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CIVIL ENGINEERS ARE IN THE MIDST of a construction revolution. Heavy structures are being located in areas formerly considered unsuitable from the standpoint of the supporting power of the underlying soils. Earth structures are contemplated that are of unprecedented height and size; soil systems must be offered to contain contaminants for time scales for which past experience is either inadequate or absent. Designs must be offered to defy the ravages of floods and earthquakes that so frequently visit major population centers.
All structures eventually transmit their loads into the ground. In some cases this may be accomplished only after circuitous transfers involving many component parts of a building; in other cases, such as highway pavements, contact is generally direct. Load transfer may be between soil and soil or, as in retaining walls, from soil through masonry to soil. Of fundamental importance is the response that can be expected due to the imposed loadings. It is within this framework that geotechnical engineering is defined as that phase of civil engineering that deals with the state of rest or motion of soil bodies under the action of force systems.
Soil bodies, in their general form, are composed of complex conglomerations of discrete particles, in compact arrays of varying shapes and orientations. These may range in magnitude from the microscopic elements of clay to the macroscopic boulders of a rock fill. At first glance, the task of establishing a predictive capability for a material so complicated appears to be overwhelming. Although man’s use of soil as a construction material extends back to the beginning of time, only within very recent years has the subject met with semiempirical treatment. In large measure, this change began in 1925 when Dr. Karl Terzaghi published his book Erdbaumechanik . Terzaghi demonstrated that
soils, unlike other engineering materials, possess a mechanical behavior highly dependent on their prior history of loading and degree of saturation and that only a portion of the boundary energy is effective in producing changes within the soil body. Terzaghi’s concepts transferred foundation design from a collection of rules of thumb to an engineering discipline. The contents of the present section offer, in a concise manner, many of the products of this and subsequent developments.

15 Soil Relationships and Classification
**15.1 Soil Classification
Grain-Size Characteristics of Soils
Atterberg Limits and Plasticity
The Unified Soil Classification System (USCS)
The AASHTO Classification System
AASHTO Classification System Examples

**15.2 Weight, Mass, and Volume Relationships of soil
The Phase Diagram of soil
Volume Relationships of soil
Weight and Mass Relationships of soil
Unit Weight of soil
Density of soil
Specific Gravity of soil
Conversion of Unit Weight and Density of soil
Weight–Volume Problems Involving Defined Quantities of soil
Soil problems Weight, Mass, Volume
Weight–Volume Problems Involving Only Relationships
Defining Terms Weight–Volume Problems