2016-08-29_REVISION - M1985001 - Laserfiche WebLink

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16)Where there is the potential for off-site impacts due to failure of any geologic structure or constructed earthen facility, which may be caused by mining or reclamation activities, the Applicant shall demonstrate through appropriate aeotechnical and stability analyses that off-site areas will be protected with appropriate factors of safety incorporated into the analysis. The minimum acceptable safety factors will be subject to approval by the Office, on a case-by-case basis, depending upon the degree of certainty of soil or rock strength determinations utilized in the stability analysis, depending upon the consequences associated with a potential failure, and depending upon the potential for seismic activity at each site. Objective: To determine and establish criteria for safe mining and reclamation of the site which maximizes extraction of construction materials and provides maximum post-reclamation usable work areas for future development and use of commercial activities (including light industrial operations) on the reclaimed floor of the pit, without creating on- or off-site impacts due to lack of slope stability. Situation and Setting: The J&J Pit is located on the eastern portion and slope of Ewing Mesa, above the Animas River Valley, with geology shown in Figure 1, area (geologic) hazards in Figure 2, and soils shown in Figure 3. This has been modified by man since development of the "waste land" site began in the early 1980s and extraction of construction materials in 1985. Pre-1985 mapping (Figure 4) shows that the original terrain was similar to that to the south on the Helmericks property. A typical cross section of the geology and associated soils is shown in Figure 5E. In general: 9. The top of Ewing Mesa is loess-derived soil, from wind deposits and sands and silts. Depth ranges from five to six feet thickness, including the topsoil derived from it. 10. This loess rests on a thick, calcareous bed of terrace deposits and glacial till (outwash), ranging in size from glacial dust (silt) to large boulders, but primarily consisting of a gravelly to sandy material with occasional sand and clay lenses.2 Although the standard angle of repose for loose, tipped material of this type is 450, the cementing allows a much steeper stable angle of repose, to as high as 800. 2 . This is locally classified as"Qa"Stream-channel, flood-plain, and low terrace deposits(Holocene)" and/or"Qt"Terrace deposits (Holocene) and [extract from USGS reports)includes modern stream channel deposits of the Animas River, adjacent flood-plain deposits, and low-terrace alluvium. The unit consists of poorly to moderately sorted, clast-supported, unconsolidated bouldery and cobble gravel in a sandy or silty matrix. Locally the gravel is interbedded with or overlain by sandy silt and silty sand. Clasts are well rounded to subangular. Deposits along the Animas River contain clasts with diverse lithologies, reflecting the wide variety of bedrock formations that crop out within the drainage basin. Clasts along the Animas River consist of more equal percentages of sandstone, igneous and metamorphic rock type, and reflect a larger, more diverse provenance area. The maximum thickness of alluvium in the Animas River Valley is 70 ft(water well records, Colorado Division of Water Resources, 1999)and more than 250 ft deep just north of the Animas City end moraines. Unit is a source of sand and gravel and is currently being mined (1998). More detailed geological investigation and mapping (immediately to the north)distinguishes between various types of terrace and outwash deposits, which share generally similar characteristics but differ in origin from water and glacial deposits. Technical Revision to M1985-001 J&J Pit, LaPlata County, dated 24 AUG 2016 Page 24