Ch. 13] STRESS DISTRIBUTION 239 



of possible clayey gouge or loose breccia along the fault surface to aid 

 movement. AVide fault zones, in which the material is highly brecci- 

 ated, are always potential sources of trouble in deep excavations of any 

 kind. Even geologic contacts between different types of rocks may 

 act as sliding surfaces. Sharp (1931) describes a large slide in Wyo- 

 ming in which Tertiary sand and gravel apparently slid along the 

 sloping contact with underlying crystalline pre-Cambrian rocks. 



In short, the effect of discontinuities of any kind must be evaluated 

 in the analysis of the stability of slopes. This is especially true if the 

 fractures are so disposed as to create an unfavorable distribution of 

 stress within the deposit. 



Stress Distribution 



Some general remarks on the distribution of stress within earth ma- 

 terials are perhaps in order. In determining the factor of safety of a 

 slope, one must compare two sets of forces: those that tend to produce 

 failure and those that tend to prevent it. The force of gravity is cer- 

 tainly the prime source of stress tending to produce failure, though it 

 may be aided by forces arising from frost action or from the hydration 

 of such minerals as gypsum or the clays. Gravity is a force that 

 affects failure because internal friction on any surface within the mass 

 depends on the effective normal stress on that surface. The resistance 

 to sliding on any arbitrary surface in general consists of two parts, 

 cohesion and friction, and is given by the well-known Coulomb 

 formula: 



s = c + p tan cj> 



The meanings of c, p, and <f> vary somewhat with the material in- 

 volved and the loading conditions. In clay and uncemented sands, 

 the cohesion c is zero; p is the normal pressure due to the weight of 

 overlying material; and <£, the angle of internal friction, is about the 

 same as the angle of repose of the sand. If the sand is cemented, it is 

 regarded as having a kind of cohesion. If it is moist, the sand has 

 an apparent cohesion due to capillary tension of the moisture films 

 between the grains. This vanishes if the sand is immersed. In addi- 

 tion, if the sand is immersed, the effective normal stress is equal to 

 the total normal stress lessened by the buoyant effect of the water on 

 the overlying material and by the neutral stress or pore-water pres- 

 sure. If the pore water is in motion, seepage pressures must also be 

 considered. 



The shear strength of saturated sands depends on many factors; 

 among the most important are the original state of aggregation or 



