CAP. 17

SOIL ENGINEERING FOR HIGHWAY DESIGNHighway engineers are interested in the basic engineering properties of soils because soils are used extensively in highway construction. Soil properties are of significant importance when a highway is to carry high traffic volumes with a large percentage of trucks. They are also of importance when high embankments are to be constructed and when the soil is to be strengthened and used as intermediate support for the highway pavement. Thus, several transportation agencies have developed detailed procedures for investigating soil materials used in highway construction.

This chapter presents a summary of current knowledge of the characteristics and engineering properties of soils that are important to highway engineers, including the origin and formation of soils, soil identification, and soil testing methods.

Procedures for improving the engineering properties of soils will be discussed in Chapter 19, Design of Flexible Pavements.

17.1 SOIL CHARACTERISTICS

The basic characteristics of a soil may be described in terms of its origin, formation, grain size, and shape. It will be seen later in this chapter that the principal engineering properties of any soil are mainly related to the basic characteristics of that soil.

17.1.1 Origin and Formation of Soils

Soil can be defined from the civil engineering point of view as the loose mass of mineral and organic materials that cover the solid crust of granitic and basaltic rocks of the earth. Soil is mainly formed by weathering and other geologic processes that occur on the surface of the solid rock at or near the surface of the earth. Weathering is the result of physical and chemical actions, mainly due to atmospheric factors that change the structure and composition of the rocks. Weathering occurs through either physical or chemical means. Physical weathering, sometimes referred to as mechanical weathering, causes the disintegration of the rocks into smaller particle sizes by the action of forces exerted on the rock. These forces may be due to running water, wind, freezing and thawing, and the activity of plants and animals. Chemical weathering occurs as a result of oxidation, carbonation, and other chemical actions that decompose the minerals of the rocks.

Soils may be described as residual or transported. Residual soils are weathered in place and are located directly above the original material from which they were formed. Transported soils are those that have been moved by water, wind, glaciers, and so forth, and are located away from their parent materials.

The geological history of any soil deposit has a significant effect on the engineering properties of the soils. For example, sedimentary soils, which are formed by the action of water, are usually particles that have settled from suspension in a lake, river, or ocean. These soils range from beach or river sands to marine clays. Soils that are formed by the action of wind are known as aeolian soils and are typically loess.

Their voids are usually partially filled with water, and when submerged in water, the soil structure collapses.

Soils also may be described as organic when the particles are mainly composed of organic matter or as inorganic when the particles are mainly composed of mineral materials.