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<br />Woodward.Clyde Consultants <br /> <br />v;;8:3 <br /> <br />affect structures founded on them. We believe that for final <br /> <br />design purposes these characteristics of Pierre shale should be <br /> <br />investigated further. <br /> <br />Also, the Pierre shale on the left abutment was observed <br /> <br />to contain veins and seams of gypsum (selenite). This mineral <br /> <br />is soluble, thus can be dissolved or leached from the formation <br /> <br />by percolating groundwater. We realize that the veins and <br /> <br />seams are probably scattered and the permeability of the <br /> <br />surrounding shale is low. However, we are of the opinion that <br /> <br />this should be investigated further, since there are dams in <br /> <br />ColoradO which have had failure or near-failure problems <br /> <br />related to solution of gypsum existing in similar form to the <br /> <br />occurrence at Narrows. <br /> <br />5. Landslides on Abutments and on Reservoir Perimeter <br /> <br />Our review of the site geology indicates that the <br /> <br />topography and lithology of the reservoir area is such that the <br />probability of landslides is low. We believe that the alluvial <br /> <br />materials may soften and slump locally but such movements <br /> <br />should not be large enough to pose a threat to the dam safety. <br />6. Seismologic Probability Considerations <br /> <br />We understand the Bureau has not yet prepared a report on <br /> <br />the seismOlogy of the site. We did not make a study of the <br /> <br />seismologic probability of the site area because this was <br /> <br />beyond the scope of our work. The Algermissen seismic <br /> <br />intensity maps show the site as being in Zone l,'which is <br /> <br />- 9 - <br /> <br />~ <br />~ <br />