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Slope Evaluation and Remediation <br />West Elk Mine <br />Somerset, Colorado <br />The contractor should take care when making excavations and not compromise the <br />bearing or lateral support for the foundations of the adjacent, existing roadways or other <br />improvements. <br />Good surface drainage should be provided around the temporary excavation to direct <br />surface runoff away from the slope faces. A properly designed drainage Swale should <br />be provided at the top of the remedial excavation. In no case should water be allowed to <br />pond at the site. Slopes should also be protected against erosion. Erosion along the <br />slopes will result in sloughing and could lead to a slope failure. <br />Buttress Back Drains Each bench should be provided with a bench drain at the toe of <br />the ascending slope generally as shown on Figure 4. The anticipated positions of the <br />recommended bench drains in plan view are shown on Figure 5. <br />Each back drain should consist of perforated PVC collection pipe at least 4 inches in <br />diameter, non - perforated PVC discharge pipe at least 4 inches in diameter, and '/2 -inch <br />nominal crushed rock constructed in a minimum 12 inch wide trench. <br />A typical, cross - section detail of a back drain is provided on Figure 6. <br />The high point(s) for the collection pipe flow lines should be installed at least 10 feet <br />below the overlying bench surface. Each drain should be sloped at a gradient of 2 <br />percent or more toward and beyond the lateral margin of the buttress fill. Farther than <br />10 feet beyond the lateral margins of the buttress fill, the drain lines may be extended to <br />`day light' as non - perforated, discharge pipe. <br />The back drain discharge pipes should be extended to appropriately located outlets for <br />gravity discharge and provided with erosion protection. <br />The actual layout of the back drains should be designed by the civil engineer. <br />Each back drain system should be tested by the contractor after installation and <br />backfilling to verify that the system functions properly. <br />Excavation Considerations Test holes for subsurface exploration were advanced to <br />the depths indicated on the test hole logs by means of conventional truck - mounted, <br />drilling equipment. We anticipate some excavation difficulties for the proposed <br />construction in these materials with conventional, heavy -duty excavating equipment in <br />Job No. 11 -6014 GROUND Engineering Consultants, Inc. Page 9 <br />