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,~. ,... t ~ r) ,;;.",",W . Geology of the Damsite Geologically, there are three important types of subsurface materials found at the Narrows damsite--the Pierre shale bedrock, alluvial valley-fill sand, and the terrace silt. The present South Platte River channel has eroded a channel into the north wall of the older and much deeper channel, the thalweg of which lies approximately three miles to the south. The alluvial sand with which this ancestral channel is filled is exposed below the high water level in the reservoir area. Since the sand has a significant coefficient of permeability, it is expected that an appreciable seepage loss would occur through the sand into the ancestral channel. Overlying the alluvial sand, on the right side of the present channel, is a layer of terrace silt of varying thickness. This terrace silt has been proposed for embankment material and for possible use in blanketing upstream from the damsite. Foundation Conditions . The foundations at the damsite are adequate to support the proposed earth-dam. The foundation conditions will require special attention in that the Pierre shale formation slopes down from the river channel toward the right end of the dam to such depth that it is not feasible to seal the impervious embankment to the bedrock. Probable water losses through the fairly clean, per- vious sand and gravel, of streaillWash type, overlaying the shale along the right abutment cannot be fully controlled; but, relief wells and toe drains provided as shown in exhibit 5 will allow structural safety and prevent downstream seepage damage satisfactorily. At the left abutment and across the river channel, the shale is at shallow depths and the impervious embankment as well as the conduits through the dam will be sealed to the shale. The com- paratively soft condition of the ancient surface exposures of the Pierre shale indicates weathering effects to considerable depth. This was confirmed by the cores recovered during the drilling operations. This condition, in addition to swelling tendencies revealed in tests on the shale when subjected to saturation re- quires the use of a sprayed protective coating and mortar on excavated shale surfaces for the conduits and spillway structures. The high permeability of the ancient sand-filled South Platte River channel under the right abutment of the dam would permit percolation and could Cause seepage on farm lands along the right bank of the river below the dam. This subject is discussed under GENERAL INFORMATION - Geology - Seepage Effects. . 20