PERMFILE121519

• Permit No: M2003016
• IBM Index Class Name: Permit File
• Doc Date: 8/1/2003
• Doc Name: Pit Wall Stability and Setbacks from Valuable Structures
• From: DMG-ACS
• To: DMG-KAP
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
DATE: August 1, 2003
TO: Kate Pickford
FROM: Allen Sorenson
COLORADO
DIVISION O F
MINESRALS
GEOLOGY
0.ECLAMATION•MINING
SAFETY•SCIENCE
Bill Owens
Governor
Greg E. Walther
Executive Director
RE: Pit Wall Stability and Setbacks from Valuable Structures, Heit Pit, Ronald W. Canany
FdeNa. M--2003-016 Division DireLTOr
Natural Resource Trustee
The Division of Minerals and Geology (DMG) has completed a stability analysis for the mined pit walls at the
proposed Heit Gravel Pit. The results of the analysis aze attached to this memo. The DMG's stability analysis differs
from the analysis provided in the pemrit application in three ways.
1. The DMG modeled the pit wall as a vertical face whereas the Applicant used a O.SH:1 V pit wall. It is virtually
impossible to control gravel mining at a highwall so as to assure the face will no exceed an azbitrary steep
angle. The DMG frequently observes vertical and neaz vertical highwalls at gravel pits.
2. The DMG does not agree that the bedrock underlying the Heit sand and gravel deposit is "infinitely strong" as
was assumed for the purpose of the Applicant's analysis. The bedrock in this location is the Cretaceous Pierre
Shale. The upper three or more feet of this bedrock is weathered and is an over consolidated or stiff fissured
clay. The weathered shale, upon unloading, may degrade to a fully softened or residual strength condition and
exhibit very low sheaz strength. The DMG ran a number of models setting the weathered bedrock at its
residual strength. However, as the attached analytical results show, the bedrock material is not involved in the
critical failure surfaces in this particulaz case.
3. The applicant modeled a progressive failure with initial pit face sloughing followed by analysis of the resulting
slope configuration. The DMG analysis considers a bulk failure that could impact structures at the proposed
25 foot setback line during the initial slumping.
DMG's analysis shows that the 25 foot setback proposed by the Applicant is inadequate to protect valuable structures.
At a 40 foot setback a metastable slope with a 1.01 safety factor results. Givett the conservatism of the assumed sheaz
strength parameters input to the analysis and the temporary nature of the mined highwalls, DMG has determined that a
40 foot setback is sufficient. To verify this fording, an analysis was conducted at a 40 foot setback with the internal
friction angle for the sand and gravel deposit set at 35 degrees (increased from 30 degrees used in the other analyses).
A 35 degree friction angle for the sand and gravel material is a reasonable and still conservative expectation. A safety
factor of 1.22 was the result, which is quite sufficient for temporary slopes.
The Applicant should commit, or the DMG should stipulate, mining may occur no closer than 40 feet to structures
afforded protection under §34-32.5-I IS(4)(e) C.R.S., Land Reclamation Act for the Extraction of Construction
Materials. This commitment or stipulation will only apply where the limits of mining illustrated on the maps provided
with the pemrit application aze closer than 40 feet to protected strnctures. Where the limits of mining shown on the
maps aze greater than 40 feet, those depicted mining limits will be enforceable.
attachment(s)
cc: Cazl Mount, DMG (via email & w/o enclosures)
Tug Martin, Banks and Gesso (via fax, w/attachments, and per KAP direcrion)
c:~acsVvly DocumentsVteit pit.doc
Office of Office of Colorado
Mined Land Reclamation Active and Inactive Mines Geological Survc-