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<br />r,^ <br />J,,'u <br />9.7 <br /> yN. Tan <br /> <br /> <br />c i <br />I <br />i <br />yHF <br />Tan 4 <br />F <br />Find this value on the outer circular scale of the <br />chart. <br />W ?{ Step 3: Follow the radial line from the value found in step 2 <br />to Its Intersection with the curve which corresponds <br />to the slope angle under consideration. <br />?? Q ?7? Step 4: Find the corresponding value of TanO/F or Off, de- <br />pending upon which is more convenient, and calculate i <br />the factor of safety, i <br />cJ,? C+ Consider the following example: <br />?"'???s }y^?e A 50 ft. high slope with a face angle of 400 is to be excavated <br />in overburden soil with a density r= 100 Ib/ft 9 , a cohesive <br />CA its strength of 800 lb/ft! and a friction angle of 30° Find the <br />factor of safety of the slope, assuming that there is a surface <br />water source 200 ft. behind the toe of the slope. <br />.i. <br />Figure 9.4 : Sequence of steps involved in using circular failure <br />charts to find the factor of safety of a slope. <br />The groundwater conditions indicate the use of chart No. 3. The <br />value of c/7H.Tanjd = 0.28 and the corresponding value of Tano/ <br />F, for a 40° slope, Is 0.32. Hence, the factor of safety of <br />the slope is 1.80. <br />Because of the speed and simplicity of using these charts, they <br />are Ideal for checking the sensltlvity of the factor of safety <br />of a slope to a wide range of conditions and the authors sug- <br />gest that this should be their main use. <br />J