2015-04-13_REVISION - M1974086 (6)

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BRIERLEY ASSOCIATES Creating Space Underground April 2, 2015 File No.: 514104 -000 Asphalt Paving Co. 14802 W 44th Ave Golden, CO 80403 Attention: M.L. Richardson Director of Strategy and Sustainable Development Subject: Geotechnical Stability Evaluation Ralston Reservoir Golden, Colorado Ladies and Gentlemen: Brierley Associates (Brierley) has conducted additional geotechnical evaluation for the Asphalt Paving Co. (APC's) rock quarry located at 6569 Highway 93, just west of the intersection with West 64th Parkway, in Golden, Colorado. Brierley has previously produced a Geotechnical Evaluation and Initial Opinion Report — Phase 1, dated August 2011, and a Geotechnical Evaluation Report - Phase II, dated November 2013, for the same site. Based on discussions with APC and Greg Lewicki and Associates, PLLC on July 8, 2014, Brierley understands that the Division of Reclamation, Mining and Safety required minimum factor of safety (FOS) is 1.3 for mine slopes. Brierley has evaluated the allowable slope configuration based on the minimum FOS to allow for flexibility for the quarry operator during mining. This letter presents the results of this analysis. 1.1 Background and Understanding The Ralston Quarry is operating within the western half of a basalt intrusion called the Ralston Plug. Lava was extruded through the Pierre Shale from the Ralston Plug during Paleocene time, resulting in an altered zone of the Pierre Shale adjacent to the Ralston Plug. The contact between the altered Pierre Shale and the basalt has been historically mapped as dipping from 60 to 80 degrees to the east (Drewes, 2006). Brierley confirmed this dip range on the west side of the plug during field mapping and at depth from geotechnical borings. Figure 1 illustrates a schematic geologic cross - section and shows the shale /basalt contact at depth. Observations of the shale /basalt contact indicate the arrangement of the contact undulates irregularly both laterally and vertically. The southern end of the quarry limits will also be dictated by the shale /basalt contact. Traces of the contact were evident at the surface, but the contact was not encountered at depth. It is anticipated the dip of the contact is near vertical or dipping to the east. In general, on the west side of the mine a thin veneer of basalt overlaying the shale has been left in place during mining operations. It is in the best interest of the quarry to prevent contamination of the basalt by keeping shale out of the mined material. We understand mining 990 S. Broadway, Ste. 222, Denver, CO 80209 1303.703.1405 1 www.BrierleyAssociates.com