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~ Yeh and Associates, Inc. <br />~~ Geotechnical Engineering Consultants <br />5700 E. Evans Avenue 420 Seventh Sheet, Suite !00 <br />' Denver, CO 80222 Glenwood $prinCS. CO SI602 <br />Tel.: (303) 781A390 Tel: (970) 384-1500 <br />Fsx: (303) 78IA383 Fax: (970) 384.1501 <br />Mazch 11, 2004 <br />Job No. 24-009 <br />Mr. Collin Stewart <br />Bowie Resources, Limited <br />P.O. Box 1488 <br />1855 Old Highway 133 <br />Paonia, CO 81428 <br />Subject: Response to CGS Utter, Rockfall Hazazd Potential Induced by Coal-mine Subsidence at the <br />Bowie No.2 Minc, Paonia, CO <br />Deaz Mr. Stewart: <br />We have reviewed the letter by CGS dated March 3, 2004 and offer the following responses: <br />Report comments: <br />1) CGS questions whether a 5-foot design rock is large enough, based on report figures 1 and 3 <br />showing 10 to 12 foot rocks in the slope below the outcrops aad from lazger blocks experienced at the <br />Twenty-mile Mine following subsidence. <br />Based on the experience of the Bowie Resources, Ltd. and studies that have been conducted at the <br />site, they have recommended that a rock size of 5-feet be used as the design rock size. Our <br />observations and analysis of the site generally support this assumption. <br />The large blocks in fig(tres 1 and 3 stopped in the area where the slope flattens.111is was considered <br />in the model with respect to the chosen slope coefficients and used to locate the berms downslopc <br />from the natural topographic benches to minim; ~e the impact energies on the berm. The berms are <br />located at the distil end of the deceleration zones, which is well below the point where the two rocks <br />in figures 1 and 3 stopped. <br />The rock fall evaluation included the analysis of 1-, 3-, 5-, 10- and 20-foot racks with CRSP. The 5- <br />foot rock was the critical rock size that was analyzed. Smaller and lazger rocks generally stopped on <br />the slope above the berm location. <br />2) Drop heights of 5 to 40 feet were used in our rock fall simulafions using CRSP, which we believe <br />conser/atively accounts for the accelernted velocity of a rock block that is toppling. <br />3 & 4) GGS is concerned that the rock fall hazard will be elevated over time and that the cliff should <br />be evaluated before and after the subsidence event. <br />We agree that the outcrop source area should be evaluated before and after the subsidence event. The <br />berm could be made permanent after the cliff has been re-evalllatcd some time aRcr subsidence. <br />/1'I R-3 ti <br />/{PPROV6,l~ 3/~.y/o ~f <br />