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I I I I I I I I I I I I I I, I I I I I The Applied Technology Council presented peak acceleration coefficient maps and seismic hazard concepts which place the site in a zone with an assigned horizontal acceleration at the bedrock surface equivalent to 0.05 g. Bulletin 46 of the Colorado Geologic Survey by Kikham & Rogers indicates one felt earthquake of Intensity VII approximately 25 miles from the site and one earthquake of magnitude 2.5 mb within 25 miles of the site. The same bulletin indicates over 200 earthquakes in the Denver area with magnitude to 5.3 mb which is approximately 50 miles from the site. The bulk of these earthquakes, however, has been attributed to well injection at the Rocky Mountain Arsenal. In BULLETIN 43 of the Colorado Geologic Survey by Kirkham & Rogers, they estimate the maximum credible earthquake for the Plains Province at a magnitude 5.5 mb to 6 mb. Bulletin 43 Plate 3 indicates the closest potentially active fault to be approximately 25 to 30 miles from the project site in northern Larimer County along Lone Pine Creek and Elkhorn Creek. Based on the relationships developed by Seed & Idriss, a magnitude 6 earthquake at 25 miles from the fault would produce a maximum rock acceleration of approximately 0.08g. 2. Desilm Acceleration. Based on the information in Bulletins 43 and 46 of the Colorado Geologic Survey, we believe using a base rock acceleration of 0.1 g to be adequate for the design of the diversion structure. Although this structure is critical to The New Cache La Poudre Irrigating Company, its failure would not be expected to risk lives; and it is not necessary for the overall safety of the community. A.6. GEOTECHNICAL EVALUATION 1. Existinl! Structure. Under Seeoal!e. The condition of the contact between the structure base and the bedrock are not known for sure. We are assuming the structure bears on the shale bedrock. This is based on the information we have concerning the elevation of the rock as found in the test borings and the measured elevation of the top of the concrete base being within five feet (5') of each other. The exact thickness of the base is not known but it is at least four feet (4') in thickness based on our measurements of the depth to the bottom of the river channel. If the structure is repaired, we would recommend the structure be cored through the base into the foundation to determine the condition of the contact and to verify the foundation slab thickness and the condition of the foundation material. We do know there is significant seepage under the structure even at low flows. This seepage can be seen as it exits upward through holes in the downstream concrete apron. If the structure is repaired, this seepage would need to be cut off by an upstream concrete wall embedded into the shale. PAGE 6 . Cbapa::rV