2009-08-06_PERMIT FILE - M2009018

Home Browse Search

Memo to Michael Cunningham 2 August 6, 2009 Heintzelman Pit Slope Stability Review File No. M-2009-018 The shear strength parameters for sand and gravel input to the stability analysis provided are zero cohesion and an angle of internal friction of 49 degrees. The friction angle selected is extremely high. For example Design of Small Dams published by the U.S. Bureau of Reclamation (3rd Edition) includes Table 5-1 presenting average engineering properties of compacted soils. In that table, only poorly graded gravel with an average friction angle of 41.4 degrees approaches the friction angle used in the analyses provided. In that same table, average friction angles presented for silty gravel and clayey gravel are 34 degrees and 27.5 degrees respectively. The analysis provided incorporates an assumption that the bedrock underlying the sand and gravel deposit is high strength (cohesion of 700 psf, 28 degree friction angle) such that a slope failure would not penetrate into the bedrock. The DRMS has observed that the Pierre shale bedrock in the vicinity of the proposed pit is weathered to deeply weathered, and weathered shale can behave like a low shear strength soil. Also, the weathered shale in this location is over- consolidated so its shear strength may be further reduced by unloading as a result of gravel mining. For more than a decade, the DRMS has employed a standard set of shear strength parameters for analysis of pits in Adams and Weld Counties. A 2003 memo describing the employment of these parameters is attached. Unless the Applicant chooses to conduct shear strength testing of the onsite materials, the DRMS will require an analysis using the standard parameters to establish appropriate setbacks from protected structures. If the Applicant decides to conduct shear strength testing, the DRMS should be consulted on appropriate test methods. On another matter relating to adequacy of the application, DRMS has reviewed the Applicant's memo dated June 18, 2009 addressing the carrying capacity of the seep ditch. The memo was provided to address the issue raised in the DRMS preliminary adequacy review directing the Applicant to provide "specifications as to how much water the ditch can carry without experiencing erosion." The memo contains an adequate demonstration that the seep ditch has the capacity to convey anticipated dewatering discharges from the pit, but does not address the erosion issue. The DRMS will require an analysis of flow velocity in the seep ditch considering maximum dewatering discharge rates added to peak existing flows in the ditch, actual (not assumed) ditch gradient, and frictional attributes of the ditch perimeter. The velocity determined must be demonstrated to be non-erosive under current ditch conditions or the ditch must be improved as needed to prevent erosion. enclosure(s) cAacs files\My Documents 4-19-06 thru\heintzelman stability analysis review.docx