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"ANk <br />MI I '"Oil "I; <br />September, 20, 2012 Project 74201125G <br />Timm Comer <br />Cripple Creek & Victor Gold Mining Co. <br />P.O. Box 191 <br />Victor, CO 80860 <br />Re: Mill Platform Machine Vibration Effects <br />Dear Mr. Comer: <br />Machine vibrations associated with the ball mill and rod mill at the proposed plant site will impart stresses <br />to the underlying soil strata. Concern has arisen that these stresses could (1) densify granular foundation <br />fills, (2) produce strains on the geomembrane liner located beneath the foundations, and (3) contribute to <br />instability in nearby slopes. This letter addresses these concerns through analysis and literature review <br />and presents our opinion. <br />Subgrade Response to Machine Vibrations <br />The settlement of granular soils from repeated vertical loading on foundations from machine vibrations <br />have been investigated by several laboratory studies (Raymond and Komos, Brumund and Leonards). <br />The studies show that settlement is a function of machine vibration amplitude and number of load cycles. <br />Brumund and Leonards (1972) showed that settlement is a function of energy transmitted to the soil from <br />the machine vibrations. Each of the aforementioned studies were conducted in laboratory settings with <br />scale foundations and idealized sand. Techniques of extrapolation of settlement of prototype foundation <br />from the laboratory model tests are not currently available. <br />Methods for predicting settlement in dry sands due to cyclic shear stress have been published by Silver <br />and Seed (1971) and state that the controlling parameters for settlement include (1) relative density, (2) <br />maximum shear strain induced, and (3) number of shear cycles imparted on the soil. This method utilizes <br />a dynamic analysis of the cyclic motions (typically ground accelerations from the design earthquake) to <br />construct a profile of average shear strain due to cyclic loading with depth. Once the shear strain profile <br />has been constructed, laboratory simple shear tests on representative soil specimens are conducted to <br />evaluate the vertical strain resulting from cyclic strain at corresponding vertical stresses. The total <br />settlement is calculated as the sum vertical strains times the corresponding layer thicknesses. This <br />method involves extensive laboratory testing and can be implemented only with materials that can be <br />tested with traditional laboratory equipment therefore is not suitable for our application. <br />For our evaluation, the computer software QUAKE= by Geo -Slope (2008) was used to conduct finite <br />element analyses of the cyclic foundation vibrations with an equivalent - linear elastic model. The <br />equivalent - linear elastic model modifies the soil stiffness in response to computed strains. This model <br />accurately estimates elastic strains from the dynamic stress imparted by the foundation vibrations but <br />AMEC Environment and Infrastructure <br />2000 South Colorado Blvd. <br />Suite 2 -1000 <br />Denver, Colorado 80222 <br />Tel: (303) 935 -6505 <br />Fax: (303) 935 -6575 www.amec.com <br />S:\projects \1125g squaw gulch valley leach facililty design\h2 - design \mill platform machine vibration letters sept 2012 \mill platform machine vibrations (3).doc <br />