GENERAL34948

• Permit No: C1982057
• IBM Index Class Name: General Documents
• Doc Date: 11/23/1993
• Doc Name: MEMO C-82-057 SENECA IIW PIT ADVANCEMENT STABILITY CONCERNS
• From: DMG
• To: SUSAN BURGMAIER
• Permit Index Doc Type: GENERAL CORRESPONDENCE
• Media Type: D
• Archive: No

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DIVISION OF MINERALS AND GEOLOGY
Department o(Nalural Resources
1313 Sherman SI., Room 215
Denver, CO 80203
Phone: [phone]
fAX: [phone]
STATE OF COLORADO
To: Susan Burgmaier, Environmental Protection Specialist
From: Bruce Stover, Technical Support
Date: November 23, 1993
Re: C-82-057 Seneca IIW Pit Advancement Stability Concerns
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Roy Romer
Governor
Michael B. Long
Division Dnecmr
I have reviewed the photographs and materials you gave me pertaining to our discussion of
the stability concerns on the north side of the Seneca IIW pit wall near the recent landslide.
The photographs and drill log clearly suggest a potential for surface movement and
disruption beyond the edge of the pit as it is advanced northwards, east of the recent
landslide.
The photographs show that the brown overburden materials exposed in the pit wall at the
recent slide were moving downslope toward and into the pit, due in part to removal of down
slope support by the pit excavation. There is reason for similar concern that this same
mechanism could create slope instability farther east of the recent event. Northward
advancement of the pit in that location may result in eventual loss of support for
approximately 47 feet of 'brown", presumably overburden materials, consisting of old
landslide deposits and weathered shale above bedrock. If this slope fails in similar fashion
to the recent event, propagation of surface movement and disruption could potentially
progress far enough upslope above the pit to adversely affect the topsoil stockpile near the
top of the slope.
If the pit is to be advanced farther northwards, I suggest that two or three inclinometers, (or,
alternatively, a time-domain-reflectometer {TDR} set up), be installed into the
overburden/bedrock contact between the pit wall and the topsoil stock pile to monitor any
movement which could occur. If checked every few days, these devices should be able to
detect slope creep and slope movement incipient to a major slope failure event prior to
initiation, allowing appropriate action to prevent damage to the topsoil pile and undisturbed
areas beyond the limits of the pit. In lieu of this approach, or of a slope stability analysis
specific to this concern, it's a gamble on whether or not a serious slope instability problem
would occur which could cause adverse effects for adjacent areas and facilities. When
mining in landslide-prone terrain, these situations can be expected to occur.