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<br />., <br /> <br />as seismic refraction and! or resistivity. It is estimated that these geophysical <br />investigations would cost about $15,000. <br /> <br />b. As mentioned above, even if it is found that, once the seepage enters the <br />bedrock it remains contained in the bedrock, it is necessary to assure that there are <br />no erodible zones in the bedrock. From observations in the outlet timne~ it appearB <br />that seepage water flows through jointed and fractured sandstone layers and is <br />perched on top of relatively thin claystone or shale layers sandwiched between the <br />sandstone layers. It would be expected that the claystone/shale layers are the most <br />subject to erosion. Removal of material from the bedrock is not near as critical as <br />the possibility of void development in the embanl.<rnent or foundation soils. <br />Removal of clayey formational zones would occur in very thin layers which would <br />be unlikely to collapse because of the strength of the overlying sandstones. If they <br />did collapse, it would not be expected that the resulting subsidence would have a <br />catastrophic impact on the embankment. There are three methods for investigating <br />possible void development in the foundation bedrock: <br /> <br />i. The highest hydraulic gradients and, therefore, the greatest seepage <br />velocities and erDsional forces would be expected to occur at or near the <br />seepage discharge in the tunnel. Therefore, if erosion of bedrock material is <br />occurring, it should be evident in the discharge areas and a careful <br />examination of these areas 8hould indicate any erosion history. Doing so . <br />would involve mapping the seepage areas with the reservoir level near full <br />followed by a subsequent examination of the seepage discharge areas at a <br />reservoir level below that which initiates the seepage. The associated costs <br />are estimated to be $3,500. <br /> <br />ii. It would be possible to drill test holes in the bedrock, obtain samples <br />of the clayey formational zones along the suspected seepage paths and <br />perform laboratory erosion potential tests. Piezometers could also be <br />installed in these test holes with little additional cost. The estimated cost <br />associated with this sampling and testing is $12,000. <br /> <br />w. Geophysical method described below for use in detecting voids in <br />embankments and soil foundations could also be used for identification of <br />voids in bedrock. <br /> <br />2. If the methods previously described do not satisfactorily locate the seepage entry <br />areas, if it is found that the seepage enters at critical areas, or if other conditions are <br />identified which indicate the likelihood of existing voids, geophysical methods such as <br />ground penetrating radar, seismic or resistivity might be used to detennine if such voids <br />exist and, if so, their approximate 10catiDn. Estimated cost for these surveys is $25,000. <br /> <br />The above information should be reviewed with the follOwing considerations in mind: <br />