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<br /> <br />Il <br />L. <br /> <br />1 <br /> <br />1 <br /> <br />LI <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />1 <br />1 <br />During the field inspection, the failure mechanism was determined for <br />each of the slide areas. Three principal failure mechanisms were <br />identified. They are: <br />• Slab type failure <br />• Rotation type failure <br />• Ravelling slopes <br />The slab type failures consistently include a cut slope where the <br />bedrock bedding planes dip toward the road so that excavation of the <br />road and the cut areas remove material from the toe of the slope and <br />cause an open or free-face in the cut slope. This slope usually becomes <br />unstable when the cut slope intersects a thin coal seam or an area which <br />contains a higher amount of moisture. Particularly in spring conditions <br />when snowmelt increases the moisture content of some near surface <br />materials, a plane of weakness exists along the bedding planes of the <br />upper cut slope and allows sliding on the bedding plane into the road <br />surface. This results in a slab of material sliding into the road. <br />In areas which do not exhibit undercut adverse bedding planes, a form of <br />rotational failure has been identified. Due to weathering of bedrock <br />material or other planes of structural weakness throughout the bedrock <br />with high moisture conditions, a rotational failure has occurred. All <br />cases exhibit a cut slope which a relatively small rotational failure <br />pushes material onto the road surface. In some instances, adverse <br />bedding planes dipping toward the road as well as thin coal seams and <br />planes of weakness in the bedrock, help to reduce the overall strength <br />of the slope and promote slippage in a rotational manner of a small <br />block of overburden. <br />The third type of slope instability was identified along the haulage and <br />access road which is characteristic of the Grassy Gap Mine area. This <br />type of instability consists of ravelling slopes. During periods of <br />higher snowfall and increased soil moistures which have occurred over <br />the past three (3) years at the Grassy Gap Mine area, unconsolidated <br />surface soil material loads the slope and causes small landslides to <br />-4- <br />