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• Analysis or design related to temporary or permanent surface water control was not <br />included in the scope. Evaluation of potential acid or toxic properties of the coal <br />refuse was also not within the scope of this investigation. <br />SUMMARY OF CONCLUSIONS <br />The existing RDA configuration is judged to be stable. This <br />investigation indicates that generally satisfactory compaction has been <br />achieved for coal refuse placed since about 1992. This corresponds <br />to improvements in processing performed in 1991 and 1992, and also <br />may be due to an increased area of the working surface. Groundwater <br />was found within the upper portions of the RDA. Significant water was <br />not found near the base of the RDA in monitoring wells installed on the <br />lower three benches. This data, along wRh information obtained from <br />discussions with the contractor who performed the initial drain and <br />RDA construction, indicate the drain is operating as intended. <br />2. Based upon the results of stability analysis we recommend a proposed <br />RDA configuration with 2.2:1 (H:V) slopes between benches. This <br />• configuration has an estimated total volume of 12.7 million cubic yards. <br />The proposed configuration is judged to be stable for the various <br />strength, water and earthquake conditions modelled. In our opinion, <br />water conditions are the most important factor which could affect RDA <br />stability. The report provides recommendations for compaction, <br />surface grading, drain installation and continued monitoring which <br />should be implemented to control development of excessive water <br />within the RDA and confirm conditions through the construction <br />process. Stability will be dependent on the qualky of the construction. <br />3. We have provided recommendations for construction of "roads" or <br />stabilized areas on the working surface, staging of refuse placement, <br />or elimination of fines from the refuse. Each of these alternatives are <br />considered acceptable practices subject to the limitations discussed in <br />the report. <br />4. We have provided recommendations for drain construction, additional <br />montoring wells, and compaction testing frequency. We believe these <br />recommendations are important to the stability of the proposed RDA <br />and should be performed and well documented. The quarterly <br />inspections should summarize information addressing these items to <br />identify significant changes so they can be addressed. <br /> <br />2 <br />