Soil Characteristics in Relation to Highway Engineering

The road builder is primarily interested in the performance of soil as influenced by moisture content and changes of moisture content. All soil is derived from bedrock which forms the outer shell of the earth by gradual weathering and the disintegrating action of air, moisture, heat, cold, frost, and chemical agencies. Soils that exist where they are form'ed are said to be "residual"; those that have been carried from their place of origin by glaciers, wind, or water are called . "transported soils." Residual soils and those transported by wind, such as loess, and by lakes and rivers, such as silts, clays, and mucks, occur in the earth's crust in series of layers forming whatis knownas the "soil profile," figure 1. Soils of a profile graded by mechanical analysis are designated as "sands," "clays," "loams," "silt loams," etc. Sandy materials are described as "light-textured" and clays as "heavy-textured" soils. The chart, figure 2 developed by A. C. Rose (l) shows the mechanical grading of the various designations of texture.