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0 <br />Tfie "Factor of Safety" is the estimated capacky divided by the estimated flow of <br />40 gpm. This analysis, although approximate, shows the Valley Fill minimum drain size <br />is very oversized. A significant safety factor is desirable considering the uncertainties <br />in the analysis and in drain construction. <br />The mine plan includes frequent excess spoil fills around the perimeter of the <br />surface mine. The "major" excess spoil fills are not large volume (less than about 0.5 <br />mcy). We believe adequate drainage would be provided in all six fills (Fills 4 through 9) <br />using a 6' x 12' triangular section (area = 64 sf). If this variance is not accepted, then <br />the two Valley Fills (Fills 4 and 8) will require larger drains (16' x 22') with the remaining <br />fills using the 6' x 12' section. For smaller fills over existing drainages (Fills 2-1 and 8-1) <br />the smaller 3' x 6' drain is recommended. Two drain sections will be required in Fill 2-1, <br />one in each drainage covered with fill. Refer to Fig. 6 for a summary of drain <br />recommendations. <br />- We obtained samples of sandstone from an outcrop (RB-3, Fig. 1). This material <br />• exhibited a slake durability in excess of 95% in two tests and an L.A. Abrasion loss of <br />30 percent. We judge this sandstone sufficiently durable for use as drain rock. As <br />mining proceeds drain rock sources should be identified and tested to confirm durability <br />prior to placement. Based on our observations and testing we believe other thick, hard, <br />durable "channel" sandstone deposits will be exposed as the surface mining proceed. <br />Considering the difficulty of generating filter material, we believe the use of filter <br />fabric will be a more practical choice. For the low gradient, intermittent types of flows <br />anticipated into the drains we believe filter fabric will perform satisfactorily. Filter fabric <br />should be Mirafi 140 or equivalent. <br />• LORENCITO COAL COMPANY, LLC <br />LORENCITO SURFACE MINE -PHASE I <br />CTLR 32, 611 16 <br />