2005-12-13_REVISION - M1992054 - Laserfiche WebLink

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_ ~,~ TRAUTN ER 1SlON OF RIGON9L~N .+'C Mr. Matt Carnahan 1~P.0. Box 1951 Bayfield, Colorado 81122 Subject: Proposed Ignacio Gravel Pith Slope Stability Comments v' Neaz La Boca, Colorado ~G~~v~a D~ember 8, 2005 Of0 ~ 3 ?005 ~" OF~C~/~~D F~e,a DivisiOUaf~i?RS'als~~~ O'Y~'on~f6fi~~~~~ `~ C'@0(OgY ~~eo PN: 503i°d-3PE IM- 19gL -Oda--V (shVerS~ ow GN -'ol/ Mr. Carnahan, This letter presents our comments based on our November 14, 2005 site meeting and observations with you at the subject proposed gravel pit and our December 8, 2005 telephone conversation. As requested this letter provides comments in accordance with the Colorado Division of Mining and Geology, Rule 6, 6.14.19(b), regarding the proposed post-mining geometry of the gravel pit slopes neaz the existing subsurface natural gas pipeline. We understand that the natural gas pipeline is about four (4} feet below the ground surface and the crest of the post-mined gravel pit slope will be about 20 feet from the pipeline. The proposed slope inclination will be 3:1, horizontal to vertical. The gravel pit is located on a terrace that generally consists of about 2 to 4 feet of overburden overlying a 10 to 12 foot thick gravel deposit. The total post-construction mined depth wilt be about 16 feet maximum. Using the proposed 3:1, h:v slope geometry, the depth of mining, and the 20 foot set-back of the slope crest from the pipeline, the toe of the slope will be about 68 feet from the pipeline. We performed a cursory analysis of the proposed post-construction slope stability conditions at the project site. Our analysis is based solely on our surface observations, our experience with similaz projects, and on assumed soil strength values, therefore the analysis should be considered appropriately. There aze numerous methods and techniques available for slope stability analysis. Most methods include an evaluation of; • the strength of the soil materials within the slope, • anisotropies within the slope materials, such as formational material bedding planes, and anomalous soil contacts, • the subsurface water and soil moisture conditions, and, • the pre-construction and post-construction geometry of the slope areas where development and construction is proposed. ., Y ;-4 N }~ G EDTECH NICAL ENGINEERING, MATERIAL TESTING AND ENGINEERING GEOLOGY 2 i 4 8oD0 DRIVE DIJRANGO, COLORADO 81 303 970-259-5095