REV02042

• Permit No: C1996083
• IBM Index Class Name: Revision
• Doc Date: 3/15/2004
• Doc Name: Adequacy Comments
• From: J.E. Stover & Associates
• To: DMG
• Type & Sequence: MR34
• Media Type: D
• Archive: No

• ~ Yeh and Associates~Inc.
~~ Geotechnical Eegineering Coasuliants
5700 E. Evans Avenue 420 Sevrnth Street, Suite 100
' Denver, CO 802]2 Glemvcod SpvinES, CO 81602
Tel.: (303) 783 A590 Tel: [phone]
Fati (303) 781A583 Fax: [phone]
March 11, 2004 Job No. 24-009
Mr. Collin Stewart
Bowie Resources, Limited
P.O. Box 1488
1855 Old Highway 133
Paonia, CO 81428
Subject: Response to CGS Letter, Rockfall Hazazd Potential Induced by Coal-mine Subsidence at the
Bowie No.2 Mine, Paonia, CO
Dear Mr. Stewart:
We have reviewed the letter by CGS dated Mazch 3, 2004 and offer the following responses:
12enort comments:
1) CGS questions whether a 5-foot design rock is large enough, based on report figures 1 and 3
showing 10 to 12 foot rocks in the slope below the outcrops and from lazger blocks experienced at the
Twenty-mile Mine following subsidence.
Based on the experience of the Bowie Resources, Ltd. and studies that have been conducted at the
site, they have recommended that a rock size of 5-feet be used as the design rock size. Our
observations and analysis of the site generally support this assumption.
The large blocks in figures 1 and 3 stopped in the area where the slope flattens. This was considered
in the model with respect to the chosen slope coefficients and used to locate the berms downslopc
from the natural topographic benches to rn;nim;.e the impact energies on the berm. The berms are
located at the distil end of the deceleration zones, which is well below the point where the two rocks
in figures 1 and 3 stopped.
The rock fall evaluation included the analysis of 1-, 3-, 5-, 10- and 20-foot rooks with CRSP. The 5-
footrock was the critical rock size that was analyzed. Smaller and lazger rocks generally stopped on
the slope above the berm location.
2) Drop heights of 5 to 40 feet were used in our rock fall simulations using CRSP, which we believe
conservatively accounts for the accelerated velocity of a rock block that is toppling.
3 & 4) CGS is concerned that the rock fall hazard will be elevated over time and that the cliff should
be evaluated before and after the subsidence event.
We agree that the outcrop source area should be evaluated before and a8er the subsidence event. The
berm could be made permanent after the cliff has been re-evaluated some time after subsidence.