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