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n <br />• <br />• <br />H5.2 Piezometers <br />Historic piezometer levels are used to establish the general groundwater flow pattern and hydrostatic heads <br />for use in stability analyses and for evaluating effectiveness of dewatering activities. Graphs, showing <br />historic water levels for piezometers, are provided in Appendix H3. Plan locations of the piezometers are <br />shown on Figure H 1. Table H 1 shows response of piezometers during the 1998 and 1999 spring seasons. <br />Tables in Figure H3 also provide a historical companson of piezometer water levels. It should be repeated <br />that horizontal dewatering trenches were installed after the 1998 Spring season, and dewatering of stone <br />columns was practiced during the 1999 spring season. <br />Table H 1 indicates that water levels in the spring of 1999 at the head of the slide rose approximately 1/3 <br />less than in [he spring of 1998. Tables provided in Figure H3 further show that long term reductions in <br />water levels at the head of the slide, from November 1997 to Jttne 1999, th the order of 5 to 8 feet were <br />achieved with the construction of the horizontal trenches. <br />During the spring of 1999 in the body of the slide, water levels rose % less than levels in f 998. Overall <br />water levels in the body have remained the same over 1998 and 1999, indicating that drying of the slide <br />mass will be a slow process and that dewatering measures now in place at the head of the slide are <br />necessary to prevent further saturation of the slide mass. <br />To confirm the effectiveness of the horizontal trenches and stone column dewatering, it was necessary to <br />confirm rainfall and runoff in the 1997, and 1998 seasons. Figure H 12 shows the rainfall amounts for the <br />last 17 years, indicating that 1997 was a wetter the normal yeaz and that 1998 was a dryer than normal <br />year. Figure H 13 shows that discharges in the North Fork of the Gunnison reached similar peaks in 1997 <br />and 1998, indicating [ha[ the rate of runoff was similar in the two years. Because of the dryer than normal <br />year in 1998, it is inconclusive that the reductions in water levels can be entirely attributed to the presence <br />of the horizontal trenches and the pumping of stone columns. Likely, the dryer than normal 1998 year also <br />provided some benefit to landslide dewatering. Rainfall data reviewed for 1999 indicate dryer than normal <br />conditions. <br />Barr Engineering Company <br />PCDOCS1208220ICAK <br />July 71,1999 <br />17 Appendix H <br />'1999 Addendum to <br />1997/98 Landslide Corrective Measures Report <br />