PERMFILE108164

• Permit No: C1982057
• IBM Index Class Name: Permit File
• Doc Date: 5/5/2005
• Doc Name: -A- Pit Stability Analysis
• Section_Exhibit Name: Tab 13 Attachment 13-12
• Media Type: D
• Archive: No

Q MWH
March 22, 2005
Mr. Robert J. Shevling P.E.
Engineering Manager
Seneca Coal Company
Re: Seneca II-W Mine A Pit Slope Stability Analysis
Dear Bob:
This letter is in response to the Division of Minerals and Geology's (DMG) TR-49 3~~ Adequacy
Review, dated, Febmary 28, 2005. The adequacy review requested that a previous analysis
submitted by MWH for the "A" Pit slide area be re-analyzed using a circular failure mode rather
then an infmite slope analysis. This previous analysis was completed and submitted in a letter to
Seneca on January 13, 2005. The previously submitted infinite slope stability analysis assumed the
material had a friction angle of 21° and a nominal amount of cohesion (60 ps fl.
As requested by DMG, slope stability model was constructed to run a circular failure analysis. A
plan view and section of the area is shown in Figure 1, `A Pit" Slide Area Plan and Section. In
addition, a phreatic surface was input into the slope stability analysis which was also requested by
DMG. The location of the phreatic surface is based on the groundwater information available Eor
the area. The estimated phreauc surface for [he section is based on the potentiometric surface
map that is included in A[[achment A, Poteutionretric Surface Map. It should be noted that the
Potentiometric Surface Map is an estimate of the potendometric surface based on very few data
points.
The section geometry, soil strength parameters, and phreatic surface were input into the
SLOPE/W slope stability software. The Morgenstern-Price method to calculate interstice forces
was used Eor this analysis. A grid search routine was used to determine the most critical failure
location for the slope.
Because DMG requested a circular failure analysis be completed, a new back analysis was
completed to ensure that the correct strength parameters were determined. As par[ of the back
analysis, both a block and circulaz failure geometry were used. In addition, [he failure surface was
limited to the geometry of the size of the failure as surveyed in the field. The results of the back
analysis resulted in the following strength parameters which show good agreement between the
two types of analysis:
Circular Failure Block Failure
c=Opsf,~=23° c=2psf,~=24°
Based on the above noted parameters, circular and block slope stability analysis were completed
for various slope angles which aze as follows:
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