106 kaye. PRINCIPLES OF SOIL MECHANICS [Ch. 5 



consolidation pressure p. This point is found in the following manner. 

 At point a, which is the point on the o-log p curve with the smallest 

 radius of curvature, a tangent aB and a horizontal line aD are con- 

 structed. Point b lies on the bisector aC of the angle between these 

 two lines and is the intersection of the bisector with the upward ex- 

 tension of the steeply dipping straight part of the e-log p curve. 



In an interesting paper on some postglacial clays, Skempton (1948) 

 has recently demonstrated that this method for estimating precon- 

 solidation loads is essentially correct. Other workers, however, have 

 experienced difficulties in reconciling the computed preconsolidation 

 loads with what was known of the histories of the clays. Greater 

 interest of geologists in this work may reveal that present imperfec- 

 tions are due to limitation in our knowledge of the properties of clay 

 and in our knowledge of the complexities of the geologic past. 



RATIONAL BASIS OF SOIL MECHANICS 



The rational method of soil mechanics is a process that should be 

 familiar to the geologist. The long road that starts with facts, leads 

 through operations, and terminates with inductive inferences — : the road 

 that is typical of much of geological thought — also characterizes soil 

 mechanics. 



At the beginning of every new job the engineer is confronted simply 

 with a certain volume of soil, its properties unknown, its exact dimen- 

 sions depending more upon the nature of the construction and to a 

 lesser extent upon the nature of the soils; for instance, heavy structures 

 stress soils to greater depths than light structures, etc. The engineer's 

 problem is, therefore, to determine the pertinent physical properties 

 of the unknown, which is the soil mass. This is done, as we have seen, 

 by (1) observation, (2) sampling, (3) testing, and (4) analyzing. As 

 a result of the sampling, testing, and analytical procedure, the en- 

 gineer hopes that he has found out all he has to know about the stress- 

 strain relationships of the soil mass in order to insure the safety of his 

 structure. However, he arrives at this position by virtue of a number 

 of assumptions, which, it must be acknowledged, may affect the ac- 

 curacy of his prediction. It is assumed, for example, that all sig- 

 nificant soil types in the mass are known and that the samples tested 

 are typical of each type and are in the same condition as the soil in 

 place. Furthermore, it has to be assumed that the mechanical theories 

 applied are pertinent to the problem and that the rigors of the mathe- 

 matics are applicable to the statistical qualities of the soil mass. These 

 qualifications do not, however, affect the validity of the analytical 



