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<br />San Luis Project - Phase n Raise 1 Hxpsnaion Comtruction Statue Report <br />' the layer thus having no affect on the function of the drain. Smaller oversized material, which <br />aze present in small quantities in some portions of the borrow, are typically well rounded and <br />the statistical probability that an angular oversized particle is in direct contact with the liner is <br />' extremely low and not considered to be significant to the function of the facility. <br />Permeability tests are not required in the specifications. However, permeability tests conducted <br />on the Type 2 material utilized during this phase of construction have indicated that the material <br />' may exhibit a lower permeability than originally anticipated. While this reduces the flow <br />capability of the material and has the potential for resulting in increased head on the liner and <br />subsequently an increase in seepage from the facility, the tailings and clay liner layer also exhibit <br />' lower permeabilities than anticipated and the net result appears to be a decreased seepage <br />potential. <br />' The head on the liner is a function of the relative difference in permeability between the drainage <br />' blanket layer and the overlying tailings and the spacing between drainage pipes. <br />InFlow to the drainage layer will be dominated by the presence of the lower permeability <br />' portions of the tailings through which the water must pass. The lower permeability layers will <br />control the rate at which water can pass through to the drain layer. Due to the deposition <br />' methods practiced at the facility, thin layers of relatively homogeneous tailings are deposited <br />over a large areal extent of the facility and the ability for water migrating through the tailings <br />to bypass low permeability zones in the tailings is negligible due to the greater seepage path <br />distance and tortuosity required. Furthermore, the tailings immediately overlying the drainage <br />layer will be well consolidated due to the very short seepage path into the drain layer and weight <br />' of overlying tailings. Thus, the tailings immediately above the drain layer are expected to <br />exhibit permeabilities towazds the lower end of the range found to exist in the tailings. <br />1 As described in the initial design documents, the performance of the drainage blanket will <br />' change over time following the initial covering with tailings. The initial tailings deposited on <br />the liner has a permeability at or greater than the drainage material, as water drains from the <br />tailings and subsequent deposition increases the pressure on these tailings, consolidation and a <br />' decrease in permeability will occur. At the point where the drain and tailings have an equivalent <br />permeability the head on the liner should reach its maximum. Following this, the decrease in <br />' permeability of the tailings will begin to limit the supply of solution to the drain and the head <br />on the liner should decrease over time. In the initial design documents it was eslima[ed that this <br />' Ocwber, 1993 I9 SR1C Project No. 1425 <br />