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• 4 • <br />voids and related sinkholes are sometimes present in areas underlain by the Eagle Valley <br />Evaporate in western Colorado because of solubility of the gypsum and anhydrite. <br />Ten sinkholes were observed near the top of the northern terrace escarpment in <br />the Phase 2 through 6 areas. The sinkholes are located on the terrace surface about 20 to <br />100 feet to the south of the top of the escarpment. They aze generally circulaz in shape <br />with diameters between 20 and 80 feet and depths between about 2 and 8 feet below the <br />surrounding unaffected terrain. They appear to have resulted from piping of the outwash <br />gravel into solution voids in the underlying Eagle Valley Evaporate. It appeazs that some <br />sinkholes will be encountered in the final pit wall and others will be excavated during <br />gravel mining operations. <br />GEOLOGIC SITE ASSESSMENT <br />The geologic conditions will have some influence on the operation and <br />reclamation of the proposed gravel pit, but they should not make the project infeasible. <br />Possible geologic conditions that should be considered are: 1) the possibility of <br />encountering subsurface voids and sinkhole rubble during mining, and 2) river terrace <br />escarpment erosion and instability in the northwestern part of the Phase 2 area. The <br />stability of the proposed final pit wall is discussed later in the Pit Wall Stability,9nalysis <br />section of this report. <br />SINKHOLES AND VOIDS <br />Sinkholes are present in the northern part of the Phase 2 through 6 areas and there <br />is a potential that subsurface voids that have not developed into sinkholes at the surface <br />are present elsewhere in the proposed gravel pit area. The sinkholes appear to result from <br />an hourglass style of piping of the outwash gravel into subsurface voids in the Eagle <br />Valley Evaporate. The voids in the Eagle Valley aze probably solution widened joints <br />with widths of no more than several feet. The rock voids are probably small in <br />comparison to the hourglass-shaped (inverted cone-shaped) rubble zone in the overlying <br />outwash gavel. The likelihood is low that large subsurface voids are present, <br />H-P GEOTECH <br />