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...,-- ~ • ~AGRA Earth & Environmental August 28, 1996 Mr. Mike Owens Owens Brothers Concrete 5550 Sheridan Boulevard Arvada, CO 80004 RE: Geotechnical Engineering Opinion for Post-Mining Stability of the Sinclair Pipeline Easement Owens Brothers Concrete Company Del Camino Sand and Gravel Mine Weld County, Colorado Dear Mr. Owens: AGRA Eanh & Environmental. Inc. 17301 W. Colfax Avenue Swfe 170 Golden. Colorado 80401 Tel (3031277-9988 Fax (303)277-9955 AEE Job No.: 6317-0015 At your request, we have evaluated the slope stability of the Sinclair Pipeline easement through the planned sand and gravel mine in Weld County. In the preparation of this study, we used the pre and post-mining contour maps contained in our previous study, Job. No. E95-7040, dated November 6, 1995, and borehole logs from the Gravel Investigation, by Western Testing Labs, Inc., Job No. A-12122, dated March 19, 1985. As we understand the situation, the pipeline runs essentially north-south through the area to be mined, and will, along with the Last Chance Ditch, define the four mining phase boundaries. The pipeline lies 4 to 8 feet deep, within a 50-foot wide easement that is further widened by another 30-foot setback to form the nearly level top of the un-mined berm. Mining would leave side slopes of 3H:1 V (18.4 degrees), between 24 feet (Phase 1) and 30 feet (Phase 2) tall. Dewatering would allow mining in the dry. After mining, the resulting excavations would be expected to fill with water, leaving about 8 feet of freeboard to the top of the embankment. The side slopes would be composed of dense sand and gravel materials with minor discontinuous lenses of clayey sand or sandy clay. Stability of the berm embankment was evaluated using limit equilibrium methods, considering infinite slopes, cohesionless material and drained conditions. Drained soil conditions were chosen because rapid drawdown will not be used to empty the ponds or dewater the excavation area. A factor of safety of at least 2.3 results for the planar failure mode, using a conservative friction angle of 38 degrees and no cohesion for the slope material. Generally, slope factors of safety in excess of 1.5 are considered acceptable for high hazard slopes. Even considering rapid drawdown conditions, it would be unlikely that the pond embankment slopes would be subject to mare than very minor surface sliding, which would be easily repaired. The minimum distance from the pipeline to the slope edge is 70 feet. Engineenng 3 Environmental Services