Laserfiche WebLink
L.G. Everist <br /> Response to DRMS Comments on Cannon Lands Slope Stability <br /> percent of Standard Proctor density at the optimum moisture content. The tests were performed as <br /> consolidated, undrained tests with pore pressure measurements. We interpreted the test results using <br /> Mohr circle plots and different failure criteria, including maximum principal stress ratio, maximum deviator <br /> stress, and 10 percent strain. Depending on the criteria, interpreted effective cohesion ranges from 150 <br /> to 750 pounds per square foot and the effective friction angle ranges from 18.5 to 30 degrees. The lab <br /> data from this testing is attached to this memorandum. <br /> Earlier testing was done on the Laramie bedrock at the Fort Lupton Pit in 2009 in association with design <br /> of a pump station, interconnect tunnels, and discharge conduit when several mined cells were converted <br /> to water storage reservoirs. This data is also attached. The triaxial test indicated an effective cohesion <br /> ranging from 150 to 750 pounds per square foot, and an effective friction angle ranging from 18.5 to 30 <br /> degrees depending on the failure criteria used for analyzing the raw data. <br /> The best method for obtaining the fully softened shear strength is to utilize the geometry and pore water <br /> pressures of a field failure, set the factor of safety at 1.0, and back-calculate the shear strength. A field <br /> failure serves as a large-scale test that provides more accurate results than a small laboratory test. This <br /> back-calculation analysis was done for a slide on an inner berm (cell divider) of Women Creek Reservoir <br /> that occurred in October of 2011. This reservoir is located on Laramie bedrock. Geometry of the slide <br /> was measured in the field and the water levels were known from the nearby wells. The resulting back- <br /> calculated shear strength for the bedrock was an effective cohesion of zero and effective friction angle <br /> ranging from 18 to 21 degrees. This correlates well with analyses we have performed on other dams in <br /> the Laramie Formation, such as Standley Lake and Great Western Reservoir. Bedrock strength testing <br /> for both Standley Lake and Great Western are also attached. <br /> With the supplied strength test data supporting that the strength parameters we have selected (effective <br /> friction angle of 18 and effective cohesion of 0 for fully softened bedrock and 500 for fresh bedrock) are <br /> conservative for Laramie bedrock at Cannon Lands, we stand by the use of the lower factors of safety <br /> from MLRB Policy No. 30 (1.3 for static and 1.15 for seismic). We have demonstrated through industry <br /> standard geotechnical index properties, published geologic mapping and physical proximity that the sites <br /> can and should be considered geologically and geotechnically the same. Therefore, the data supplied in <br /> this and the previous memorandum are sufficient to provide a high degree of confidence in the strength <br /> parameters used in the Cannon Lands slope stability analyses. We propose no further testing is <br /> necessary. The presented slope stability analyses are a conservative yet realistic representation of slope <br /> stability for the Cannon Lands site, based on actual site conditions. Additional testing of the Laramie <br /> bedrock will, in our professional opinion and past experience, result in higher strength parameters than <br /> we are currently using. <br /> 2. Seismic Evaluation <br /> The 2014 USGS National Seismic Hazard Map is attached. The highest and lowest hazard zone <br /> correlate to the two-percent probability of exceedance in 50-year map of peak ground acceleration, which <br /> is also attached. This map shows peak acceleration as a fraction of standard gravity. Faint contours of <br /> peak acceleration are visible on this map. We have obtained the contours from the USGS as a shape <br /> file. When the shape file is loaded into GIS a more detailed and interactive view of the contours is <br /> available. On the attached detailed map, the City of Fort Lupton is shown as a large yellow dot. Cannon <br /> Lands is located just north of Fort Lupton, equal distance between the 8 and 10 contours (or 0.08 and <br /> 0.10 g). We have selected 0.09 g based on this detailed map. We chose 2 percent in 50-year because it <br /> is the longest earthquake recurrence interval available, and thus represents the most conservative <br /> analysis. <br /> Project DA201041.00 6/25/2020 Page 2 Deere&Ault,a Schnabel Engineering Company <br />