PERMFILE139786

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 07f-1 Rockfall Hazards Study Norther Mining District
• Media Type: D
• Archive: No

1•

CJ

N

SubTerra, Inc. Twentymile Coal Company 5/23/99
Northern Mining District Rockfall Hazards Evaluation
Scanline
(length in ft)

3 (440) Joint
Set

S1 Number of
Joints

14 Apparent
Spacing (ft)

31.4 Alpha
(degrees)

75 Terzaghi
Corrected
Spacing (ft)
8.1
S3 59 7.5 20 7.0
S4 52 8.5 15 8.2
4 (420) S1 18 23.3 75 6.0
S3 49 8.6 20 8.1
S4 39 10.8 15 10.4
Using true spacing, all three-set block-forming combinations of joint sets S1, S2, S3 and S4
were analyzed to obtain the frequency distribution of block sizes. Experience has shown that
many discontinuity sets have a spacing distribution that approximates reasonably to a negative
exponential form (Priest and Hudson, 1976, Priest and Hudson, 1981), and this was assumed to
apply in this study. The negative exponential distribution is asingle-parameter distribution that is
completely defined by its mean, so provided the mean spacing is known, the full range of
probabilities can be estimated. Analysis of the bedding plane data indicated a lognormal
spacing distribution. Using these distributions, the potential discontinuity combinations and
resulting block sizes were analyzed using probabilistic modeling software.
Based on the analysis, a worst case block size of about 6000 cubic feet was considered
appropriate for Phase I rockfall hazard assessment. An 'equivalent' disk, with a diameter of 26
ft, a thickness of 11 ft, and a volume of 5840 cu. ft. was used in the CRSP3 runs
Prior to the main hazard assessment, a number of trial runs were carried out to calibrate the
' CRSP3 model, and select appropriate site-specific slope surface parameters. These calibration
runs were carried out on sections of the slope where previous rockfalls provided a basis for
comparing observed and predicted behavior.
' Typical block sizes were determined from the boulder catalog for each calibration section.
These block sizes were converted to equivalent disks or cylinders for input to the CRSP3 model,
by setting the diameter equal to the average of the two most similar dimensions, and the
' thickness or lengths equal to the remaining dimension.
2.4 CRSP3 Parameters
CRSP3 requires that the surface of the modeled profile be described in terms of tangential and
normal (restitution) coefficients. Tables 2-2 and 2-3 show the relationship between the surface
' descriptions presented by Pfeiffer and Bowen (1989) for assigning tangential and normal
(restitution) coefficients, and inferred equivalent site conditions. Table 2~ indicates the ranges
for these parameters considered appropriate at the site.
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' 4 PN: 9915