Ch. 5] SCOPE OF SOIL MECHANICS 95 



structures using these materials. Because soils form an integral part 

 of most structures, whether as foundation or as construction material, 

 it follows that modern structural design should include a rational 

 analysis of soils. However, of all the important construction mate- 

 rials used by man, soil is the only one which, because of its complexity 

 and variability, cannot be reduced to relatively simple and universal 

 numerical values. This is apparent when the complexity of factors 

 affecting soil behavior is recognized. The mechanics of soil certainly 

 must include such considerations as compressibility, rigidity, perme- 

 ability, and the elastic and plastic properties. Furthermore, the aniso- 

 tropic nature of most soils and the difficulty of deriving a statistical 

 expression for the deviations from homogeneity complicate still further 

 any attempt to reduce soil behavior to simple terms. 



It was, however, the initial problem of soil mechanics to decide, 

 first, if any sort of reduction was possible and, then, how such a re- 

 duction, simple or otherwise, could be made. The problem initially re- 

 solved itself into the following questions: (1) What soil properties af- 

 fect mechanical behavior? (2) How can these properties be measured? 

 (3) What are the stress distributions within a soil mass? (4) How 

 do soils act when subject to stress? What was needed, in short, was a 

 knowledge of the stress-strain relationships in any soil mass. 



All soils consist of a three-phase system: solid, water, and gas. A 

 thorough understanding of soils must therefore involve an under- 

 standing of the interaction of all phases. Although there are notable 

 exceptions, it can be said that soil mechanics is mainly concerned with 

 the interaction features, or the aggregate properties, of soils. Re- 

 search into the properties of the individual phases, and in particular 

 into the properties of the solid or granular phase, is carried on mostly 

 in mineralogy, sedimentation, soil physics, and soil chemistry. 



PRINCIPLES OF SOIL MECHANICS 



Geneeal 



The many soil conditions, with their attendant engineering prob- 

 lems, that are analyzed by soil mechanics can be grouped into three 

 categories: (1) those in which overstressing produces rupture or failure 

 of the soil, (2) those in which the moderate stress conditions produce 

 only deformation of the soil, and (3) those in which permeability of 

 the soils is the important factor (Table 1). These problems can be 

 further classified into those in which the hydraulic properties (the 

 condition of the pore water) of the soil play the principal role, and 

 those in which the strength of the entire soil mass is involved, that is, 



