Investigation of the Effects of Earthquake-Induced Deformations on Seepage at Trampas Canyon Dam

9410895 Siddiqi本研究计划旨在调查地震变形可能导致或促成土坝（特别是Tampas峡谷坝）排水堵塞的假设。 这座大坝已经被几次重大地震所动摇，包括1994年1月17日北岭地震及其余震。 近年来，由于倾斜排水沟功能不当，该大坝的地下水位（潜水面）显著上升，导致下游边坡出现渗漏。 Tampas峡谷大坝是一座土石坝，位于加州橙子县的圣胡安卡皮斯特拉诺。 过去三年进行的一项全面土力调查显示，在现有高潜水面的拟静态情况下，安全系数远低于可接受的水平。 几次钻孔显示，上游堆芯材料严重堵塞了排水沟。 根据本次调查的结果，假设芯材的存在是由于地震引起的变形导致排水管和芯材之间的现有结合破坏所致。 岩芯和排水沟的相对位移，沿着岩芯材料的松动，被认为会导致细粒岩芯材料侵入较粗和多孔性更强的排水沟材料。 到目前为止，还没有发现这种机制的确切证据。 本次调查的目的是确定该大坝潜水面的变化是否由地震引起的沉降引起。 这是通过监测由于严重余震引起的堤坝内的水平和垂直运动来实现的。 监测压力计读数和水池高程，以确定潜水面是否上升。 ***

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. ***