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<br />Memo to Dave Berry <br />Bowie No. 2 Additional Geotech Study <br />page 6 <br />Page 3; "Fill Recommendations" <br />Dependent upon the results of the additional amended stability analyses <br />implied by my adequacy concerns, the recommendations for maximum <br />slope angles, compaction densities and material placement techniques <br />may prove to be inappropriate. At this point I can not complete an overall <br />opinion concerning their efficacy. However, the recommendations <br />concerning lifting, scarifying, benching, and compaction agree with <br />prudent earthwork construction practice. <br />Maxim Technologies, apparently in response to a request from the <br />applicant, reconsidered and restated its recommendation concerning the <br />use of filter fabric above basal boulder rubble blankets beneath reclaimed <br />fills. Maxim has specified that where used as a drainage blanket coarse <br />boulder rubble should be fabric shielded to prevent invasion of fines into <br />the drainage blanket. In situations were the boulder rubble is being used <br />to increase frictional bond of the basal layer of the fill "it is important <br />that rock be incorporated into a soil matrix to insure that any voids <br />between rock are filled to prevent migration of fines." I agree with these <br />statements. However, in the final design specifications of the permit <br />more detail will be necessary. First, how will the two differing <br />situations be identified and projected under reclaimed conditions? Often, <br />after backfilling, previously well-drained situations develop ground water <br />saturated conditions. <br />Further, specification for fabric installation over coarse boulder rubble <br />materials will require allowances to avoid over stressing the fabric's <br />tear resistance. Repeatedly at coal mine sites, underdrains have been <br />compromised when fabric's failed between coarse underdrain matrix and <br />overlying fine grained soils. The fabric is incapable of bridging the <br />matrix voids in the boulder rubble. At a minimum, properly graded finer <br />grained filter layers should be placed to support the filter fabric. <br />