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_ Revised March, 1992 <br />• Colowyo also committed to meeting the requirements of a <br />subdrainage system to be extended from the head to the toe of the <br />fill. CTL/Thompson had initially provided a design for a <br />controlled underdrain under a portion of the fill and a gravity <br />sorted underdrain under the remainder of the fill. The proposed <br />underdrain wi21 be extended to the head of the fill to meet the <br />requirements of Rule 4.09.2(2). It is important to note that in <br />the opinion of CTL/Thompson, the extension of the subdrain will <br />serve to meet a regulatory requirement, but will not provide for a <br />substantial improvement in mass stability. <br />Taking into consideration CTL/Thompson's site investigation, <br />proposed designs and construction plans with modifications <br />resulting from joint OSMRE/MLRD/Colowyo discussions and review, <br />Colowyo will commit to the following design and construction plan. <br />CONSTRUCTION PLAN <br />All available topsoil will be removed and either stockpiled <br />for later use or direct haul replaced to a reclaimed area. <br />Colowyo will follow the CTL/Thompson recommendation for <br />excavation of clay materials in the valley bottom to bedrock or a <br />depth of at least 20 feet (which ever is less) about 700 feet <br />• upstream of the toe of the fill. A buttress fill comprised of <br />these excavated materials in accordance with the CTL/Thompson <br />recommendations will also be constructed. <br />A controlled underdrain in accordance with the CTL/Thompson <br />recommendations will be placed in the natural drainage bottom from <br />the head to the top of the fill. The harder, available sandstones <br />obtained from the mining operation will be selectively handled and <br />placed in at least a 24 foot wide by 8 foot high configuration to <br />serve as the underdrain before covered by spoil material. The <br />natural spoil sorting which will occur by utilizing the thicker <br />lifts recommended by CTL/Thompson will be sufficient to protect the <br />drain from clogging. <br />CTL/Thompson also believes that it is desirable to create a <br />permeable fill to reduce the risk of developing free water levels <br />within the fill. Colowyo will follow CTL/Thompson's <br />recommendations to fill construction. Lift thicknesses up to 100 <br />feet thick is acceptable and will be utilized to construct the <br />fill. This method of spoil placement also enhances the <br />construction of afree-draining layer of spoil material at the base <br />of the fill. Experience at Colowyo provides evidence that the <br />natural sorting process which occurs while dumping in higher lifts <br />is sufficient to create this drain. Inspection and documentation <br />of this natural sorting is recommended and will be conducted by <br />• Colowyo. See the Inspection Plan section for additional details. <br />I a-2 <br />