2008-07-03_PERMIT FILE - M2007087

• Permit No: M2007087
• IBM Index Class Name: PERMIT FILE
• Doc Date: 7/3/2008
• Doc Name: Review of Geotechnical Stability Exhibit
• From: DRMS-acs
• To: DRMS-db2
• Email Name: ACS
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF RECLAMATION, MINING AND SAFETY
Department of Natural Resources
1313 Sherman St., Room 215 COL 6 R A D O
Denver, Colorado 80203 D t v t s t o N o f
Phone: [phone] RECLAMATION
FAX: [phone] MINING

SAFETY
DATE: July 3, 2008 Bill Ritter, Jr.
Governor
TO: David Bird Harris D. Sherman
Executive Director
Ronald W. Cattany
FROM: Allen Sorenson
/Division Director
Natural Resource Trustee
RE: Review of Geotechnical Stability Exhibit, Albert Frei and Sons,
?Priola Pit, File No. M-2007-087 r/
The Division of Reclamation, Mining, and Safety engineering staff (DRMS) has reviewed the mine plan for
the proposed Priola Pit relative to the potential for excavation in the pit to destabilize nearby structures, .
particularly the City of Thornton's water storage reservoir and appurtenances. Engineering staff has
previously communicated verbally that the setbacks proposed in the application are conservative and
sufficient. This memorandum is being provided as I am unavailable to be present at the July 2008 hearing on
this matter. The proposed mine plan requires horizontal setbacks from potentially affected structures equal to
two times the excavation depth plus 25 feet. The Applicant has provided quantitative slope stability analyses
demonstrating that these setbacks will be protective of off site structures with sufficient factors of safety. The
DRMS has reviewed these analyses in conjunction with the proposed mine plan, and concludes as follows:
I. The slope stability analyses provided did not consider the potential for the pit slope to fail through the
weathered shale that underlies the sand and gravel deposit. Although this is an important
consideration, DRMS has determined that the setbacks proposed by the Applicant are sufficiently
conservative that the influence of potentially low shear strength weathered shale is inconsequential.
This determination is based on hundreds of quantitative analyses conducted by DRMS for simiilar
excavation scenarios in similar stratigraphy.
2. The setbacks proposed in the application, based on previously conducted DRMS analyses for similar
pits, could in fact be reduced substantially and still be protective of critical nearby structures.
3. The proposed excavation, with strict adherence to the setback criteria described in the application,
poses no risk of slope failure destabilizing the City of Thornton reservoir and appurtenances.
4. The City of Thornton letter of objection to the Priola Pit application references Colorado State
Engineer's dam safety regulation requiring a waiver to excavate a borrow pit within 200 feet of the toe
of a jurisdictional dam. This dam safety regulation applies specifically to borrow pits exploited to
provide materials to construct the jurisdictional dam, but if the State Engineer determines that the
regulation applies in this matter, the Applicant will be subject to compliance. Any DRMS or Mined
Land Reclamation Board approvals do not absolve the need to comply with any other permitting or
licensing requirements of any other jurisdiction.
5. The DRMS considered conducting an independent analysis of slope stability for the Priola Pit, 'but
given the extensive existing knowledge base for pits in the same area and with similar stratigraphy,
combined with the time consuming nature of the quantitative analyses, it was determined that the file
review and analysis resulting in this memorandum is sufficient for the pending hearing.
cc: Tony Waldron, DRMS (via email)
c:\acs files\Mv Documents 4-19-06 thru\priola Qeotech review.doc
i
Office of Office of
Mined Land Reclamation Denver • Grand Junction • Durango Active and Inactive Mines