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372 PART IV SOIL WITH WATER-~l0 FLOW OR STEADS' FLOW <br />_'w <br />A <br />N <br />0 <br />A <br />0 <br />w <br />~lx! i I <br />Water level Water level <br />at surface Ir ROad before s6p <br />Railway ~ Road aker slip <br />200 --'Z - - ~ River bank before gip <br />t • ~ ~ ~ River bank after slip <br />Coal measure t.` <br />clay shales ~ <br />100 Approx. slip surtace -~~-_- <br />Average depth to <br />slip wrlace: 17 k <br />0 I I <br />n 100 200 300 400 500 600 700 800 <br />Cnainage: (h) <br />Fig. 24.19 Landslide at Jackfield, England (From Bishop and Bjerrum, 1960). <br />equilibrium aeain after only moderate movement, <br />another analysis was made to find the strength available <br />after failure. The box marked "natural slopes" indicates <br />conditions at nature-made slopes in which landslides <br />have recently developed. <br />1500 <br />t00o <br />a 500 <br />00 500 1000 1500 2000 2500 <br />a Ilb/Ita) <br />Fig. 24.20 Strength mobilized during landslidcs in London <br />clav (After Skempton, 1964). <br />The inference from these results is that only the <br />ultimate or residual streneth of the London clay can be <br />relied upon to sustain a slope; the additional streneth <br />that exists at the peak of the stress-strain curve is Inst <br />with time. Similar conclusions apparently apply (or <br />other overconsolidated clays which exhibit fissuring, <br />notably the clay-shales of the Dakotas, Montana, and <br />Saskatchewan. The processes that cause a gradual <br />reduction in strength to the residual value are termed <br />progressive failure. Hjerrum (1967) has discussed these <br />processes and the extent to which they may develop in <br />various overconsolidated soils. <br />24.9 DESIGN OF SLOPES <br />The inclination of most man-made slopes, such as <br />those along hiuhways, are chosen primarily on fhe basis <br />of experience. Analyses are made when an unusually <br />hieh slope must be cut, or when there is some reason to <br />believe that trouble might develop. Analyses are usually <br />Table 24.2 Safety Factors from Analysis of Landslides in Intact Clays <br />Computed <br />Location Soil Type Safety Factor Reference <br />Lodalen, Norway Lightly <br /> overconsolidated 1.05 Sevaldson (195 <br />Drammen. Norway Normally <br /> consolidated 1.15 Bjerrum and <br /> Kjaernsli (1957) <br />Selset, England Overconsolidated 1.03 Skempton and <br /> Brown (1961) <br />Breckenridge, Canada Lightly <br /> overconsolidated 1.12 Crawford and <br /> Eden (1967) <br />Siburua, Venezuela Compacted <br /> plastic clay 1.02 Wolfskill and <br /> Lambe (1967) <br /> <br /> stren65o <br />peaR sat <br />laJute <br /> Cut slope <br />t s <br />uen4~~ <br /> .' .. J R~sdo- <br />: <br />: <br /> , f, <br />t~--Aker failure <br /> Natural slopes <br />~° <br />A <br />:~; <br />,~ <br />4 <br />s <br />~~ <br />~s <br />~{{c <br />4 <br />rr <br />r` <br />