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•~ <br />1•teno tc ~'.~ aP __.-~~ <br />TBM Land Use Consideration <br />pane 2 <br /> <br />The contour bench strip mining cut and fill technique results in a <br />non-structural earthen fill. Spoil material is excavated, loaded in trucks <br />and hauled to previously mined portions of the strip. The spoil material is <br />then dumped and dozed into the previous excavation. Material is iiahtly <br />compacted by the wheel and track loadino. Plormally the near surface materials <br />are more compacted due to repetitive passage of equipment. Little if anv <br />moisture or compaction control is performed in these areas durino backfillino_ <br />operations. In contrast, a fill constructed to act as a foundation for <br />structures (a structural fill) is commonly compaction controlled to assure <br />acceptable minimum soil density is achieved and to limit differential <br />consolidation. Asa result, any structures founded on a non-structural spoil <br />backfill of the surface-mined area could be subjected to significant <br />differential settlement which could result in serious structural damage. <br />Based upon a visual overlay of the boundaries of the previously surface-mined <br />areas and the subdivision plat, none of the nine affected lots appear to have <br />had more than 50; of their area surface-mined. In my opinion, the safest <br />method for precluding exposure to the potential hazard is to restrict these <br />affected portions of the lots to non-structural land use applications. <br />Further, I recommend that any development on these lots include a thorough <br />examination by a qualified engineer or enaineerina peoloaist experienced in <br />foundation and soil engineering. In addition, the preparation of any <br />development plan within this subdivision should include an evaluation of <br />potential subsidence hazard and appropriate mitigation enaineerina. I believe <br />that the Division should communicate these opinions to the Permittee, the Land <br />Developer and the County Commissioners. <br />Consideration of Alternative Land Use <br />If the Permittee were to reouest consideration of an alternative land use <br />designation, the revision application would have to comply with the <br />requirements of Rule 4.16.3. Among other things, the Permittee would need to <br />demonstrate that residential development land use is compatible with adjacent <br />land use and existing local, State and Federal land use policies and plans. <br />Since this subdivision has been approved by the Commissioners, this should not <br />be difficult. Geotechnically, the Permittee would have to demonstrate to the <br />Division that the proposed residential development land use would not <br />represent a hazard to the health and safety of the public (future residents) <br />or the environment. As discussed here above, I believe this demonstration, at <br />a minimum, should address the potential enaineerina constraints for <br />construction presented by the existence of the previous surface and <br />underground mining activity. This demonstration would probably reouire a <br />significant enaineerina and aeotechnical effort and investment. <br />cc: Mike Lona <br />Randy Price <br />