Laserfiche WebLink
::. <br />~: <br />Memorandum <br />MATERIALS AND GEOTE CHNICAL BRANCH <br />GEOTECHNICAL PROGRAM ~T <br />4ZOI EAST ARKANSAS AVENUE, DENVER, COLORADO $0222 303-757-9275 PAX 303-757-9242 ~~ <br />munie~ o u.~rwmu <br />US-85 <br />Ft. Lupton Pit <br />TO: Gary DeWitt, Region 4 Materials <br />FROM: Mark Vessely, Geotechnica] Program <br />DATE: April 22, 2005 <br />SUBJECT: Pit Slope Review Comments <br />As requested, the CDOT Geotechnical Program has reviewed the provided documents for the <br />slope stability of the excavation slopes at the proposed Ft. Lupton Open Pit Mine. Reviewed <br />geotechnical documents include: <br />A Division of Minerals and Geology Memo with stability model output figures dated <br />March 23, 2005. <br />The Civil Resources update letter dated March 22, 2005 and regarding slope analysis <br />changes. <br />A letter report from Civil Resources dated September 27, 2004 presenting the initial <br />slope stability analysis and minimum set back differences. <br />Representatives from the Geotechnica] Program also attended a meeting on April 1, 2005 with <br />other CDOT personnel and representatives of Civil Resources. <br />The Geotechnical Program reviewed the provided model data and also recreated portions of the <br />analysis using our software. The proposed slope geometry consists of a 50 foot high cut with a <br />1:1 slope and 50 feet of horizontal separation between the slope crest and US 85. The toe of the <br />slope is proposed to be located 100 feet from the highway. In the interest of brevity, our <br />opinions are as follows. <br />a. The Federal Highway Administration (FHWA) recommends a minimum factor of <br />safety of 1.3 to 1.5 for excavation slopes and various temporary earthen structures on <br />highway projects. Based on this guidance, CDOT is off the opinion that a minimum <br />factor of safety should be 1.3 and the critical slope crest is the edge of the right-of- <br />way. <br />b. The presented analyses detail circular failures through the granular materials. In our <br />experience, planar and block failures are mare probable in frictional materials with no <br />cohesion. A brief block analysis indicated factors of safety of near 1.0 for block <br />failure surfaces sliding on the claystone. <br />c. Regardless, both failure analysis methods indicate lower factors of safety for smaller <br />Failures more near the slope face. This is a more probable scenario. The modeled <br />\ViuhNroject Files\085 - 239 - F[ Lup[on Pit Review\Slope stability wmmrnts.doc <br />