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Mr. Chri Oestreich September 29, 2023 Page 2 the upper part is characterized by a weakly developed brown soil. GEOTECHNICAL CONDITIONS Based on the site investigations,the natural site stratigraphy generally consists of three main units: 1)Overburden generally consisting of near surface sandy clay to silty sand;2)sand and gravel alluvial deposits that underlie the overburden and overlie the bedrock;and 3)bedrock usually consisting of sandy claystone,siltstone,and sandstone that is commonly weathered in the top foot and is commonly interbedded. Overburden The overburden typically grades from a sandy clay to a silty sand that is of low plasticity typically ranging from approximately 2 to 5 feet in thickness. This unit is usually slightly moist to moist and stiff to very stiff or medium dense with the top 6 to 8 inches containing significant organics. Sand&Gravel The sand and gravel encountered is present throughout the site typically underlying the overburden and overlying the bedrock and ranges from approximately 20 to 43 feet in thickness. This unit typically consists of slightly silty,fine to medium grained, locally coarse grained sand with local sandy gravel also present. Local thin sandy clay to clayey sand lenses were locally encountered within this unit. Where encountered,gravels were typically 1 to 2 inches in diameter. Local cobbles up to 4-inches were also locally encountered. Bedrock The bedrock was encountered at depths ranging from approximately 23 to 46 feet below the ground surface in the exploratory borings. Bedrock was generally weathered in the upper one foot typically became harder in unweathered zones. The bedrock consisted of claystone,siltstone and sandstone that were commonly interbedded. Local lignite was also encountered. The claystones are typically moderately to highly plastic. The siltstones and sandstones are non- plastic to lowly plastic. Groundwater Groundwater was encountered in all of the borings at approximately 4 to 14 feet below ground surface.The groundwater levels will vary seasonally and will typically rise during the irrigation season and during high river stages. Groundwater will be controlled with the proposed below grade slung walls and by pumping down the water level during mining. After slurry wall construction, groundwater mounding is anticipated on the upgradient(south and west)side of the site and a groundwater shadow(deeper water table)is anticipated on the downgradient(north and east)side the site. From a geotechnical standpoint,the sand and gravel will form the majority of the mine slopes. These soils are generally strong and stable, particularly when dewatered. STABILITY ANALYSES The Colorado Division of Reclamation and Mining Safety(DRMS)drafted a policy regarding stability analyses of neighboring structures. The document summarizes adequate factors of safety(FOS)for non-critical and critical structures. The L.G. Everist Conveyor is considered a critical structure. The FOS are for both static and seismic(from an earthquake)stability analyses. For generalized strength assumptions and critical structures,an FOS of 1.5 is considered sufficient for static conditions and an FOS of 1.3 is considered suitable for seismic conditions. The stability of the L.G. Everist conveyor was evaluated at the maximum section under anticipated loading conditions as discussed below. The Galena computer program was used for the analysis. The method for selecting the critical failure surface for each analyzed loading condition is the following. The Modified Bishop's Method of Analysis was used to find the critical failure surface by randomly searching with 20 termination points, 20 initiation points,and 30 different radii (12,000 failure circles)over a broad range of the slope surface and at the conveyor to identify the most critical factor of safety failure surface. Both static stability under anticipated mining conditions and seismic stability under peak ground