PERMFILE54453

• Permit No: C1981012
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 30 STABILITY LETTER
• Media Type: D
• Archive: No

CF&I Steel Corporation •
January 20, 1982
Page 2
with a friction angle of 38.1°. The field shear strength of the overall
spoil material will be somewhat greater than that of the finer fraction.
However, this friction value will be used because, to a large degree,
the fine fraction of a soil governs its resulting behavior. In addition,
the use of this laboratory value will be on the conservative side.
Stability Analysis
As indicated to us by CF&I, the Allen coal processing waste pile is dry.
Since the refuse is "soil-like granular", the stability of this pile was
estimated by nomographic techniques described by Huang. This procedure
was recommended by MLRB in their correspondence to CF&I, dated October
14, 1981. The' portion of the nomograph used in this analysis and a
summary of the calculations is shown on Table 1. This analysis resulted
in a safety factor of 1.70.
The analysis of the Maxwell Spoil Piles was conducted on a cross section
provided by CF&I. This cross section and the material parameters used
in this analysis are shown on Figure 4. The density and strength of
the coal was selected on the basis of ]aboratory tests summarized in .
this report and afield density test of the Allen spoils conducted by
1 CF&I. The values for the other strata were obtained from a report to
CF&I entitled "Preliminary Engineering Geology and Subsoil Investigation
for a New Mine Site, Picketwire Valley, Las Animas County, Colorado", by
Chen and Associates, Inc. dated November 14, 1975.
Analysis of the slope for static loading conditions utilized the computer
program BISHOP. Ttiis program was developed by the U.S. Bureau of Mines
and is based on they Modified Bishop method of analysis. The minimum
factor of safety wens determined by performing analyses for trial circular
failure surfaces herving centers at nodal points on a grid system. At
each point of the grid system, circles with different radii were used
until a minimum value was found. The critical circle was located and the
minimum value of s~ifety factor was computed for the overall slope.
The critical circlE~ determined by that method is drawn on Figure 4. As
indicated, a minimum factor of safety of 1.56 for static loading conditions
was computed. As may be seen on Figure 4, the critical circle is relatively
shallow and relates to surficia] sloughs on the face of the pile. The
safety factor against deep rotational shear failure, such as the dash circle
shown on Figure 4, is much higher. For this particular deep failure, the
safety factor was computed to be 2.20.
The Maxwell slope was also analyzed for potential failure along the
interface between the spoil pile and the existing ground using computer
program STAB L. This method is a form of the Modified Bishop method that .
has been revised t~~ allow consideration of noncircular failure surfaces.