PERMFILE135779

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 07f-1 Rockfall part 2
• Media Type: D
• Archive: No

•

7:-~---~~~-! ~ ~ SubTerra, Inc.
!r
Mr. Karl Koehler
Cyprus-Amax Coal
Twentymile Coal Company
29515 Routt County Road #27
Oak Creek, CO 80467
Mining Engineering
Civil and Mining Rock Mechanics
Mine Subsidence
Solid Waste Disposal
Tunnel Engineering
Concrete/Shotcre[e Technology
Construction Bid Documents
Tuesday 31 October 1995
Re: Evaluation of Drainage Break in Benn at Section J on Rockfall Hazard Mitigation
Dear Mr. Koehler
This report is an extension of SubTerra, Inc.'s previous work (Reference 1) in the area
of rockfall hazards evaluation for sections of the Twentymile cliff located adjacent to
Routt County Road No. 27, in Routt County, Colorado.
The original study recommended the construction of a 20 ft high trench/berm
combination adjacent to Routt County Road 27 with a 1.5:1 (H:V) slope facing back
towards the cliff. For permit reasons, two gaps were left in this trench to permit
drainage from inside the trench. This letter report presents the results of an evaluation
of the break constructed in the rockfall hazards mitigation, trench/berm at Section J
(shown on Drawing 9404D4R8, Reference 1).
The sections used in the simulation were constructed using the survey data provided
on 10-26-95, contour data contained on Drawing 9404D4R8, and the photographs
received on 10-28-95.
The Colorado Rockfall simulation Program, Version 3.0 (CRSP3) was used to simulate
two conditions. The first run simulated the as-built gap with no remediation. The
second run simulated the as-built gap with a 10 ft high, 1:1 wall constructed using rocks
that had previously fallen from the cliff.
Assumptions in the data and information incorporated in the CRSP runs include:
1. A 10 ft diameter, 10 ft thick disk was selected for use in the CRSP runs. This
diameter is consistent with the scanline data for those sections of the cliff within
or proximate to Zone 1 (see Section 2.4 of the original report).
2. A conservative surface roughness equal to 38% of the modeled sphere diameter
was selected for use in the CRSP runs. This value accounts for the actual cubic
shape of boulders observed in the field (see Table 2-1 in the original report) and
corresponds to the middle of the range suggested by Pfeiffer and Bowen (1989).
3. The location of a rock-wall barrier as shown on the attached cross-section.