based on the <br />simplifying assumptions descn~bed below. Overhurring faihrre would occur if a <br />combination of the upstream hydrostatic forces and the seepage forces at the base <br />of dre dam are sufficiently high to cause the dam to rotate about its comer located <br />at the downstream end of the base. The attached calculation sheets show safety <br />factors against overturning. It is seen that, the overturning safety factor at full <br />reservoir level is about 2.7. A safety factor of 3.0 is indicated with a water depth of <br />about 4.2 feet. These safely factors aze conservative because they don't include <br />resisting moments due to the ends of the dam or due to fire upstream or <br />downstream gutrite levels. <br />d. Infernal Shear Friction -The mechanisms for an internal shear friction <br />failure are similar to those for a eliding far~ure except that the sliding surfaces are <br />located within the dam. Therefore, ittternal shear friction sliding potential for the <br />concrete dam is driven by the upstream hydrostatic reservoir forces and resisted by <br />the shear friction strength of the concrete mass. I3ue to the manner in which this <br />dam was constructed, an internal shoat friction failure would moat likely occur <br />along bondmg surfaces where concrete sacks were stacked on top of each other. <br />Because of this, the shear resistance along these weaker planes would be expected <br />to be similar to lift joints of a Roller Compacted Concrete dam constructed of <br />conventional concrete but without joint bonding pmvisiona Typical values of shear <br />friction for enbonded conventional concrete joints' inchrde internal friction angles <br />ranging from 35 to 51 degrees and cohesion values varying from 2,600 to 31,000 <br />lbs/sq ft. Therefore the shear friction strength of actual concrete-to-concrete <br />