Laserfiche WebLink
areas exhibited favorable geological conditions within the exposed highwalls (readily <br /> mappable) and observed within the South Quarry boreholes. <br /> • The recommended final highwall configuration within the east (west facing) <br /> highwalls consists of 40H:40V benches for the full planned depth of the mine. This is <br /> a result of the foliations within the granitic and biotite gneiss trending in an adverse <br /> manner, i.e. dipping out of the east highwall. This results in significantly reduced <br /> slope stability. <br /> • In the South Quarry expansion areas located to the northwest of active mining (Figure <br /> 1) where minimal geotechnical data had been obtained and no structural mapping has <br /> occurred, the currently permitted highwall configuration of 40H:40V has not been <br /> modeled, analyzed, or updated. As mining progresses and exposes base rock in this <br /> area, additional analyses and modeling will need to be conducted and an additional <br /> TR to M-1973-021 will be required, if highwall design alternatives other than <br /> 40H:40V are desired by Al. <br /> • The interbench highwalls are anticipated to be near vertical (85°from horizontal) per <br /> discussion with Al. While this configuration results in acceptable highwall stability, <br /> GEI recommends that, wherever possible, this angle be less (-70° recommended) to <br /> decrease the effects of local kinematic failures in addition to freeze-thaw weathering <br /> and resultant highwall spalling. This would also serve to increase the overall stability <br /> of the highwalls. <br /> • In areas where the sillimanitic gneiss is present within any highwall (active mine <br /> front or final) at any thicknesses, special care should be taken as this material has <br /> been identified as significantly weaker than surrounding granitic or biotite gneiss. <br /> Minor structural instabilities have been noted in mine front highwalls where this <br /> material is exposed by mining activities. The nature and extent of these failures is not <br /> part of the scope of this project, however, it is believed that the weak rock may fail by <br /> "crushing"when adjacent material is removed and subsequent blasting (impact <br /> shaking) disrupts the in-situ material. <br /> Sillimanitic gneiss has been observed exposed within the South Quarry and within <br /> borings advanced during various phases of quarry design with varying thicknesses <br /> noted. Areas where the sillimanitic gneiss is one-quarter of the bench height, the <br /> entire bench highwall should be sloped at a 2H:IV. We anticipate that this <br /> configuration will reduce the "crushing" instability. <br /> 6.1.2 Monitoring Recommendations <br /> Along with a robust engineering analysis and design program for highwall development, <br /> monitoring of the mine highwalls is a critical step in recognizing and reducing the potential <br /> for both small and large scale failures. we strongly recommended that A site specific <br /> Aggregate Industries Morrison Quarry GEI Consultants, Inc. <br /> South Quarry Highwall Design Technical Revision 28 August 2017 <br />