Ch. 5] 



SLOPE-STABILITY ANALYSIS 



103 



face is considered valid because detailed landslide studies have con- 

 firmed that most surfaces of sliding are strongly concave upward, al- 

 though their exact form may be conditioned by such geological factors 

 as bedding, fissures and fractures, the shape and distribution of dif- 

 ferent types of rock and soil, and the local concentration of pore-water 

 pressures. 



The force tending to produce sliding in Fig. 2 is the weight W of the 



W= Weight of soil 

 S = Shea ring strength 



Fig. 2. Slope-stability analysis. 



Disturbing moment W\Di 



Resisting moment=SiLi+S2L2 + S3L3+W 2 jD 2 



r- , , , , resisting moment 



Factor of safety = 



disturbing moment 



See text for description; also Taylor (1948, pp. 

 406-479). 



wedge of soils lying above the arcuate sliding surface and to the right of 

 the vertical through the center 0. It is measured as a moment about 

 the center of the circle. The force tending to resist sliding, or the 

 resisting moment, is the total shearing resistance which can be mobilized 

 along the entire length of the sliding surface plus the weight W of the 

 soils lying to the left of the vertical through the center and above 

 the sliding surface. It is also expressed as a moment about the center 

 0. The lever arms for both moments are the horizontal distances 

 between the centers of gravity of the two soil masses and the vertical 

 through the center 0. 



In this analysis the total shearing resistance is computed by using 

 Coulomb's equation, in which the values for cohesion and angle of in- 

 ternal friction are known from the testing results, and the normal 

 pressures on the sliding surface are scaled or computed from the draw- 



