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S*6-01 AAA iS ("A h"ek <br /> June 17, 2019 <br /> Mr. Mark Johnson, Compliance Manager <br /> Burnco Colorado, LLC <br /> 301 Centennial Drive <br /> Milliken, Colorado 80543 <br /> Re: Stability Analysis, Pond 2, Bernhardt Mine <br /> Dear Mr. Johnson: <br /> This letter has been prepared to address the Mined Land Reclamation Board (MLRB)Construction Materials Rule 6, <br /> Section 4, Subsection 19, Exhibit S-Permanent Man-Made Structures(6.4.19, Exhibit S)for the Bernhardt Gravel Mine <br /> located north of Weld County Road 48 and east of Weld County Road 23 in Weld County, Colorado. This letter <br /> describes the project and slope stability analyses utilized to evaluate the minimum distance between the edge of mining <br /> and adjacent structures. <br /> The site is located in part of the south half of Section 1, Township 4 North, Range 67 West of the 6th Principal Meridian in <br /> Wels County, Colorado. The mine consists of five(5)cells. Cells 1, 2, and 3 have been previously mined. Cell 4 is <br /> currently being mined. Cell 5 has yet to be mined. Cells 2, 3,4, and 5 are lined with slurry walls and will be reclaimed as <br /> below grade, lined reservoirs. The mine is shown on Figure 1. <br /> PREVIOUS STABILITY ANALYSES <br /> Previous stability analyses performed by Tetra Tech were documented in a letter report dated November 19,2002. <br /> Subsequent stability analyses were performed by DRMS staff and the mine permit was issued with the understanding <br /> that a setback of 55 feet would provide adequate stability for neighboring structures. The previous Tetra Tech analyses <br /> assumed mining would be at a slope of 0.5:1 (horizontal to vertical, h:v). The DRMS analyses assumed mining at <br /> vertical slopes. The mine operator mines with perimeter mine slopes of 311 (h:v). <br /> STRUCTURE ENCROACHMENT <br /> The original mine plans called for a power line located on the interior of Cell 2 to be removed. Thus the original mine <br /> plan did not provide a set back from the power line. The power line was never removed and the mine operator mined to <br /> within approximately 23 feet power line. Civil Resources performed new stability analyses(discussed herein)with the <br /> 3:1 perimeter mine slope used by the operator. The analyses indicate the power line is stable. <br /> GEOLOGY <br /> The site is located in the alluvial valley of the Big Thompson River approximately four(4) miles upstream of its <br /> confluence with the South Platte River. Geologic mapping by Colton (1978)indicates most of the mine will in the Post <br /> Piney Creek and Piney Creek Alluviums. A small part of Cell 4 may be in the Broadway Alluvium. The mapping <br /> indicates the near surface bedrock underlying the alluvium is either the Fox Hills Sandstone and/or claystones and <br /> sandstones of the Upper Transition Member of the Pierre Shale. Outcrops of the Fox Hills Sandstone form bluffs on the <br /> north side of the alluvial valley. <br /> Data from drilling performed at the time of the original permit indicates the general subsurface profile consists of <br /> approximately two(2)to ten(10)feet of overburden sandy clay to clayey sand overlying approximately eight(8)to 35 <br /> feet of sand and gravel with local clayey and silty sand lenses on top of claystone and sandstone bedrock. The alluvial <br /> deposit is thickest in Cell two(2) and thinnest in the north parts of cells 1 and 5. <br /> From a geotechnical standpoint,the sand and gravel will form most of the mine slope. These soils are generally strong <br /> and stable particularly when dewatered. <br />