2008-07-10_REVISION - M1978143 (4)

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GEOTECHNICAL STABILITY EXHIBIT This information is presented to address the plan to mine within 200 feet of the structures listed in Exhibit S. This analysis is presented in leu of obtaining agreements from the numerous owners of the noted structures. Grand County Department of Road and Bridge, has committed to mining slope adjacent to structures at their final slope rate. Under the approved plan this would be 2:1 from the original land surface to the mine floor. This leaves a stable slope of undisturbed gravel adjacent to the structure. To do this Grand County will have any buried structures (i.e., communication lines, etc.) located by the Colorado Call One locate service and then apply a 25 foot setback from the markers. For above ground structures the setback will be staked prior to mining in those areas. For example, as mining begins near the north fence the 25 foot setback will be staked. A second line will be staked from this setback line a distance equal to the mine depth times 2. This establishes the toe of the slope line. This leaves a slope envelope where mining would be done to create the slopes without backfilling. The three cross-sections on Figure 1 show what is explained above. In Section A, a vertical face is being excavated as mining approaches the structure (fence) and the slope distances have been staked. In Section B the mining has shifted from working a vertical face and is transitioning into the slope mining method. In Section C the final slopes are completed. This method of creating slopes leaves a stable slope requir- ing no backfilling or other work once complete. Site specific information shows that the natural angle of repose or friction angle for this gravel deposit is approximately 35° or 1.32:1. This falls into the parameters listed in Rock Slopes: Design, Excava- tion and Stabilization, Publication No. FHWA-TS-89-045, Table 1 - Typical Soil and Rock Properties for Gravel, uniform grain size. The deposit of gravel contains no clay lenses or non conformities that would weaken the working face. It would be possible to mine within 45 feet of any of the listed structure with a vertical face and be assured that no damage would occur to the structure by a slope failure. Instead and for safety reasons Grand County has chosen to use the method described above. This will entail more work on their part but when done will have created stable slope that protects the struc- tures. Some of the structures lie at an elevation below the mine floor and would not be effected by the mining operations or be 26 (Revised 7/9/08)