REV99230

• Permit No: C1981022
• IBM Index Class Name: Revision
• Doc Date: 9/30/1999
• Doc Name: SANBORN CREEK MINE TR-32 SUPPLEMENTAL RESPONSE 12 PN C-81-022
• From: DMG
• To: JOE DUDASH
• Type & Sequence: TR32
• Media Type: D
• Archive: No

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STATE OF iii iiiiiiiiniu iii
999
COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman St., Roam 215
Denver, Colorado 80203 DIVISION O F
Phone: [phone] MINERAL S
FA\: [phone] $c
GEOLOGY
DATE: September 30, 1999 R E C L A M A T I O N
MINING•SA FETY~

TO: Joe Dudash _ R;,, ovens
Governo.
FROM: Jim Pendleton ereg E. wz~cner
Enecunve Director
nLChael B. Long
RE: Sanbo reek , TR-32 Supplemental Response #12 Division Director
o: C-81-022)
I have reviewed the Oxbow supplemental response which you forwarded to my attention.
Response #12 addressed my earlier completeness comments. In my opinion the response is
sufficiently detailed to render the TR application complete, but inadequate to satisfy my
concerns.
Stability of Elk Creek Portal Fill
I continue to be concerned for the stability of the south-facing, down slope Elk Creek portal
bench facial out slope. First, I believe the propensity of this Williams Fork Formation detritus to
fail in shear, as demonstrated by the existing landslide occupying much of the natural slope
adjacent to the proposed Elk Creek mine portal area, suggests that this material is characterized
by a relatively low shear strength. Oxbow proposes to use [his material to construct [he earthen
portal bench. This fill will be placed within the existing channel of Elk Creek. The Flow will be
diverted into a 9-foot diameter culvert buried within that fill. My engineering geotechnical and
geohydrologic experience leads me to suspect that an elevated saturated surface will develop
within the Elk Creek portal fill, at least seasonally.
Montgomery Watson has observed that; "The landslide exists in this area as a result of the
cumulative effects of several factors including shallow bedrock, steep slopes, and most
significantly, saturated conditions at the D-Seam outcrop" (Emphasis added). I concur with
this observation. Saturated conditions characteristically render the Williams Fork Formation's
claystone unstable, not only within Elk Creek canyon, but rather throughout the North Fork
region. Unless unusual precautions are taken to seal the channel diversion culvert, and/or to
intercept and divert any ground water underflow or leakage, the portal bench fill will probably
develop an elevated saturated surface. Ground water invasion commonly occurs along subcrop
beneath alluvial fills. In addition, it is difficult to intercept, preclude, or divert the conjunctive
aquifer flow which commonly occurs down gradient within [he alluvial/colluvial deposits
adjoining youthful canyon stream channels.