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<br /> <br />~ir'~1 ~ ~r COLOZ~DO <br />DIVISION OF ~b11NERALS AND GEOLOGY <br />D~:panmenl ~~i Natural Resources <br />I J I J Sherman S[.. Room ' I S <br />Denn•r, Q~lorado NO]UJ <br />Phone 17031 Nbfi-1567 <br />F:1\ 13031 N7'_-8106 <br />MEMO <br />~~ <br />DIVISION O~ <br />I~1INERAL <br />GEOLOGY <br />aECLA MAiIOh <br />MI NING~SAFETY <br />811 Owen, <br />Gmernnr <br />C.eq E. Walcner <br />E~ecunve Ovec~ar <br />DATE: June 1, 2000 .~~~n~el e. Lnng <br />Division Gveaor <br />TO: Tom Schreiner, North team, DMG D <br />FROM: Chris Kamnikar, North team, DMG 1 <br />RE: Slope Stability Report for Lafarge Tanabe Pit, M-2000-002 <br />Having reviewed the Slope Stability Analysis submitted by Tuttle Applegate, I have found the following items <br />which render it inadequate for use as a geotechnical slope stability analysis, and its conclusions erroneous. <br />Specifically: <br />1) The use of an intemal angle of friction of 13 degrees for weathered claystone. The Division's standard for <br />untested weathered claystone is usually 14 degrees for the internal angle of friction. While the difference <br />is relatively minor, the setback for the safety factor could still be effected by about five to ten feet. <br />2) No seismic loading was done for [he temporary slopes. Since the safety factor for the reservoir was <br />affected by -0.4 in the reservoir model when this was added, it can be assumed that it is not an <br />insignificant factor. Despite the fact that they are temporary, the horizontal and vertical seismic loading <br />values must still be included in the slope stability analysis of these slopes. <br />3) The cohesion value used in every case for sand and gravel was 200 psf. This value is inconsistent with <br />that which has been approved for use in previous Division of Minerals and Geology stability analyses, and <br />is inconsistent with industry cohesion standards for sand and gravel. In the absence of empirical data to <br />the contrary, a default value of 0 psf is used (or 2 psf, as is indicated in Table 3 but never implemented, in <br />cases where the modeling sofnvare requires a positive value). If on-site data for soil cohesion at the site is <br />going to be gathered, the tests to be implemented must be approved by the Division in order for the data to <br />be considered verifiable and accurate. <br />Because of these flaws in the slope stability analysis, the conclusions drawn and proposed setbacks can not be <br />approved by the Division, since they do not meet the minimum standards for technical adequacy. Atthe very <br />least, a setback of twice the depth of the excavation should be proposed, since past slope stability analyses <br />have found that a saFety factor or I.0 to 1.05 can be obtained by maintaining this setback distance from the lip <br />of most excavations similar to this one. <br />I hope this answers your questions regarding this slope stability analysis. <br />Christina <br />