2019-09-30_REVISION - M1988108 - Laserfiche WebLink

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Arcosa,Inc. ARCOSA Exhibit D — Mining Plan Shale feed for the lightweight aggregate plant is mined from an existing quarry located in Section 5, TIS, R70W. As reserves are depleted, the mining proceeds to the south by removing overburden to expose additional shale to feed the process plant. The removed overburden will be used directly in reclaiming the mined-out portion of the quarry. Geology The regional and site-specific geologic features include surficial geologic units and bedrock geologic units. Surficial geologic units are shown on Exhibit D-1, and bedrock units are shown on Exhibit D-2. These Exhibits were obtained from the Engineering Design and Operations Report prepared by Sergent, Hauskins & Beckwith in 1992 for the Western Aggregates, Inc (former owner) proposed Shale Mine Ash Backfill Project' and updated for this permit application. Material to be mined as feedstock for the lightweight aggregate processing plant is shale of Cretaceous Age contained in the Pierre Shale Formation, which is discussed below. Surficial geologic units within the affected lands boundary shown on Exhibit D-1 include artificial fill placed by human operations, Slocum Alluvium and Verdos Alluvium. The vast majority of the surficial geology within the affected lands and mine quarry area is Slocum Alluvium which consists of 5 to 20 feet of silty, sandy and cobbly gravel that was deposited as a fluvial lag deposit during the middle Pleistocene (SHB, 1992). As mentioned above, bedrock units within the affected lands consists of the Pierre Shale formation which is subdivided into four units. The quarry area is almost entirely within the upper shale unit, and the processing area and affected lands west of State Highway 93 lie within the upper transition member portion of the formation. The Pierre Shale formation is bedded and dips an average of 56 degrees toward the east at the quarry site and is interspersed with thin beds of limey sandstone and relatively small concretionary calcareous lenses. The Pierre Formation at this location is approximately 8,000 feet thick. As shown in Exhibit D-2, the quarry in cross section only penetrates a small fraction of the upper unit of the Pierre Shale formation. The shale is generally dark greenish-grey to black in color, is well consolidated, quite soft and contains approximately 10% inherent moisture. Additionally, the material deteriorates rapidly when exposed to the natural elements, precluding any possibility of maintaining more than a few days' stockpile between mine and plant. Sufficient reserves of the shale are available within the affected lands area to sustain plant operations for approximately 18 years. The Pierre Shale Formation is extremely impervious as evidenced by laboratory and field testing. Table D-1 was extracted from a report prepared for a proposed landfill located in Douglas County on the Pierre Shale Formation. As indicated, permeabilities were measured by two methods, a packer test and a recovery test. The results indicated show hydraulic conductivities ranging from 10 -6 to 10 -8 cm/sec. These extremely low conductivities coupled with the vast thickness of remaining underlying Pierre shale make the quarry an ideal location for backfilling off-spec materials. The proposed Ash Backfill project was approved by the Colorado Department of Health in 1993, however the project was dis- approved by Jefferson County. The comprehensive summary of regional and local geology, hydrogeology and other pertinent information,including Exhibits and exhibits,from the Seargent,Hauskins&Backwith design report are discussed in this permit.