2012-09-06_REVISION - M2008070 (25)

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Design and Operations Plan Western Gravel E & P Waste Disposal Facility 2.3.2.3 Bedrock Materials Bedrock samples were obtained from core holes TH -1 and TH -2 that were advanced into bedrock beneath the site during September 2010. TH -1 was advanced at the north side of Cell A and adjacent to the edge of the terrace slope above the White River and TH -2 was advanced at the west side and south end of the east leg of the gravel pit footprint. Based upon exposed bedrock at the site and obtained core samples, bedrock at the facility appears to be Wasatch Formation. Bedrock materials at the site include interbedded sandstone, claystone, and siltstone and compare well to published information. Based upon published information, the facility is located in the vicinity of the Wasatch Formation and overlying Green River Formation contact. Bedrock at the site has been identified as Eocene age Wasatch Formation. Wasatch in the area has been characterized as claystone, sandstone, siltstone, mudstone, and shale of mostly fluvial origin up to 4,000 feet in thickness. It is estimated that over 1,000 feet of Wasatch underlies the site. Sandstones located beneath the facility are characterized as stratigrahpically lenticular that are poorly hydraulically connected between units and therefore, poor groundwater producers . 2.3.2.4 Local Structural Geology Several northwest trending normal faults located in the area at both sides of the White River Valley parallel the river. The closest fault is located approximately 1/2 mile southwest of the site and displaces rocks of the Anvil Points and Garden Gulch Members of the Green River Form ation and the Wasatch Formation. Because of no apparent displacement of Quaternary age gravel deposits located northwest of the site, recent Holocene age fault movement is not suggested. Based upon obtained bedrock core samples discussed in Section 2.4.2, bedrock dip at the facility is consistent with published information. Regional low angle bedrock dip between 5 degrees to 10 degrees to the southwest trends towards the Piceance Basin center and away from the White River. 2.3.2.5 Geologic Hazards Geologic hazards (e.g. slope instability, faulting, rockfall, landslide, subs idence, erosion) have been evaluated and addressed in the landfill design. Based upon the assessment, geologic hazards do not appear significant at the facility. Geologic hazard evaluation results are discussed below. • Except for the terrace face located along the north and east sides of the landfill, vicinity slopes are gentle to moderate, ranging from approximately 2 to 8 percent. Bedrock outcrops exposed along the terrace face generally ranges from approximately 45 percent to nearly vertical. Based upon site observations and topography, potential slope instability and/or landslides appear low to remote. • A bedrock stability investigation was conducted along highlands overlooking the White River and to the south of the river. An area approximately one to two miles in circumference from the facility was evaluated. As part of the evaluation, an extensive data search was conducted in order to help identify geologic hazards in the area. Based upon field observations, features (e.g. rock slide, bedrock fracture displacement) indicative of structural bedrock failure were not observed nor identified during data search activities. Therefore, potential slope stability hazards and/or landslide hazards appear to be very remote. • Although bedrock outcrops on the terrace face are prevalent, rockfall hazards do not appear significant. • Since the landfill cells will be constructed on minimal (less than 10 feet) thicknesses of fill placed directly on bedrock, soil subsidence is considered remote. Minimal (e.g. up to five feet) bedrock NWCC, Inc. October 24, 2011 Page 9 of 51