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<br />" <br /> <br />There is a method of spray-applying a polymer sealant Its use for sealing reservoirs is <br />relatively new and the reports of its effectiveness are varied. The cost is significantly less <br />than for synthetic liners. Tnis method consists of applying a hydrophylic polymer that <br />absorbs and bind~ about 400 times its weight in water. A second protective polymer is also <br />applied. These materials are then mixed into the upper few inches of the soil There are <br />questions about its effectiveness on exposed rock surfaces, its long-tenn petfonnance <br />(whether it would have to be re-applied periodically), the effect of human and animal foot <br />traffic as well as its overall sealing ability. It is estimated that the cost of applying this <br />material to the swface area described above would be about $9,000 including design and <br />inspection costs. If it is decided to pursue this method, the first step would be to arrange <br />for the manufacturer's technical representative to examine the site and provide an opinion <br />concerning whether the method would be appropriate. <br /> <br />4. Dwmg our February 27 site visit with you and Norm Hunt, we discussed an option <br />of creating a temporary dam aroWld the suspect area with a synthetic membrane filing the <br />impoWldment area with grout and allowing the grout to find the seepage paths. However, <br />due to the difficult physical and time constraints discussed, We have not pursued this option <br />and have not included estimated costs for it in this letter. <br /> <br />The problem of addressing the question about whether there are existing voids is a more <br />difficult one than that of future progressive void formation. There are two possible <br />conditions that need to be considered: <br /> <br />1. The pOssibility of embankment, fOWldation or other soils being carried with the <br />seepage water. <br /> <br />2. The possibility of bedrock material being carried with the seepage water. <br /> <br />Following is a discussion of potential methods for addressing the possibility of existing <br />voids: <br /> <br />1. If the methods previously described were successful in identifYing the seepage entry <br />areas and it was found that they were limited to the spillway area shown on the 1909 plans, <br />it might be possibie to conciude that voids which may have deveioped in that area wouid <br />not be catastrophic in nature. The worst case that might be expected would be for voids in <br />that area to cause subsidence resulting in damage or washout of the spillway. If a spillway <br />channel was originally excavated as shown on the 1909 plans, the most damage to the dam <br />structure that a washout of the existing spillway would create would be erosion back down <br />to the previous spillway channel. It would then be necessary to investigate two related <br />pOSSibilities: <br /> <br />a. It is possible that the seepage water could enter the bedrock exclusively in <br />-the spillway area described above but contact erodible soils on its route from the <br />spillway area to the tunnel discharge points. This could be investigated by <br />installation of the piezometers previously described or by geophysical methods such <br />