Upper Stillwater Dam is to be a roller-compacted concrete gravity structure, founded on nearly horizontally bedded sandstone and argillite rock. An overview of the testing and analyses used to evaluate the adequacy of the foundation relative to deformation, seepage, and stability is presented.
Upper Stillwater Dam will be the Bureau of Reclamation's first roller-compacted concrete gravity dam. It is located on the south flank of the Uinta Mountains in northern Utah. The maximum height of the dam is 82 m, and the crest length is 812 m at elevation 2492 m. An ungated overflow spillway will be constructed near the center of the dam. Water is diverted to Stillwater Tunnel or regulated to Rock Creek through a single intake structure. The general features of the dam are shown in figure 1. The dam will be founded on interbedded sandstone and argillite of the Precambrian Uinta Mountain group. The bedding structure is nearly horizontal at the site. The bedrock has been subdivided into five rock units for mapping purposes as follows: (1) an upper sandstone unit near the top of both abutments, (2) a middle sandstone unit with numerous interbeds of argillite and siltstone, (3) a thick argillite designated unit M extending to near the base of both abutments, (4) a lower sandstone unit which forms most of the foundation, and (5) a small but continuous argillite interbed designated unit L within the lower sandstone unit. A construction contract was per- formed to strip surficial materials, exposing seven minor faults designated F-1 through F-4 and F-7 through F-9. They are nearly vertical, cross the foundation from upstream to downstream, and generally consist of a gouge zone several millimeters wide within a fractured zone of rock about 1 m wide. Jointing, other than bedding joints, is limited to near vertical sets predominantly oriented parallel to the faults. joint spacings average between 0.5 and 3 m depending on the set and 1ocation. There is no distinct weathering profile, but most joints are filled with sand and organic materials to depths averaging 6 m. An erosional channel forms a bedrock low about [4 m deep near the right side of the foundation. This channel, termed the trough, should relieve any large horizontal stresses. Additional details of the geology are shown in figure 1.
Initial attempts to estimate in situ foundation deformation modulus were based on correlations with RMR (Rock Mass Rating, Bieniawski, 1978) and geophysical shear wave frequency. However, neither method was found to be totally acceptable for the rock at the Upper Still water damsite. In situ jacking tests were performed at the site utilizing the Goodman borehole jack. The data were reduced according to the method described by Goodman, Van, and Heuze (1968). Adjustment factors were also considered. However, the measured modulus values were low enough that corrections were considered unnecessary. The results from argillite were used directly because they were consistent and indicated a modulus parallel to the bedding consistently 1.5 times that perpendicular to the bedding. The sandstone results required further analysis.