Laserfiche WebLink
-4- <br />Backfilling and Grading - Rule 2.05.4(2)(c) <br />The stability analysis performed by Rocky Mountain Geotechnical, <br />Volume 2, For the western portal is a sensitivity - type analysis which <br />considered a range of material strengths and alternative hackfill and <br />grading configurations. Material strength properties were determined <br />for both highwall materials and existing bench fill materials. <br />The stability analysis projects that a backfilled static slope safety <br />factor of 1.3 or greater can be achieved in two manners: <br />(1) The existing portal highwall could be partially backfilled <br />(approximately half its existing height would be covered); or <br />(2) The existing portal highwall could be totally backfilled if <br />a supporting buttress toe berm was constructed throughout the length <br />of the existing bench, approximately 20 feet high by 30 feet wide. <br />Page 70 of the amended application refers to Volume 2, but does not specify <br />conformance with that study's recommendations; nor does it specify which <br />alternative hackfill and grading plan will be followed by the applicant. <br />The application should be amended to specify which configuration will be <br />accomplished. <br />The partially backfilled configuration would be a technically acceptable <br />alternative. However, this would require that Dear Coal Company request <br />a variance from approximate original contour based upon steep slope mining <br />conditions. <br />Rule 2.06.5 outlines the conditions which must be met in order for the <br />Division to approve a permit incorporating a variance from approximate <br />original contour. Included in 2.06.5 is the requirement that the applicant <br />demonstrate that the purpose of the variance is to make the affected <br />lands suitable for an industrial, commercial, agricultural, residential, <br />or public postmining land use. It is also specified that the applicant <br />demonstrate that the owner of the surface lands within the permit area <br />has requested, in writing, as part of the application, that a variance be <br />granted. <br />The partially backfilled configuration achieves a static slope safety <br />factor of 1.3 only if an internal angle of friction of 410 or higher <br />for the highwall materials is assumed. This value is higher than the <br />laboratory test determined value of 33.70 for. effective peak stress and <br />should, therefore, involve monitoring of the final backfilled configuration <br />to assure stability. Monitoring, utilizing an acceptable surface surveying <br />technique would be required for a period of at least 2 years. <br />If Bear Coal Company does not desire to file an acceptable request for <br />a variance from approximate original contour, then tl~e applicant must <br />commit to complete the total backfilling configuration including the <br />20 foot high by 30 foot wide toe buttress berm throughout the length <br />of the existing portal bench. <br /> <br />