REV13688

• Permit No: C1981015
• IBM Index Class Name: Revision
• Doc Date: 3/29/1994
• Doc Name: Memo on Stability of Portal Bench and Highwall
• From: DMG
• To: STEVE RENNER
• Type & Sequence: SF1
• Media Type: D
• Archive: No

Memo to Steve Renner
Fruits Highwall Stability
page 8
1994 Analysis (Highwall Complete Backfzlling)
The bedrock dips at approximately 7° into the highwall. Because of
the bedrock orientation, the seam which matches the original
elevation of the portal bench surface at the highwall is exposed
approximately 50 feet above that surface at the front of the bench.
In response to the Division's expression of concern, Mr. Greg
Lewicki performed a stability analysis of optional backfilling of
the portal bench aide elopes. Mr. Lewicki used the material shear
strength parameters developed for previous analyses. His analysis,
using the RBAt~ program, concluded that the necessary fill
configuration would not be sufficiently stable to satisfy the
Division's 1.3 static elope safety factor requirement. In fact,
the determined static and paeudoatatic factors of safety range from
0.787 to 0.918, suggesting the slope will not stand.
Mr. Lewicki's analysis is atraightforvard. In addition, it uses
material properties for the fill which may be higher than
actuality. Mr. Lewicki used these same strength parameters used in
his portal bench reconfiguration analysis in 1989. However, in
that situation the 2h:ly (26.6°) configuration had a sufficiently
high factor 'of safety to render concern about perfection of the
material parameters trivial. Mr. Lewicki's 1994 analysis is
limited to the "complete backfilling scenario", because that was
the specific directive in the amended NOV. His analysis does not
investigate whether partial backfilling scenarios might be stable.
Additional Analysis of Partial BackfiUing
In order. to thoroughly consider the slope stability of partial
backfilling scenarios for the Fruits mine portal highwalls, I
completed an additional stability analysis. In completing my
analysis I used the SBSLOPB modeling program included in the
software provided with the OSMRE Technical Information Processing
System. I have selected what I believe are conservative material
properties from the past analyses performed at the Fruits mine site
for the completion of the analysis.
Bedrock Material: Angle of Internal Friction (B) = 27.0°
(Chen & Assoc,'83) Cohesion (C) = 4,000 psf
Density = 135 pcf
Fill Material: Angle of Internal Friction (B) = 36.8°
(Fox & Assoc, '89) Cohesion (C) = 0 psf
Density = 116 pcf
Pseudo Static Acceleration Factor = 0.102 g
(Geo Testing, '79)