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� <br />� <br />Denver, and also obtained files from the SEO in Durango. In addition, I reviewed published <br />USGS and Colorado Geological Survey data on the geology of the area. <br />3. 0 GEOLOGY OF THE SITE <br />The site lies at an elevation of 9400 feet in the narrow valley of the Rio Grande River. Bedrock <br />in the area is primarily Tertiary volcanic ash deposits called "tuffs". At the dam site, the right <br />abutment is comprised of a hard and strong, relatively massive, welded tuff. This tuff is visible <br />in the right abuhnent outlet tunnel, the cliffs forming the stream valley downstream of the right <br />abutment of the dam, and on the right reservoir banks upstream of the dam. This has been <br />mapped on the USGS geologic map of the Durango Quadrangle Southwestern Colorado <br />(Steven, et.al., 1974) as the Outlet Tunnel Member of the La Garita Tuff. This member is both <br />underlain and overla.in at the site by other tuff members. <br />There are three primary recent Quaternary soil deposits in the area of the reservoir. These <br />include (from oldest to youngest): glacial driii, landslide deposits, and recent alluvium or stream <br />deposits. The USGS has mapped four majar landslides in the area of the reservoir, and these are <br />shown on Figure 2. These landslides appear to be rock block glides (rock avalanche) or <br />combination block glide-slumps that probably formed at the end of the last glacial period, some <br />1 U,000 years ago. The instability appears to have occurred due to down-cutting of the valley <br />during the last glacial episode cornbined with a very wet paleo-climate and ice wedging. Tuffs <br />are simply ash falI and flow volcanic deposits; and it is possible given the correct chemistry, for <br />volcanic ash to alter to pure bentonite. I was not able to discern during the reconnaissance the <br />exact bed that this slippage has occurred on, but hypothesize there may be a weak horizontal bed <br />within tliese tuff formations (possible rich in bentonite). <br />The left abutment of the dam sits entirely upon one of these block glide sIide masses. Based on <br />my observarions during the site reconnaissance, the landslide mass appears to be made up of <br />chaotic large displaced blocks of rock surrounded by a silty gravel soil matrix. The landslide <br />mass on the left abutment has generally slid to a fairly stable configuration with an overall slope <br />of 10:1 (horizontal to vertical). Locally, however, downstream of the dam, due to the cutting of <br />the Rio Grande River, the toe of the slide slope is at about 1.5:1. Piezameters in the left <br />abutment of the dam indicate that the landslide mass is quite permeable as the water level in the <br />left abutment mimics the water Ievel changes in the reservoir; although at a reduced elevation <br />due to head losses_ Historic seepage on the left bank downstream of the dam has been reported <br />as high as 3 cfs. <br />In 1993 when the reservoir was generally full, an instance occurred where dirty water was <br />issuing from the le$ abutment with total flows of about 2 cfs through the landslide mass. This <br />was remedied by work directed by HARZA Engineering where short horizontal drains typically <br />40 feet Iong were drilled into the left abutrnent downstream of the dam and a rock buttress was <br />placed in the general area. It should be noted that due to the chaotic blocky nahzre of the <br />landslide mass, the drains could only be installed to depths of approximately 40 feet when the <br />original plan caIled for 100-foot long drain holes. This area of instability was very small as , <br />compared to the overall landslide mass. <br />� <br />i ,_ . _2_ <br />_,� = <br />.e-- <br />