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PN: 55743PE <br /> October 11, 1029 <br /> Page 4 <br /> We suspect that perhaps more critical considerations in regard to the long-term stability of this <br /> deposit are related to the nature of the interface between the deposit and the substrate,and <br /> subsurface water considerations. We understand that the project drainage design includes <br /> development of positive surface drainage at the site including diversion of surface water from <br /> waste rock deposit areas. Long term maintenance of the surface drainage will improve the long <br /> term stability and performance of the slopes at this site. <br /> We typically recommend that embankment fill be placed on sloped surface with a toe-key and <br /> periodically benched to promote an intimate contact of the fill with the underlying support <br /> material. This concept is shown below. <br /> Pre-construction ground New waste rock fill <br /> surface <br /> Benches <br /> Regolith <br /> Toe Key <br /> 41006, <br /> Periodic <br /> Bench Drain <br /> Toe Key Drain <br /> Toe Key and Bench Concept <br /> No Scale <br /> The shallow depth to the underlying bedrock on this site will influence the geometry of the <br /> benches developed within the slope. The regolith that is proposed for the project may be <br /> incorporated into the toe of the slope as the same time as development of the toe key early in the <br /> development of the site and at the initiation of the deposition of the waste rock. As material is <br /> dumped, it should be periodically prepared and incorporated into the slope with a bulldozer, or <br /> other excavation equipment to develop an intimate contact between the two materials. The <br /> "stair-stepped"appearance of the substrate shown in the sketch above will never actually be <br /> visible in the field, but this geometry, or a facsimilia of it, should be the goal when placing and <br /> spreading the waste rock. <br /> The width of the toe key should be at least 8 to 10 feet. We suspect that the shotrock will be <br /> free-draining,therefore,the toe key and benches should be sloped to allow for lateral water <br /> migration. Accumulation of water within these areas will promote localized failure. The cross- <br /> slope gradients of the toe key and benches should be 2 percent or steeper. <br /> The observations and recommendations contained in this letter are based on our surface <br /> observations at the time of our site visit and do not include subsurface exploration, laboratory <br /> TRAUTNER .lei 41114 <br />