Laserfiche WebLink
Asphalt Paving Co. <br />Geotechnical Stability Evaluation <br />April 2, 2015 <br />Page 2of4 <br />limits are dictated along the contact when shale is encountered during drilling of a blast hole. <br />The blast holes are approximately 38 -ft deep and typically drilled on a 14 -foot by 14 -foot pattern. <br />If shale is encountered, the hole will not be shot, and generally the adjacent holes are not <br />loaded or shot either. <br />1.2 Stability of the Shale /Basalt Contact <br />Brierley's observations of the shale /basalt contact indicate a stable interface that is not prone to <br />slip -dip rockslides; the stability of the basalt at the contact is stable. Additionally, the April 1994 <br />Amuedo and Ivy, Inc. report for the quarry concluded that large -scale planes of weakness were <br />not found along the contact. <br />In 2011, Brierley performed 2D slope stability analyses using a computer program (Slopem, <br />GEO -SLOPE International) based on limit equilibrium theory to compute FOS. Four scenarios <br />were evaluated for basalt overlaying shale on the west side where a thin veneer of basalt is left <br />in place over the shale. One scenario specifically evaluated the existing bench configuration on <br />the west side; the model included a basalt joint at an unfavorable orientation as determined <br />during our analysis of discontinuity data. Block failure analysis was used to mimic failure along a <br />basalt joint which then intersects the shale /basalt contact. The bench analysis resulted in a FOS <br />of 2.9, and full slope analyses resulted in FOS greater than 1.3 for all scenarios evaluated, <br />including rapid drawdown. <br />1.3 Stability of the Basalt Jointing <br />General observations during previous mapping indicate that existing slopes have been mined <br />nearly vertical with a series of benches and haul roads traversing the site. The basalt across the <br />site does present a few localized slides along dip -slope joint orientations, however no large - <br />scale slope failures were observed at the mine site. <br />To evaluate a more probable failure along jointing within the basalt rock mass, in 2013 Brierley <br />utilized the computer program Swedge by Rocscience, Inc. to model the stability of potential <br />rock wedges formed by discontinuities. Material properties for the rock mass and its joints for <br />this evaluation were consistent with those provided in Brierley's February 10, 2015 letter titled <br />"Summary of Material properties for Rock Mass and Joints". Brierley analyzed three - <br />dimensional wedge failures for bench -scale and overall slope angles for the four primary dip <br />directions proposed for final slope configurations. For each dip direction, the steepest allowable <br />slope dip angle that maintained a FOS greater than 1.3 was established for the proposed final <br />slope configurations. <br />Two geometries were analyzed for the wedge failure analysis: a bench -scale and the overall <br />slope - scale. The bench -scale geometries were used to analyze local failures and consisted of a <br />double bench up to 80 -ft tall. These benches were modeled as a vertical rock face. The overall <br />mine slope -scale geometries were determined based on the anticipated overall height to the <br />final floor elevation of the quarry, either 200 or 400 ft depending on existing elevations and <br />aspect. <br />BRIERLEY <br />ASSOCIATES <br />Creating Space Underground <br />