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Geotechnical Evaluation of Mine-Induced Seismicity on Monument Dam <br />Dorsey 8 Whitney, LLP <br />shallow slumps on the downstream slope, high water levels in piezometer P-1 (which is <br />located near the maximum section of the dam and on the dam crest) that respond rapidly to <br />changes in reservoir level, and cracks on the dam crest and downstream slope of the lefr part <br />of the earthfill dam that have been correlated with landslide movement. <br />The active landslide is the uppermost part of a series of landslides that have repeatedly failed <br />from the lefr valley slope and are expected to also underlie and form the foundation of the <br />earthfill dam (Western Engineers, Inc., 1992). No movement has been associated with the <br />underlying, older, landslides. <br />The active landslide that forms the left abutment of the dam, and through which the <br />emergency spillway channel is cut, is an integral component of the overall dam and reservoir <br />system. Documented site observations and instrument data during the period 1985 to 2004 <br />provide atime-line of conditions associated with landslide movements, including <br />inclinometer movement measurements, observations of cracks in the dam and landslide, <br />surface movement monuments, and piezometer water levels, as summarized in Table 2.2. <br />Note that only a limited amount of the available data was obtained by GEI for some of the <br />years summarized in Table 2.2. Note that the three inclinometers installed in 1992 were not <br />measureable by 1995 due to landslide movements of about 3 inches total occurring during <br />this period. Since 1995, estimation of landslide movement has been accomplished with <br />• lower precision using surveyed surface movement monuments and observations of any <br />cracks that propagate through to the ground surface. Documented site observations and <br />instrument data during the period 1985 to 2004 indicate a continuing pattern of landslide <br />movement with some locations showing 0.5 to greater than 1.0 foot of movement. <br /> <br />GEI.~ 6 <br />C60350GY-OJ-I6 Monumml Darn Gmtmhni W Eval <br />March 2006 <br />