9410895 Siddiqi This research program is to investigate the hypothesis that seismic deformations can cause, or contribute to, drain clogging of earth dams, specifically, the Tampas Canyon Dam. This dam has been shaken by several significant earthquakes, including the January 17, 1994 Northridge earthquake and its after shocks. In recent years it has been observed that the water table (phreatic surface) has been rising significantly at this dam due to improper functioning of the inclined drains, resulting in seepage on the downstream slope. The Tampas Canyon Dam is an earth and rockfill dam, and is located in San Juan Capistrano, Orange County, California. A complete geotechnical investigation during the past three years has indicated that the factor of safety under pseudo-static conditions with the existing elevated phreatic surface is well below the acceptable level. Several borings have revealed significant clogging of the drains with upstream core material. Based on the results of this investigation, it is hypothesized that the presence of the core material has been caused by disruption of the existing bond between the drain and the core due to seismically induced deformations. Relative displacement of the core and drain, along with loosening of the core material, are thought to result in the intrusion of the fine-grained core material into the coarser and more porous drain material. No definitive proof of this mechanism has so far been found. The objective of this investigation is to determine if changes in the phreatic surface at this dam are caused by seismically- induced settlements. This is being accomplished by monitoring horizontal and vertical movements within the embankment due to significant aftershocks. Piezometer readings and pool elevations are monitored to determine if a rise in the phreatic surface occurs. ***
