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I I I I I I I I I I I I I I I I I I I-~ c Recommended groin protection details for both upstream and downstream slopes are also shown on Fig. 3. This provision will provide protection at the dam abutment contact where wave action and runoff otherwise result in difficult future maintenance. Foundation. Considering the generous base width of Zone 1 (greater than the head) the minimum core trench depth of 10 feet is believed to be more than adequate if foundation contact materials are impervious. If alluvial channels or other pervious foundation materials are encountered, the trench will have to be deepened to insure an adequate cutoff. Should this depth become excessive, considering dewatering and trench slope stability, an acceptable alternative would be an upstream impervious blanket as shown in dashed lines on Fig. 2. Provided the proposed Zone 1 extension along the abutments is tied into the natural ~mpervious materials now existing on the valley floor, very little additional construction would be required. The present core trench design is considered reasonable and appropriate for anticipated foundation conditions. The impervious blanket for the valley bottom should be retained as an alternative to fall back on, should extensive pervious zones or serious dewatering problems be encountered. The topography of the upper left abutment appears to result in adverse slopes for the dam foundation. The Zone 1 contact should be shaped during cutoff trench excavation to provide a relatively horizontal contact as viewed in cross-section. Earthwork Details. The following comments,relate to both specification and drawing provisions, In general, they do not affect abutment stability but-are offered as clarification or simplification which should improve the design and facilitate construction.