Physical Properties of Soils
The subject matter of Part I is divided into four chapters. The first deals with the procedures commonly used to discriminate among different soils or among different states of the same soil. The second deals with the methods and program of soil exploration. The third is concerned
with the hydraulic and mechanical properties of soils and with theexperimental methods used to determine numerical values representative of these properties. The fourth chapter deals with the physical processes involved in the drainage of soils.
Index Properties of Soils
ARTICLE 1 PRACTICAL IMPORTANCE OF INDEX PROPERTIES
In geotechnical engineering, more than in any other field of civil engineering, success depends on practical experience.The design of ordinary soil-supporting or soil-supported structures is necessarily based on simple empirical rules, but these rules can be used safely only by the engineer who has a background of experience. Large projects involving unusual features may call for extensive application of scientific methods to design, but the program for the required investigations cannot be laid out wisely, nor canthe results be interpreted intelligently, unless the engineer in charge of design possesses a large amount of experience. Since personal experience is necessarily somewhat limited, the engineer is compelled to rely at least to some extent on the records of the experiences of others. If these records contain adequate descriptions of the soil conditions, they constitute a storehouse of valuableinformation. Otherwise, they may be misleading. Consequently, one of the foremost aims in attempts to reduce the hazards in dealing with soils has been to find simple methods for discriminating among the different kinds of soil in a given category. The properties on which the distinctions are based are known as index properties, and the tests required to determine the index properties areclassification tests. The nature of any soil can be altered by appropriate manipulation. Vibrations, for example, can transform a loose sand into a dense one. Hence, the behavior of a soil in the field depends not only on the significant properties of the individual constituents of the soil mass, but also on those properties that are due to the arrangement of the particles within the mass. Accordingly, it isconvenient to divide index properties into two classes: soil grain properties and soil aggregate properties. The principal soil grain properties are the size and shape of the grains and, in clay soils, the mineralogical character of the smallest grains. The most significant aggregate prop-
erty of cohesionless soils is the relative density, whereas that of cohesive soils is the consistency. Thediscussion of soil grain and aggregate properties is preceded by a description of the principal types of soil, and it is followed by a condensed review of the minimum requirements for adequate soil descriptions to be incorporated in the records of field observations.
ARTICLE 2 PRINCIPAL TYPES OF SOILS
The materials that constitute the earth’s crust are rather arbitrarily divided by the civilengineer into the two categories, soil and rock. Soil is a natural aggregate of mineral grains that can be separated by such gentle mechanical means as agitation in water. Rock, on the other hand, is a natural aggregate of minerals connected by strong and permanent cohesive forces. Since the terms “strong” and “permanent” are subject to different interpretations, the boundary between soil and rockis necessarily an arbitrary one. As a matter of fact, there are many natural aggregates of mineral particles that are difficult to classify either as soil or as rock. In this text, however, the term soil will be applied only to materials that unquestionably satisfy the preceding definition. Although the terminology described in the preceding paragraph is generally understood by civil engineers,...