Using Post-Tension Anchors to Stabilize the Abutment Section of a Concrete Arch Dam

Tarryall Dam, a concrete arch dam, is located on Tarryall Creek approximately eighteen miles south east of Jefferson, Colorado. The dam, built in 1929, is owned and operated by the Colorado Division of Wildlife and is a popular fishing and recreation destination. The arch dam is founded on rock except at the right abutment where a gravity section was constructed due to poor rock foundation material. The dam is currently experiencing seepage at the abutment. As a result, Boyle Engineering was retained and proceeded to send divers to inspect the upstream face of the dam. The inspection revealed cracks that corresponded to the previously noted seepage downstream. Tarryall Reservoir was subsequently drained. A field investigation of the dry abutment revealed the aforementioned cracking and exhibited displacement at the lift joints. A structural engineering analysis was performed to remedy the situation. A finite element model of the arch and foundation rock was developed to determine the arch thrust loads for the abutment. The abutment carries both the axial arch thrust and the perpendicular hydrostatic forces. This resulted in a biaxial overturning stress situation that caused an impending stability problem. Due to the sloping foundation rock interface at the abutment, and the existing vertical construction joints, the abutment was evaluated in sections. The existing gravity abutment stability was considered unacceptable. Foundation rock samples were tested to evaluate bond strength and the friction angle of the rock. As a result, Boyle Engineering proposed a post-tensioned anchor solution to provide the required abutment stability. This paper will focus on the design process for the anchors and evaluating the stability. 9 pp., 2 tables, 6 figures.