2022-10-27_PERMIT FILE - M2022001 (3)

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Mr. Drew Damaniano December 30, 2021 Page 2 commonly interbedded within the sand and gravel unit;and 4)bedrock usually consisting of sandy claystone and sandstone that is commonly weathered in the top 1 foot and is commonly interbedded and interlaminated. Overburden The overburden is typically a fine to medium grained sand that locally grades to silty or clayey sand typically ranging from 0 to 7 feet in thickness. This unit is usually slightly moist to moist,loose to medium dense with the top 6 to 8 inches containing significant organics. Of the overburden samples tested,the range of percent passing the No. 200 sieve ranged from 4.1%to 31.9%. Atterberg Limits testing ranged from granular non-plastic to Liquid Limits of 26 and Plasticity Indices of 9. Sand&Gravel The sand and gravel is present throughout the site locally occurring at the surface, but typically underlying the overburden and overlying the bedrock. This unit typically consists of slightly silty,fine to medium grained sand overlying or grading to a a fine to coarse grained sand and/or gravelly sand. This unit is typically medium dense but locally dense. This unit is commonly fifty(50)feet thick but is locally as thick as eighty-five(85)feet. Mud Lens The mud lens unit is locally intedensed within the sand and gravel unit. This unit typically consists of clayey to silty sand. Thickness varies from one(1)foot to as much as fifteen (15)feet. This unit tends to be thicker on the south part of the site. Bedrock The bedrock encountered in the exploratory borings was generally weathered in the upper foot typically became harder in unweathered zones. The bedrock consisted of sandy claystone. Groundwater Groundwater was encountered in all of the borings at approximately 15 to 25 feet below ground surface.The groundwater levels will vary seasonally and will typically rise during the irrigation season and when recharge is applied. Groundwater will be controlled with the proposed below grade slurry wall and by pumping down the water level on the interior of the slurry wall. After slurry wall construction, groundwater mounding is anticipated on the upgradient(south and southeast)side of the site and a groundwater shadow(deeper water table)is anticipated on the downgradient(north and northwest)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. With the exception of the silt pond,dry mining is planned in the cells as the slurry wall will control water level in the main cell and mining will be above the water table in the other cell. STRUCTURES WITH 200 FEET OF DISTURBED AREAS The known, permanent,man-made structures within 200 feet of the proposed mine areas that are not owned by the miner are shown on the attached map(Exhibit C-3) STABILITY ANALYSES Division of Reclamation and Mining Safety(DRMS)staff drafted a policy regarding stability analyses of neighboring structures. The draft summarizes adequate factors of safety(FOS)for non-critical and critical structures. The structures around the Section 20 mine are,for the most part,considered critical structures. 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 structures within 200 feet of the proposed mining limits was evaluated at three representative sections under anticipated loading conditions around the perimeter of the site as discussed below. The computer program