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Mr. Phillip Courtney <br /> February 15, 2023 <br /> Page 2 <br /> surface bedrock beneath the site is most likely the Lower Member of the Pierre Shale. Tweto (1979)describes the <br /> Lower Member as organic rich shale with numerous bentonite beds. The regional mapping indicates the bedrock is <br /> overlain by modern alluvium including the Piney Creek and younger deposits. <br /> GEOTECHNICAL CONDITIONS <br /> Based on the site investigations by others in the mine area, the natural site stratigraphy generally consists of three main <br /> units: 1) Overburden: generally consisting of near surface, loose to firm, silt, clayey silt and silty sand; 2) Sand and <br /> Gravel: alluvial deposits that underlie the overburden and overlie the bedrock; and 3) Bedrock usually consisting of silty <br /> claystone. <br /> Overburden <br /> Overburden was encountered in all of the borings. Overburden thicknesses range from approximately two (2)to twenty- <br /> two (22)feet averaging nine (9)feet. The thickest overburden occurs on the southwest part of the site. <br /> Sand& Gravel <br /> The sand and gravel unit was encountered in all of the borings underlying the overburden and overlying the bedrock. <br /> This unit thicknesses ranges from approximately five (5)to twenty-seven (27)feet averaging nineteen (19)feet. The <br /> sand and gravel unit is thinnest on the southwest part of the site. <br /> Bedrock <br /> The bedrock was encountered in all of the borings at depths ranging from twenty-six(26)to twenty-nine (29)feet. <br /> Groundwater <br /> Groundwater levels were not recorded on the drill logs but were recorded on DWR permit records at three (3) monitoring <br /> wells drilled in late November and early December of 2012. The reported depths to groundwater for these wells ranged <br /> from five (5)to seven (7)feet. Groundwater levels will vary seasonally and will typically rise during the irrigation season <br /> and when recharge is applied. Groundwater will be controlled in the Phase 2 and 3 cells with the proposed below grade <br /> lining systems. After liner construction, groundwater mounding is anticipated on the upgradient(north and northwest) <br /> side of the site and a groundwater shadow(deeper water table) is anticipated on the downgradient(south and southeast) <br /> side the site. <br /> From a geotechnical standpoint, the sand and gravel will form the majority of the mine slopes. These soils are generally <br /> strong and stable, particularly when dewatered. Dry mining is planned at the Phase 2 and 3 cells where <br /> lining/dewatering will occur. Wet mining will occur in the Phase 1 area. <br /> STRUCTURES WITH 200 FEET OF DISTURBED AREAS <br /> The known, permanent, man-made structures within 200 feet of the proposed mine limits that are not owned by the <br /> miner are listed below. <br /> 1. The Excelsior Ditch. <br /> 2. Overhead Transmission lines. <br /> 3. Baxter Road. <br /> These structures are closest to the planned mine limits and will be stable per the stability analyses described herein. All <br /> other known structures are at greater distances and, therefore, will also be stable. Known structures within 200 feet of <br /> the permit area are listed on Exhibit C-1 of the permit application. <br /> STABILITYANALYSES <br /> Division of Reclamation and Mining Safety (DRMS)drafted a policy regarding stability analyses of neighboring <br /> structures. The policy summarizes adequate factors of safety (FOS)for non-critical and critical structures. The <br />