2009-03-06_REVISION - M1987049 (11)

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BRIERLEY October 19, 2007 Project No. 00036-003 ASSOCIATES Mr. Bill Timmons Limited Liability Company General Manager "Creating Space Underground" Ready Mixed Aggregates 14585 Old Brighton Road Brighton, CO 80601 Re: Ready Mixed Pit No. 1 Slope Stability Analysis at Tower Pit Boundary Dear Mr. Timmons, As requested, Brierley Associates has performed a global stability analysis of the proposed mined face north of Tower Lake, which comprises the subject quarry's southern boundary. The analysis was done using the Spencer "method of slices" within the limit equilibrium computer program "'S1opeW". We started by back-analyzing a full height, vertical excavation face without the influence of the lake to determine appropriate soil shear strength properties based on observed field conditions. Because these excavations are known to stand for months to years with only minor sloughing, we assumed a factor of safety of slightly over one and determined that reasonable soil properties for the sand and gravel layer are a friction angle of 45° with cohesion of 200 psf. Using these properties, we then analyzed the proposed mining excavation near the lake, considering potential slope failure planes between the property line and the mined excavation. Assuming a minimum allowable factor of safety against global slope failure of 1.3, we determined that the minimum offset distance from the property line to the crest of the mined slope is approximately 25 ft. As shown on the attached calculation, the potential failure surface originating from the toe of the excavation and exiting at the ground surface just inside the property line has a factor of safety greater than 1.3. Potential failure surface that exit to the right of this surface, i.e. larger failure wedges closer to the lake, have a higher factor of safety, while smaller potential failure surfaces closer to the excavation have lower factors of safety. However, if the slope does fail, it will 2329 West Main Street likely occur near the theoretical Rankine failure angle for a frictionless slope (= Suite 201 45 degrees + friction angle divided by 2 as shown on the calculation). This failure slope will likely be well within the property line. Littleton, CO 80120 It should be noted that this analysis also indicates that near vertical, full height 303-703-1405 faces in the sand and gravel layer are only marginally stable. Therefore, the areas Fax: 303-703-1404 on the mine floor at the bottom of this slope should not be considered safe and it should be assumed that sloughing of material from the crest of the slope could www.brierleyassociates.com occur at any time.