|The right embankment, the spillway, and approximately 100 feet of the left embankment adjacent to the spillway are founded upon Cloverly sandstone. On the lert side of the dam, the bedrock dips downward to the west. The westerly portion of the left embankment is founded upon a deposit of sand and gravel.|
The right aburment consists of Thermopolis shale. Both the Cloverly and Thermopolis Formations dip gently in a downstream direstion with a strike approsimately parallel to the axis of Gray Reef Dam. The left abutment of the dam is sand and gravel underlain by Cloverly sandstone.
The location of the dam axis was shifted approximately 290 feet downstream as a result of subsurface conditions revealed by exploratory borings. The second axis was chosen because there was a smaller amount of overburden above bedrock and the quality of bedrock was superior. The bedrock at the original dam axis way a claystone of the Morrison Formation.
There was approximately 6 feet of gravel overlying the sandstone bedrock in the streambed along the second axis. To the west of the left abutment, the sand, gravel and silt comprising the terrace of an old flood plain rises to an elevation higher than the top of the dam. This gravel deposit thickens towards the left valley wall.
The foundation preparation for the spillway structure consisted of stripping the overburden and exposing the bedrock. For the embankment sections, the soft, loose overburden was stripped to expose competent sands or gravels and a cutoff trench was excavated down to the rock surface. The cutoff trench is located under the crest of the dam and has a bottom width of 20 feet at the contact with bedrock. Near the left abutment, where the bedrock dips downward, the cutoff trench was extended to bedrock until the depth to rock exceeded 5 feet. The cutoff trench then became a key trench, excavated to a maximum depth of 5 feet.
There has been a problem with seepage through the foundation gravels underlying the left abutment. A study into this seepage was completed in 1962. Conclusions noted that the seepage is related to the reservoir level and probably poses no threat to the safety of Gray Reef Dam. The seepage surfaces downstream of the left abutment and creates an unsightly area that is troublesome to visitors because it is difficult to walk or drive on the soft ground. The area affected by the seepage is a popular public fishing spot. Ten observation wells have been installed to monitor water level measurements and a shallow, subsurface drainage system has been installed to collect and discharge the seepage to the North Platte River.