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1 • <br />crushed rock comprises the first few feet of material directly above the liner. This free draining <br />rock fill is typically augmented with a network of perforated pipes. Assuming the leach solution <br />application rates do not exceed the capability of this drainage layer to convey fluids to the <br />pregnant solution storage area, only minimal hydrostatic head will be developed and, unless there <br />are large holes or rips in the liner, leakage will be minimized. In order to assure that the drainage <br />layer and perforated pipe network are functioning properly, and that solution application rates are <br />not in excess of drainage network capacity, the head on the liner should be monitored in the areas <br />of the pad above the toe dam crest elevation. Vibrating wire type piezometers are ideally suited <br />to this purpose (Figure 3.3). <br />In order to minimize the hydrostatic head applied to the lowermost liner in the area of the pad <br />below the elevation of the toe dam crest, a two geomembrane liner system with an intervening <br />leak collection layer is typically employed (Figure 3.3). The leak collection layer consists of a <br />clean sand or gravel layer, possibly augmented with perforated pipes, or a geonet. Leakage <br />through the primary geomembrane is conveyed through the leak collection layer to a sump where <br />the collected fluid is pumped back to the heap or to the processing plant. Monitoring of the leak <br />collection system involves measuring to hydrostatic head in the sump to assure that the frequency <br />and rate of pumping is adequate to prevent flooding of the leak collection layer, and tracking the <br />volume of water that is pumped out to assure that leakage through the primary liner is within an <br />acceptable range. The acceptable range for leakage into a leak collection system depends on the <br />head applied to the primary liner, the square footage of liner to which this head is applied, and on <br />the number and size of holes in the liner. Giroud and Bonaparte (1989) present equations for <br />estimating the amount of leakage that can be expected through a primary geomembrane liner. <br />They recommend the following equation for flow rates through a geomembrane hole where the <br />subsoil offers no resistance to flow through the hole, as would be the case with a leak collection <br />layer: <br />q = CB a(2gh) <br />where q is the flow rate in m /s; CB is a flow coefficient of approximately 0.6; a is the area in <br />square meters of a circular hole; g is the acceleration due to gravity (9.81 m/s and h is the head <br />in meters on the liner. Giroud and Bonaparte (1989) report that with good quality control one <br />small hole, less than 0.1 cm per acre is typical for a geomembrane installation. Using this <br />information, an acceptable leakage rate through the primary geomembrane may be calculated. If <br />the rate at which fluid is pumped from the leak collection system exceeds the acceptable leakage <br />rate, then the system may have to be shut down while the source of the excessive leakage is <br />located and repaired. <br />The leak detection system is differentiated from the leak collection system both by its <br />location in the liner system profile and by its essential purpose and function. While the leak <br />collection system functions to reduce head on the lowermost geomembrane liner in the liner <br />system profile, and can be expected to handle relatively large volumes of leakage through the <br />uppermost geomembrane, the leak detection system is located at the bottom of the liner profile <br />and should only collect the low volume fluid that leaks through the composite liner system. The <br />equations and methods for estimating the acceptable leakage rate into the leak detection system <br />are described in section 3.3.1.3. The volume of process fluid reporting to the leak detection <br />7 9 <br />