An Uplifting Experience: Mansfield Dam Stability Assessment

Mansfield Dam is located on the Colorado River about 13 miles northwest of downtown Austin, Texas. The dam impounds Lake Travis and consists of a 278 foot tall concrete gravity section flanked with a zoned earthfill wing dam up to 100 feet tall. The dam was designed by the Bureau of Reclamation (Reclamation) and constructed by the Lower Colorado River Authority (LCRA) and Reclamation in two stages between 1935 and 1942. Mansfield Dam, the tallest dam in the State, is classified as high hazard by the Texas Commission on Environmental Quality and serves as a major water supply and recreational reservoir for central Texas.
Reclamation was prominently involved in the operation and maintenance of Mansfield Dam until 1997 when the LCRA assumed full ownership. The original design of the dam incorporated two grout curtains and two lines of foundation drains. Sixty-two uplift monitoring stations were installed as part of original construction. The uplift monitoring stations consisted of up to 100 feet of 2-inch diameter steel pipe connecting the drainage gallery to holes drilled into the foundation bearing stratum. Subsequent reviews of the performance of the dam and comparisons to state-of-practice design criteria revealed concerns that the data from the uplift monitoring was not reliable. Several of the uplift monitoring points have failed due to corrosion. Stability analysis of the dam indicated that effective foundation drains are critical to the dam stability. As part of the current assessment, 18 vibrating wire piezometers were installed in twelve core borings drilled beneath the lowest drainage gallery and well into the foundation rock. Material testing including direct shear testing of the concrete, rock, and concrete to rock interface provided site specific values for revised stability analyses. The new piezometers indicate that uplift pressures in the foundation and along the dam baseline are within acceptable values suggesting an effective drain system.
This paper discusses the challenges in obtaining site specific information below the base of a large concrete dam, the methods and procedures used to collect the data, testing results including direct shear tests, and the results and conclusions of the stability assessment.