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<br />June 3, 1992 <br />Page 2 <br />STEFF~.JBERTSON AND KIRSTEN <br />collect drainage from the deposited tailings and promotes consolidation. Testing of the site subsoils <br />(Appendix D.1, 112 Permit, November 1988) indicated that naturally occurring material existed for <br />this purpose. The laboratory testing indicates that selected samples of potential drainage material <br />exhibited permeabilities in excess of 10~' crrt/sec, In situ testing of these materials (Appendix E, <br />112 Permit, November 1988) indicated some variability in the subsurface profiles, however, values <br />of in excess of 10~ cm/sec are measured. <br />The material specked for construction (Section 6.2.2, Technical Specifications, Appendix J, 112 <br />Permit Amendment, September 1989) considered a material which would both provide suitable <br />permeability for drainage and provide filter compatibility with the tailings. Testing of the material <br />installed during construction (Appendix A, Type 2 Material, Construction Report, SRK, July 1991) <br />indicates that, on the basis of 47 passing tests, an average fines content of 20 percent existed in the <br />material. Based upon accepted relationships for particle size for silty sand and gravel and <br />permeability, this value would correspond to a permeability of approximately 10'' am/sec. Ranges <br />of permeability indicated by the quality control testing may be one order of magnitude plus or <br />minus 10'' ccn/sec. <br />Tailings are deposited in the impoundment in slurry form. Initially, the tailings exist in a low <br />density state with a dry unit weight of less than 70 pounds per cubic foot (PCF). As deposition of <br />tailings continues, drainage of tailings solution through the underdrain pipe network coupled with <br />increased surface loading ultimately results in an increase in tailings density through consolidation. <br />In addition, thin lift placement techniques are employed to enhance desiccation and consolidation <br />of the tailings. Laboratory testing indicates that an average dry unit weight of 90 PCF can be <br />anticipated following consolidation of the tailings. The increase in tailings density is associated with <br />a decrease in hydraulic conductivity. <br />Physical testing of the tailings (Appendices D and E, 112 Permit Amendment) indicated a hydraulic <br />conductivity of 1 x 10' cm/sec for tailings immediately following deposition. Following drainage <br />and consolidation, the hydraulic conductivity of the tailings would decrease to approximately 5 x 10' <br />cm/sec, which is significantly less than the hydraulic conductivity of the drainage layer. Therefore, <br />the quantity of seepage reporting to the underdrain pipe network would be primarlly controlled by <br />the hydraulic conductivity of the tailings as opposed to the drainage layer. <br />The spacing of underdrain pipes was selected to limit the development of hydraulic head on the <br />synthetic liner by allowing the rate of drainage within the drainage layer to exceed the rate of <br />seepage through the tailings. The calculations for drain spacing (Appendix G, 112 Permit <br />