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GENERAL40854
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Last modified
8/24/2016 7:59:58 PM
Creation date
11/23/2007 10:53:59 AM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
C1980007
IBM Index Class Name
General Documents
Doc Date
12/3/1997
Doc Name
LANDSLIDE STATUS AND PLANNED MITIGATION WEST ELK MINE PN C-80-007
From
MOUNTAIN COAL CO
To
DMG
Permit Index Doc Type
GENERAL CORRESPONDENCE
Media Type
D
Archive
No
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~. <br />12-Oa- 1 997 8: 57at~t <br />~cr-Le~1771 Lt-~+~ <br />FRO[-1 ENG. WEST ELK f4IPJE 970 9295050 <br />HHrcuiivu ~HwSUrv <br />Page Three <br />Mr. Norm Every <br />Mountain Coat Company <br />CMl ~bs aavb F~.ea <br />P. d <br />Mar0ing Lawson Assocrares <br />drained. as would appeaz to be the present case. then during times of high groundwater <br />pressure from snow melt and itrfiliruion, this layer would also act to collect and distribute <br />groundwater and high pore pressures to a large portion of the slide failure plant. <br />~. <br />Lying above the stream bed deposits azc low density silts and clays that compose the <br />majoory of the existing slide mass. Recent inclinometer installations have confirmed the <br />location of the poncipal failtrre plane at the interlace between [hest silo and clays and the <br />stream bed deposiu (i.e., at the very base of the existing slide ma+s). These inclinometers <br />have also disclosed the existence of shear zones at higher levels within the slide mass that <br />ate believed to represent the movement of discreet independent blocks within the slide <br />mass that can be cottel:ncd with surface scarps and the segmentation of the larger slide <br />mass. <br />The silt and clay mateoals. that compose the main slide mass are believed to have been <br />accumulated through successive mud flow/debris flow events deposited on the surface of <br />the older stream bed deposits. In fact these cvertts may well have been responsible for the <br />displacctttent of the existing North Fock of the Gunnison River northward to rtes cturcnt <br />position. Due to the nature of the depositional mechanics, the clay soil deposits are loose <br />normally consolidated materials with very high void ratios and law shear strength- Yct <br />due co the high clay content they should also exhibit low pentreabilities (assumed to be in <br />the range of 10-6 cm/s ro lU-8 cm/s). The back analysis of the slide mass. assuming the <br />failure to exist at the basal imerface, indicates a coefficient of friction of 9.6 degrees arid <br />cohesion of 320 psf. This is consistent with the weak low density natttrc of ehe clay mud <br />flow/debos flow materi:s. <br />w <br />It is our understanding that the existence of a landslide macs has been known from the <br />inception of the mine m the carry 1980's and that local surficial slides near the portal area <br />have plagued this arcs from the star. However, facilities located on the larger landslide <br />mass tmnh of the portal facilities did not experience any significant distress that <br />detrimentally affecnd operations until approximately 1992-93. Monitoring of movements <br />in this target landslide mass did not begin until 1993. !t is also otu understanding that <br />pillar robbing of a portion of the F-scam panels known as the riw Subrnains was <br />terminated in 1991 and sealed. Recent inspection of the seals around these abandoned <br />worlings have disclosed seepage in the rock around the seals indicating that presence of a <br />situtificant quarrtiry of water and pressuue in the old workings at this location Thetef'ore. <br />it is likely that these old workings may represent a sotucc of groundwater and pore <br />pressure contributing to the reactivating and atccleration of the slide mass since 1992. <br />If in fact these old workings aze co6ccting end accumulating water during significant <br />snow melt and infiltration events, then it would be expected to have two principal effccu <br />on the behavior of the slide tea»: <br />
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