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17 CONTROLLED SEEPAGE FIGURE 3.4-7 ZONED WITH DRAIN The drain is a method of controlling the seep- age of water through a dam. Water entering the drain flows freely through the drain and exits safely beyond the dam without wetting the material in the downstream zone. Most older dams in Colorado are homogene- ous. A method used to control seepage on some of the older dams was to stop the water at the source, by placement of a barrier of steel or concrete on the upstream face of the dam. NO CUTOFF UPSTREAM BARRIER FIGURE 3.4-10 FLOW PATH IN COARSE MATERIAL WITHOUT CUTOFF Extending the barrier into the foundation to control the flow of water under the dam is important. If the foundation has low resistance to the flow of water, for instance, through fractured rock or a sand layer, the most effective means of reducing the flow of water through the founda- tion is a cutoff. Often the core is extended into the foundation by digging a trench along the length of the dam and filling it with the flow- resistant material. The sketch below shows how the cutoff trench controls the flow ofwater through the foundation. Controlling flow is necessary to prevent excessive loss of reservoir water and assure the staj,;lity ofthefoundation. FIGURE 3.4-8 UPSTREAM BARRIER 3.4-2 FOUNDATION CUTOFF Extending the barrier into the foundation, as shown below at Point A, also helps to control the flow of water through the foundation. SAND 0\ GRAVEL a. Cutoff Too Shallow ~ ~.. :....'. .-:. ~.~ :0 SAND 0\ GRAVEL b. Effective Cutoff FIGURE 3.4-9 BARRIER EXTENDED INTO FOUNDATION Without the extension (cutoff), water will go under the barrier and exit at the base of the embankment, defeating the purpose of the barrier. FIGURE 3.4-11 CUTOFF TRENCH Many of the principles of earth dams also apply to the behavior of rockftll dams.