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25  Unprotected, erodible core material was placed directly on a poorly treated, weathered, jointed, and (in places) soluble foundation.  The initial response of foundation grouting to address seepage observed during first filling sealed deeper foundation seepage pathways and forced the seepage closer to the embankment/foundation contact. Seepage along this contact introduced an unfiltered exit through the rock toe drain. Study of the Quail Creek Dike failure shows the importance of applying the correct design features commensurate with foundation geology and the importance of recognizing how geologic conditions can affect seepage, internal erosion, and piping failure modes. TARRYALL DAM Tarryall Dam is located in Central Colorado downstream of the town of Jefferson. It was designed in 1929 as a thin arch dam to be founded on rock. Figure 17 – Downstream Face of Tarryall Dam from the Right Abutment The dam has a structural height of 70 feet and a hydraulic height of 38 feet, meaning that the dam itself extends 32 feet below the ground surface as shown on Figure 17. Figure 17 also shows that while the left abutment is a thin arch against jointed rock, the right abutment arch section is supported by a large gravity buttress. As previously stated, the dam was intended to be a thin arch structure supported by competent abutment rock. Granitic igneous rocks outcrop on both abutments and although moderately jointed, were thought to be suitable for support of this type of dam. However, during the excavation of the cutoff trench to bedrock, the field engineers encountered unsuitable, highly weathered and fractured rock in the right abutment. Realizing this rock would not support an arch, the design was modified to include a large concrete gravity section to provide both increased anchorage for the arch, and a gravity section to prevent overturning or sliding of the dam on this abutment. This configuration worked relatively well for nearly 70 years, until cracking was observed in the arch section monolith joint closest to the gravity buttress section. In 2001, the reservoir was ordered lowered to 5 feet below the spillway to reduce the load on the dam while additional studies were performed. In 2002 the additional studies showed that in fact the dam was unstable at high loading conditions and a zero-storage restriction order was imposed, forcing the reservoir to be fully drained until repairs could be made. Repair designs focused on providing additional anchorage of the right abutment arch and gravity sections to better support that end of the dam against movement and also to provide additional reaction force for the remaining arch section. A series of eight, multi-strand rock anchors were designed and installed through both the arch section and gravity section to depths up to 111 feet. The anchors secured the dam on the abutment, providing the necessary resistance to overtopping and sliding, and also provided additional reaction for the remaining unanchored arch. Figure 18 – Detail of Plan for Anchors through Right Abutment Arch and Gravity Sections