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• <br />• <br />• <br />• <br />• <br />• <br />• <br />• <br />• <br />• <br />• <br />• <br />FOUNDATIONS AND RETAINING FALLS <br />step 9: Calculate the factor of safety, F, from <br />equation 10.49. For submerged clays, the <br />buoyant. force should be subtracted from the <br />clay density. <br />F = ^^°r 10.49 <br />PeHH <br />Pex =Peal - 62.4 (if submerged) 10.50 <br /> <br />~,~r~d <br />t ~~ <br />i l <br />l I <br />____. ___. °r~~rtia ~ / I ~ I <br />n°x ~~rci.. i <br /> <br /> tl ! ~ ~ I ' <br /> / / i ! I i i <br />~ ~ nrt~ ' ~ ~ <br />In ~ ~ i <br />f <br />~ <br /> <br />~„mew. <br />ma` ~ ~ <br />Po <br />~ 1 <br />' <br />I <br /> ar <br />i I / <br />~ OJ <br /> 1 <br /> <br /> I ~ /I I G , <br />1' / ,', ~. <br /> ~ V' 1 <br /> / _J_- - I <br />~ _ __ _ _ _ _;. x-... ~S SJ C.m <br />---- - ~~ <br /> I _ artle i <br />I <br /> <br />Example 10.5 <br />10-15 <br />An underwater trench is excavated in soft bay mud. <br />The walls of the trench are sloped at 4.5 vertical:3 hori- <br />zontal. The mud has a saturated density of 100 Ibm/ft3 <br />and a cohesion of 400 lbf/ft'. There is a layer of dense <br />sand and grave] 59 feet below the surface of the mud. <br />Vdhat is the depth of cut that can be used while main- <br />taining a 1.5 safet}• factor? <br />The cut. angle is <br />,~ <br />9.5 ° <br />Q = arctan 3 = 56 <br />ro <br />9 a Flom figure 10.13. the stability number for p = 56° is <br />approximately 5.4. <br />6 c <br />3 <br />a <br />m <br />> Z <br />° ~ ~~ <br />]0 b° 5° ro JV [V ~V ~ <br />s~ooe anB~e. 0. tleg rees <br />Figure 10.13 The Tav)or Chart for <br />Slope Stability (¢ = O) <br />Figure 10.13 shows that toe circle failures occur for al] <br />slopes steeper than 53°. For slopes less than 53°, slope <br />circle /allure, toe circle (allure, or base circle /allure may <br />occur. 't'hese possibilities are illustrated in figure 10.14. <br />H ~ <br />slope circle <br />H / <br />ID=01 ~/ <br />toe circle <br />~i <br />l <br />D ~ <br />~_. <br />base circle <br />Figure 10.14 Failure Modes in Clay <br />sand antl gravel <br />Since the clay is submerged, the effective unit weight is <br />(100 - 62.4) = 37.6. <br />From equation 10.49, the maximum cut depth is <br />H _ A'°c - (5.4)(400) = 38.3 ft <br />Fp (1.5)(31.6) <br />B. HOMOGENEOUS, COHESNE SOIL (c > 0, <br />~ > 0°) <br />For cohesive, non-granulaz soils, the failure will be <br />through the slope toe for ¢ > 5°. <br />PROFESSIONAL PUBLICATIONS, INC. ~ P,O. Box 199, San Carlos, CA 94070 <br />