2011-01-25_REPORT - C1980007

• Permit No: C1980007
• IBM Index Class Name: Report
• Doc Date: 1/25/2011
• Doc Name: 2010 Springs Subsidence Report Operators Rsponses to DRMS 1/21/11 Adequacy Review Letter
• From: Gary Witt
• To: DRMS
• Permit Index Doc Type: Subsidence Report
• Email Name: TAK
• Media Type: D
• Archive: No

MEMORANDUM
To: Kathy Welt and Ryan Sweetwood
Mountain Coal Company, LLC
Via Email
From: Wright Water Engineers, Inc.
Gary D. Witt, P.G., CPG and C. Richard Dunned, P.E.
Date: January 25, 2011
Re: Response to CDRMS Comments Regarding Angle of Draw Calculations
The following represents Wright Water Engineers, Inc. (WWE) response to a letter from Tom
Kaldenbach of the Colorado Division of Reclamation, Mining and Safety (CDRMS) to Ryan
Sweetwood of Mountain Coal Company, LLC (MCC) dated January 21, 2011.
The subject CDRMS letter indicates satisfaction with MCC's responses to comments 1 and 2 as
outlined in an adequacy review letter from them dated December 29, 2010. However, comment 3
(angle of draw calculations) remains a concern. According to the letter, CDRMS is of the opinion
that the angle of draw for longwall panel E -1 may be greater than 18 degrees based on their
independent analysis. Data used in their analysis was presented in a table showing ground
elevation, top of E -seam elevation, vertical distance, horizontal distance and the calculated angle of
draw for each of the five locations used in a similar analysis by WWE.
Upon review of the data presented by CDRMS, it is apparent that there are vertical and horizontal
distance differences between their analysis and the information presented in Table 1 of a
memorandum to MCC by WWE dated August 2, 2010 (also included as Appendix B of the Spring
2010 Subsidence and Geologic Field Observations report). There appears to be agreement on the
horizontal distances at three of the five locations (i.e., Station 10 [10 foot difference], 57, and 80). It
is the opinion of WWE that the horizontal distances reported by CDRMS at Station 37 and 101,
upon further review, might better reflect the perpendicular distances from the monument points to
the coal extraction boundary. Similarly, there is vertical distance agreement on three locations (i.e.,
Station 37, 57, and 80). However, the differences represented at Stations 10 and 101 are substantial.
We believe these differences are the result of a misunderstanding of the location at which to
quantify the vertical distance between the ground surface and the top of the E -seam. As stated in
our August 2, 2010 memorandum, WWE used a vertical distance representing `the overburden
thickness from the ground surface to the top of the coal seam at the longwall panel boundary".
CDRMS appears to have used a vertical distance representing the overburden thickness from the
ground surface to the top of the coal seam at the subject monitoring station. In the case of Station
101, the E -seam overburden thickness changes rather rapidly near the eastern edge of the E -1 panel.
Wright Water Engineers, Inc., 2490 W. 26`" Avenue, Ste. 100A, Denver, CO 80211
Tel. [phone]; Fax. [phone], e- [e-mail]