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Memo to Greg, Squire 2 September 6, 1999 causes a reduction in cohesive strength. In a fully softened state, cohesion in the shale drops to zero without a significant drop in the angle of internal friction. Alignment of clay platelets in the shale, due to stress relief or in apre-existing shear zone, would reduce the friction angle and result in very low shear strength called residual strength. Potentially, fully softened or residual strength bedrock could affect the overall stability of a mined gravel bank and should be considered on a case-by-case basis when analyzing gravel pit wall stability. The applicant has provided logs for 5 borings taken at the site of the proposed pit. These borings indicate that the depth to bedrock may be in excess of l6 feet at certain locations in the pit, particularly the northeast comer. Since the stability analyses conducted are based on a maximum pit depth of l6 feet, the Division should stipulate increased mining setbacks if the depth of excavation exceeds 16 feet within 200 feet of a protected structure. See proposed stipulation no. 1 below. Bond for Pit Lining The applicant has elected not to provide the Division with specifications and a quality assurance plan for pit lining as an enforceable component of the permit. Without the regulatory control over liner installation that specifications and a quality assurance plan would provide, the best way [o assure that the post mining land use is achieved is through bonding for a complete liner installation job. Once the Operator can demonstrate that an adequate liner or portion of liner has been completed, the bond may be reduced accordingly. The bond for liner installation is based on the estimated costs to complete the following tasks. • Dewater the pit to provide access for liner installation. In the event of permit revocation and bond forfeiture at a pit with anon-existent or a leaky liner, the pit will likely be filled with ground water. Given that the mine plan for the Timnath Connell Pit requires the Operator to mine and line the perimeter prior to mining the interior of the pit, it may be safely assumed that the maximum dewatering cost to be incurred by the State would be fora 300 foot wide moat around the property. With a 9600-foot pit perimeter, and a 10-foot water depth, the moat would contain 215 million gallons (m.g.). Colorado Department of Transportation 1994 unit costs from bids received for dewatering are $201.50 per m.g. after deducting 12.41 percent indirect costs, which yields a dewatering cost for the Timnath Connell Pit of $43,322.50. • Rip shale from the pit floor for the construction of the liner. 131,200 compacted cubic yards (ccy) of material will be required to construct a 9600 foot long perimeter liner with a 369 square foot cross section, including the cross-sectional area of the keyway. The estimating form for ripping work attached to this memo yields a cost of $19,509 for this task. • Excavate ripped shale and place on pit perimeter in thin lifts. Include costs for moisture conditioning, blending and grading of lifts, and compaction. The estimating form for scraper work attached to this memo yields a cost of $ 131,155 for this task. • Quality assurance testing and observation at $500 per day for 20 days yields a cost of $10,000 for [he project.