104 kaye. PRINCIPLES OF SOIL MECHANICS [Ch. 5 



ing. The ratio of the resisting moment to the disturbing moment is 

 the factor of safety of the circle. To complete the analysis, other 

 circles are drawn and their factors of safety are computed. The circle 

 having the smallest factor of safety is called the critical circle and is 

 the surface most likely to fail. The factor of safety of the entire slope 

 is that of the critical circle, and for a stable slope the factor of safety 

 must exceed 1. 



A number of factors limit the accuracy of the stability analysis. 

 In the first place, the cohesion and angle of internal friction of many 

 clays are far from constant, and the determination of their values is 

 subject to much uncertainty. In fact, studies of many slides in homo- 

 geneous clays have indicated that the stability analysis can be ap- 

 preciably simplified by assuming that the shearing resistance along 

 the sliding surface is equal to half the unconfined compression strength 

 of the clays. Secondly, in some circumstances the exact value of the 

 stresses that are effective in any plane though the soil mass cannot be 

 readily appraised, particularly when the soil is subject to seepage 

 forces. In fact, when any part of the sliding mass lies beneath the 

 water table, seepage forces have to be taken into consideration in the 

 stability analysis. This calls for the construction of a flow net, an 

 operation often characterized by considerable uncertainty. Thirdly, 

 it is difficult to evaluate the many geological factors, such as the 

 structural heterogeneities (cracks, fractures, cemented layers, small 

 pervious beds, etc.) present in many soils. However, despite these 

 limitations, shearing strength analyses play an important role in soil 

 mechanics and have been used with considerable effectiveness. 



Consolidation 



The slow compression of saturated clays under load is a deformation 

 problem based on the hydraulic properties, rather than on the strength 

 properties of the soil system. There has been deep interest in the 

 solution of this type of problem in soil mechanics, and of direct interest 

 to the geologist is that out of it has been developed a technique of 

 value to historical geology. 



It will be recalled that soils are a skeleton of solid particles with 

 fluids filling the interspersed voids. If a volume of soils with voids 

 entirely filled with water is confined laterally and subjected to vertical 

 pressure, any reduction of volume (strain) occurs only if either of the 

 following conditions is fulfilled: (1) if there is a reduction in volume 

 of the solids making up the soil skeleton, or (2) if there is a reduc- 

 tion in volume of the voids. As it is assumed that the soil grains and 

 the water within the voids are incompressible, a reduction in volume of 



