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_.. <br />Tom Gillis <br />-7- February 15, 1982 <br />proposal. No material strength laboratory testing results are reported, <br />with wiu ch tc a.:alyz~ the pile's static slope safety factor. <br />Two mathematical slope stability analyses are included within the supplem`ntal <br />submittal. The first analyzes a potential planar slip failure along the <br />foundation/waste material contact. Assumino a conservative angle of internal <br />friction of 2vo and a 0 cohesion, t:,is stability analysis projects a "weighted <br />area safety factor" of 7.58, ~~ith a minimum cross-section static slope safety <br />factor of 1.47. Considering the conservative nature of the calculation <br />technicue utilized and the conservative material assumptions made, this analysis <br />should suffice to demonstrate that the static slope safety factor of a <br />projected failure along the foundational contact would exceed a required <br />minimum of 1.5. <br />The second mathematical slope stability analysis included within the supple- <br />mental submittal presents a :•ir:plistic mathematical calculation of the static <br />slope safety factor for ones :_rcular rotational surface passing tl,roueh the <br />pile's foundational layer. T:~'s analysis projects a static slope safety factor <br />of 1.4 for that surfaces if a.: angle of internal friction of 30° is assurr~d. , <br />Through back-calculation, t?.. applicant determined that an angle of internal <br />friction of 28.So would result in a static slope safety factor of 1. 3. <br />Unfortunately, Rule 4.09.1(7) requires that all underground development waste <br />and excess spoil piles attain a static slope safety factor of at least 1.5. <br />I believe that the applicant may be confusing the slope stability requirements <br />of the backfilI and grading regulations (Rule 4.I4.2 (I)(b)) with those of <br />the excess spoil and underground development waste pile regulations (Rule 4.09.1(7)). <br />The application is, therefore, still deficient. The applicant should repeat <br />the stability analysis for the underground development waste pi 1e. I do not <br />believe that difficulty will be encountered in demonstrating that the pile will <br />achieve a static slope safetu factor in excess of I.S, if an appropriate and <br />more sophisticated analysis is performed, based upon actual material strength <br />properties determined through appropriate laboratory testing. The use of <br />any one of a number of readily available computerized analytical slope stability <br />techniques would allow the evaluation of multiple failure surfaces, in order <br />to determine the critical surface of lowest static slope safety factor. A , <br />geotechnical analysis for the site should first be completed utilizing test <br />pits or angering, in order to determine the actual foundational materials and <br />stratigraphy. Samples of the rock and soil materials encountered should be <br />collected for Laboratory testing. Monitoring wells should be installed in <br />order to verify ground water elevation and occurrence. In addition, monitoring <br />wells should be installed above and below the proposed waste disposal site to <br />monitor ground water quality and elevation throuyhout the pile's life expectancy. <br />The possibility e:c sts, in this specific instance, that completion of the <br />required geotec'~,nical analysis and stability analysis could be stipulated to <br />follow approval of the permit. The applicant could be required to temporarily <br />deposit underground development waste materials within the existing site in <br />the east yard. Co:-pletion of the analysis and redesign, if necessitated, could <br />be stipulated ro be ne:_'ormed bit some: certain date, such as 6 months follo+ring <br />approval of tt.~e psr-i_, FOl_:::i:,:, 'Jpi:.'.'::1 of that analysis and redesian, the <br />applicant couZ:= t..-i .'... ~Ilo:,<,d _. ...._~.:~ deposition of material in the newly <br />approvsd site. Suc;: _-; app_cve_'. ~~r,v_:. ::-quire concurrence by other reeulatory <br />agencies involv:. -', ....ic:'. mi_a:t present .;o:-ne difficulty. <br />