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Mining Methods and Slope Stability Evaluation for Expanded Development of the Walstrum Aggregate Quarry_ <br /> 3.0 REVIEW OF GEOLOGICAL CONDITIONS AND HISTORICAL SLOPE <br /> STABILITY IN THE WALSTRUM QUARRY <br /> The geology of the Walstrum Quarry site is documented in a very thorough investigation report <br /> prepared by Schwochow (1989). Precambrian-age metamorphic gneiss is the principle rock <br /> located within the quarry boundaries. Some igneous rocks consisting of pegmatite and monzonite <br /> are found in intrusive dikes. The igneous rocks are volumetrically insignificant. <br /> Of most concern with respect to the slope stability issues is the structural nature of the rock at the <br /> site. The gneiss is extremely foliated in moderate to steeply dipping layers. The strike of the <br /> foliation planes is generally east-west. The rock dips from 40 to 60 degrees to the north. In early <br /> 1989 the author visited the Walstrum quarry to investigate a planar failure that occurred in a wall <br /> striking N 60' E, Iocated near the quarry entrance. In this slide, a large block slid down an <br /> undulating foliation plane towards the Quarry's interior roadway. At that time, it was quite <br /> apparent that the slide occurred due to the unfavorable steepness and orientation of the wall with <br /> respect to the strike and dip of the foliation planes. <br /> In the 12 years that have passed since that failure, the orientation of the principal highwalls in the <br /> quarry have been mined towards the east, hence creating walls striking north-south across the <br /> foliation planes of the gneiss formations. Another steep and stable wall with a slope less than <br /> 1/2:1 has been created on the northeast side of the buffer ridge between the quarry and U.S. <br /> Highway 6. The strike of this wall is about E 40' S. These walls, as shown in Figures 3.1 and 3.2 <br /> have been very stable. <br /> Inspection of the rock walls within Walstrum Quarry revealed that the there are literally thousands <br /> of foliation planes within the gneiss formations. The walls also reveal some relatively small areas <br /> where the walls have broken back slightly to joint planes intersecting the foliations at acute angles. <br /> However, there is no evidence that any major wedge failures have occurred in the walls due to <br /> unfavorable intersections of joint and foliation planes. The geological structure of the quarry and <br /> the wall failure in 1989 indicate that efforts to enhance rock stability in the Walstrum Quarry <br /> should be directed toward methods and controls designed to prevent planar slide failures. <br /> REVEY Associates, Inc. Page 4 of 14 October,2001 <br />