2011-08-29_REVISION - M1986015 ‎(11)‎

• Permit No: M1986015
• IBM Index Class Name: REVISION
• Doc Date: 8/29/2011
• Doc Name: Amendment Submittal
• From: Continental Materials Corporation
• To: DRMS
• Type & Sequence: AM4
• Media Type: D
• Archive: No

Depth
(feet below
existing grade)
Material
Effective
Friction Angle
0' (degrees)
Effective
cohesion
c' (psf)
Moist Unit
Weight
X (pcf)
Saturated Unit
Weight
X (pcf)
10
Overburden
28
50
114
--
10-38
Sand and Gravel
31
0
--
130
38-40
Weathered Shale
14
0
--
142
40+
Clayey Shale
28
100
--
142
•
•
Transit Mix
May 18, 2011
Page 4 of 4
Analysis 1
This stability analysis involves modeling the proposed excavation with a near- vertical highwall and evaluating the
variety of failure surfaces which result in a FOS less than 1.01. Both sections were evaluated with a near - vertical
highwall of 38 feet. Where the failure surface which extended the furthest behind the highwall with a FOS less
than 1.01 daylights is the minimum required setback.
According to the analysis, the proposed pit wall offset is 85 feet from the property line for Section D -D', and the
failure surfaces which daylight 24 feet beyond the property line have FOS greater than 1.01; failures originating
in front of that point may have FOS less than 1 (Figure 3). The proposed pit wall offset is 50 feet from the
property line at Section E -E' and failure surfaces which daylight 44 feet beyond the property line have FOS
greater than 1.01; failures originating in front of that point may have FOS less than 1 (Figure 4).
Analysis 2
The second analysis evaluates the reduced setback which could be realized from mining to a specific slope as an
alternative to a vertical highwall. Specifically, the required maximum setback that could be allowed while
maintaining a 1:1 (horizontal: vertical) slope was evaluated. The distance through a daylight point offset from the
slope crest where a FOS greater than 1.01 was determined. The offset using the 1 H:1 V slope configuration is 55
feet. Figure 5 demonstrates the factor of safety for the critical failure surface through the point 55 feet offset from
the crest of a 1 H:1 V slope; failures originating in front of that point may have FOS less than 1. Therefore, to
maintain a minimum 55 -foot setback between the top of the excavated slope and the adjacent structures, the mine
side slopes should be flatter than 1 H:1 V. Material beneath and behind the 1 H:1 V slope should not be removed,
even temporarily. The stability of the slope appears to be sensitive to the phreatic surface, and therefore
comprehensive development and monitoring of the dewatering system should be conducted during mining.
RECOMMENDATIONS
This stability analysis is intended to demonstrate that the temporary highwall will not adversely affect the stability
of any significant, valuable and permanent man-made structure according to rule 6.4.19 of the Colorado Division
of Reclamation Mining and Safety (DRMS) Mineral Rules and Regulations of the Colorado Mined Land
Reclamation Board for the Extraction of Construction Material (Reference 3). Therefore, for a highwall mined to
a near- vertical slope we recommend the minimum required setback from the highwall to a structure is 94 feet.
However, if a drainage ditch is located between the highwall and the property line we recommend the minimum
required setback from the highwall to a structure is 109 feet. For a highwall mined at a I H: 1 V slope we
recommend the minimum required setback from the highwall to a structure is 55 feet.