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Page 13 4.0 CASE STUDIES AND CRITERIA 4.1 Introduction The experiences gained from multiple-seam coal and oil shale mining in the Piceance Creek Basin provide the preliminary criteria for the assessment of the solution mining impact on the overlying strata. Section 4.2 presents the case studies for the multiple-seam longwall mining experience and discusses the qualitative indication from these experiences. Qualitative and quantitative criteria derived from these experiences and other sources aze summarized in Section 4.3. 4.2 Multiple-Seam Coal Mining 4.2.1 Impact of Minabilitv of the Upper Seam from the Lower Seam Minine The 1991 Longwall Census (Merritt 1991) shows that of the 78 mines with longwall faces in the United States, 32% of these mines have mining in adjacent strata. About half of these mines report some type of interaction problem with the adjacent coal beds (see Table 4-1). In all but one case, the interburden between the mines is 200 ft or less and the overburden above the upper mine ranges from 800 to 2000 ft. Three mines report severe interaction problems, but in the remaining cases, problems aze moderate and manageable. Table 4-1. Multiple-Seam Longwall Mines Number Reporting Type of Interaction Number Problems Interaction with longwall workings ......... .. 16 - Mines with overlying workings ........... .. 9 - Superpositioned gateroads .............. .. 2 - Offset gateroads ........................ .. 2 - Slightly offset .......................... .. 5 4 Mines with underlying workings .......... .. 7 - Superpositioned gateroads .............. .. 2 1 Offset gateroads ........................ .. 2 - Slightly offset .......................... .. 3 2 Interactions with room-and-pillaz workings . .. 9 - Mines with overlying workings ........... .. 8 4 Mines with underlying workings .......... .. 1 1 Agapito Associates, Inc.