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Memo to Jared Ebert 2 June 18, 2009 <br />Bromley Lakes Setback Review Permit No M1987 049 <br />Table 5-1 presenting average engineering properties of compacted soils. In that table, only poorly graded <br />gravel with an average friction angle of 41.4 degrees approaches the friction angle used in the AM-02 <br />analyses. In that same table, average friction angles presented for silty gravel and clayey gravel are 34 <br />degrees and 27.5 degrees respectively. <br />Back calculation to obtain shear strength parameters was done using the assumption that the vertical pit <br />walls had a safety factor of one based on the observation that there have been no bulk failures of any <br />vertical pit walls at the site. The back calculated shear strength values were then input to an analysis of a <br />1 H:1 V slope, and of course a safety factor substantially greater than one resulted. The back calculation <br />method to determine shear strength in this case ignored the critical consideration that most bulk <br />landslides and slope failures occur during extreme conditions such heavy rainfall, rapid snowmelt, or <br />freeze-thaw. Therefore, a slope that is stable most of the time would in fact have a safety factor of less <br />than one under critical conditions. One related attribute to this discussion is the presence during normal <br />conditions of a shear strength enhancement due to apparent cohesion which is created by matric suction <br />in unsaturated soil. Once a soil becomes saturated, the strength imparted by apparent cohesion is <br />eliminated. <br />The analysis provided in AM-02 incorporates an assumption that the bedrock underlying the sand and <br />gravel deposit is high strength such that a slope failure would not penetrate into the bedrock. The DRMS <br />has observed that the claystone bedrock in the vicinity of Bromley Lakes is weathered to deeply <br />weathered, and weathered claystone behaves like a low shear strength soil. Also, the weathered <br />claystone in this location is over-consolidated so its shear strength may be further reduced by unloading <br />as a result of gravel mining. <br />For more than a decade, the DRMS has employed a standard set of shear strength parameters for analysis <br />of pits in Adams and Weld Counties. A 2003 memo describing the employment of these parameters is <br />attached. Unless the Applicant chooses to conduct shear strength testing of the onsite materials, the <br />DRMS will require an analysis using the standard parameters to establish appropriate setbacks for the <br />AM-02 areas of the pit. If the Applicant decides to conduct shear strength testing, the DRMS should be <br />consulted on appropriate test methods. <br />enclosure(s) <br />cc: Tony Waldron, DRMS, via email and w/ enclosures <br />Peter Hays, DRMS, via email and w/ enclosures <br />CAacs files\My Documents 4-19-06 thru\bromley lakes setback review.docx