2006-12-15_REVISION - M1978208 - Laserfiche WebLink

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~_ „ N33W ~ _ 35NE 7,* u~_ .u~ <zc= J-f Foliation rim N58W 74NE J-1 Second N54E 75SE J-2 Second N38W 77SW J-3 Second N54E 76NW J-4 Secon N27W 41SW J-5 Terti N20E 40NW J-6 Terti Stability evaluations for proposed highwall slopes were performed using the average joint orientations listed above. These were based mainly on the primary foliation and secondary steeply dipping joints, but also considered the tertiary daylighting joint sets. Because the tertiary joint sets aze not very common or persistent, the two sets were averaged into one westerly dipping orientation for the analyses. Also because of the lack of occurrence and persistence of these tertiary features, they are considered to be relevant only to individual highwall, and not overall slope, stability. The stereonet analysis and summary included in Attachment D shows potential sliding plane and wedge failures. These all assume continuous, through-going joint features, which is a conservative assumption for this rock mass. Analysis shows potential slope failures if the slope were steepened to 1:4 from the originally proposed 1:2 configuration, but acceptable slope conditions for a quarry development if steepened to 1:3 as Elk Creek is presently proposing (12 ft benches by 35 ft highwalls = 1:2.9). For slope failures to occur, planaz or wedge failures must first be kinematically (geometrically) possible or admissible. In addition, for given possible planar or wedge failure modes, the material strength properties of the rock must be low enough for these failures to be mechanically possible or admissible. Factors of safety less than one were considered as cases where failure was mechanically possible or admissible. Such analyses were run for all kinematically admissible planes and wedges for both weathered and unweathered rock using internal friction angles of 30 degrees and 40 degrees, respectively. For the proposed 1:3 slopes, our analyses indicate that sliding failure is kinematically admissible only on the averaged westerly dipping joint set, but that this is mechanically admissible only in the upper weathered rock. Toppling failure is not kinematically admissible for any of the six joint sets analyzed. Of five wedge failure modes identified in the analyses, three aze kinematically and mechanically admissible in the upper weathered rock, but not in the general rock mass. Two of the wedges might require spot bolting, but limited individual wedge failure in final highwalls is usually considered acceptable in quarry operations. As an example, DMG permitting for the Morrison Quarry in a similaz rock mass assumed up to 15 to 20 percent such failures to be acceptable. The upper weathered rock should be laid back to 1:1 for long term stability.