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<br />.' <br /> <br />" <br />, <br /> <br />. <br /> <br />. <br /> <br />\, v (I <br /> <br />1.0 INTRODUCTION <br /> <br />Gurley Reservoir Dam, located just south of Norwood, Colorado, has <br />been restricted to zero storage by the Colorado Office of the State <br />Engineer due to the recent discovery of sinkholes in both the upstream <br />shell of the dam embankment near the outlet structure and in the <br />reservoir bottom just upstream of the embankment. Steffen Robertson and <br />Kirsten (SRK) plans to conduct an investigation of selected portions of <br />the embankment, foundation, impoundment, and outlet structure to <br />determine the probable causes and sustaining mechanisms of the <br />sinkholes. The results of this investigation will be used to develop a <br />teChnically acceptable, cost effective, remedial design which will <br />address the dam safety issues raised by the Office of the State Engineer <br />in the zero storage restriction letter of December 15, 1986. The design <br />will include an evaluation of alternate approaches to upgrading the <br />safety. and effectiveness of the outlet works in conjunction with the <br />sinkhole remediation program. <br /> <br />The causes of all of the existing sinkholes at Gurley Reservoir are <br />not known with certainty at this time. Based on discussions with <br />representatives of the Farmers Water Development Company (FWDC) and a <br />review of the Colorado State Engineer's files, we understand that at <br />least some of these sinkholes appeared approximately ten years ago, were <br />backfilled and patched, and have since reopened. The results of earlier <br />tracer dye tests indicate that some of the sinkholes located near the <br />outlet structure in the embankment have developed as a result of fill <br />t <br />materials infiltrating into the outlet conduit through existing <br />"weepho I es". As the outl et condui t appears to have been i nsta 11 ed by <br />tunnelling through the sandstone bedrock of the valley wall, these <br />sinkholes have apparently propagated through both embankment fi 11 and <br />foundation bedrock materials to reach the outlet structure. Normal <br />seepage forces in combination with surge-induced suction forces during <br />operation of the outlet conduit, would probably be sufficient to create <br />many of the observed sinkholes. However; some of the sinkholes have <br />appeared in the natural foundation materials just upstream of the <br />embankment and far enough east of the outlet structure to indicate that <br /> <br />-1- <br />