Ch. 7] SLIDES 137 



They state that, in homogeneous, cohesive soils, material with 



a shearing resistance, 



s = c + p tan 4> 



can stand with a vertical slope at least for a short time, provided the height of 

 the slope is somewhat less than H c [the critical height]. If the height of the 

 slope is greater than H c , the slope is not stable unless the slope angle /3 is less 

 than 90°. The greater the height of the slope, the smaller must be the angle /3. 

 If the height is very great compared to H c , the slope will fail unless the slope 

 and /3 is equal to or less than 4>. 



Terzaghi and Peck say that slope failures in cohesive material are 

 preceded by tension cracks near the top of the slope and that at some 

 subsequent time sliding along a concave surface occurs. When the 

 failure on this surface intersects the toe of the slope or some point 

 above the toe, the slide is known as a slope failure. If failure occurs 

 on a surface some distance below the toe of the slope, it is- known as a 

 base failure. In Sharpe's classification both of these types would be 

 termed slump. 



Terzaghi and Peck further point out that because failures of slopes 

 are common during the construction period, such failures can be con- 

 sidered as large-scale shear tests, thereby offering an opportunity for 

 evaluating minimum shearing resistance and avoidance of similar 

 failures by a design change in further slopes in that vicinity and ma- 

 terial (see Fig. 1). 







Fig. 1. Slumping due to slope failure. (After Terzaghi and Peck, 1948, p. 182.) 



. . . The depth z c of the tension cracks and the shape of the surface of sliding 

 are determined by field measurements. The line of sliding is then replaced by 

 the arc of a circle having a radius r and its center at 0. Equilibrium requires 

 that 



from which 



