2023-02-08_REVISION - M2003016 (2)

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Mr. Brennan Middleton September 30,2022 Page 2 finer than the 200-sieve ranging from 41.9 to 77.5, Liquid Limits ranging from 35 to 45,and Plasticity Indices ranging from 18 to 26. Sand&Gravel The sand and gravel unit was encountered in the borings underlying the overburden and overlying the bedrock. This unit thicknesses ranges from approximately seventeen(17)to forty(40)feet. Bedrock Claystone bedrock was encountered in the borings at depths ranging from sixteen(16)to forty-five(45)feet. Percentages finer than the 200-sieve ranged from 62.5 to 97.9, Liquid Limits ranged from 52 to 54,and Plasticity Indices ranged from 24 to 29. Groundwater Groundwater was not encountered in borings within the slurry wall. Groundwater levels measured in the slurry wall investigation borings ranged from approximately two(2)to seven (7)feet below the ground surface. Groundwater levels will vary seasonally and will typically rise during the irrigation season and when recharge is applied. Groundwater is controlled with the below grade slurry wall and an underdrain constructed on the south side of the slurry wall near the southwest part of the mine. Slung wall construction results in groundwater mounding (higher water levels)on the upgradient(south and southwest)side of the site and a groundwater shadow(deeper water levels)on the downgradient (north and northeast)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. STRUCTURES WITH 200 FEET OF DISTURBED AREAS The known,permanent,man-made structures within 200 feet of the proposed mine limits that are not owned by the miner are listed below. 1. Underdrain, 2. Concrete Ditch adjacent to the County Roads, 3. Weld County Roads 20 and 25, 4. A pond and driveway at the 45 Acres LLC site, 5. The Koenig Pond and a building at the Central Colorado Water Conservancy District(CCWCD)south of the site, 6. A Fiber Optic Line owned by Century Link, 7. A gas line owned by DCP, 8. The Meadow Island# 1 Ditch, 9. Ponds owned by Platte Land and Water. 10. Overhead Electric Lines owned by United Power, and 11. A gas line owned by Western Midstream. All other known structures are greater than 200 feet from the mine highwalls. 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 at the Heit Mine are,for the most part,considered critical structures. The FOS are for both static and seismic(ground accelerations from an earthquake)stability analyses are listed in the Draft letter. 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.