2023-05-24_REVISION - M1998013 - Laserfiche WebLink

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Mr. Mark Johnson January 2 , 2014 Page 2 GEOTECHNICAL CONDITIONS The mine site is located on a terrace of St Vrain Creek. The Piney Creek Alluvium most likely overlies sandstone and claystone bedrock of the Denver Formation. Exploratory borings drilled by Northern Colorado Geotech (NCG, 2003) primarily along the southern boundary of the site, encountered three to seven feet of overburden topsoil and silty clayey sand overlying sand and gravel ranging in depth from 17 feet to the maximum depth drilled of 20 feet. Weathered siltstone/claystone was encountered in one boring at 17 feet in depth below the existing ground surface. 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. SITE GROUNDWATER Civil Resources conservatively modeled the groundwater at five feet below the ground surface, based on the NCG geotechnical exploration completed by Terracon in September 1997. 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 Bestway Concrete &Aggregates are listed below: • Last Chance Ditch relocation (western, northern, and eastern boundaries) • WCR 11.25 (western boundary) • Stinar of Longmont, residence,sheds, barn,dairy milking parlor, metal grain bin, and fences(western boundary) • Vogl Trustees of Longmont, residence and utility shed (northwestern boundary) • Overhead power line (northern boundary) • WCR 26 (northern boundary) • Kerr-McGee Burch 3-1A well (northeast corner of site) • Rural Ditch Return relocation (eastern boundary) • Rural Ditch Return—Phase 1 (eastern half of site) • Kerr-McGee oil road (southern boundary) • Rural Ditch Return relocation (southern boundary) • Kerr-McGee gas line (southern boundary) • Lee of Longmont(Globoker Owner), residence, sheds, dairy milking parlor,grain bin (southwest boundary) STABILITY ANALYSES The stability of the proposed mining limits was evaluated at six sections under anticipated loading conditions around the perimeter of the site and is discussed below. The computer program XSTABL 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 is used to find the critical failure surface by randomly searching with 12 termination points and 12 initiation points (144 failure circles)with 7 foot line segments over a broad range of the slope surface. This procedure is repeated over different initiation and termination locations until the most critical factor of safety failure surface is identified. The range is narrowed and 12 initiation points and 12 termination points (144 failure circles)with 7 foot line segments for the final run of 144 circles to determine the lowest factor of safety.