Civil and Building Engineering.

Residual soils are derived from the in-place weathering of rocks and are found on every continent, including the subcontinents of Australia and Greenland (Sowers, 1963). Residual soils exist in southeastern North America, Central America, the islands of the Caribbean, South America, southern Asia, and Africa. The soils occur in humid, warm regions that promote rapid decomposition of the parent rock.

There are many processes of weathering, depending on the climate during the geologic time at the site. The processes can be differential expansion and contraction, freezing and thawing, chemical action, rainfall and leaching, and the action of organisms.

As is to be expected, the decomposition is not uniform, with residual soil varying in thickness. At certain places in India, some areas of the parent rock resisted decomposition and were left in residual soil as boulders. Residual soil can sometimes be expansive, such as the soil shown in Figure 2.2. The site had been excavated, and drying of the clay generated the pattern of cracks shown in the foreground. The structure and physical characteristics of residual soil are related to those of the parent rock, but careful sampling and testing can obtain properties for use in design. In situ tests are frequently very useful.
Deposits of organic material can weather into peat, muck, and muskeg and can pose severe problems for the engineer.

Weathering of soils such as limestone has resulted in karst geology where caverns and openings in the rock are prevalent. Karst geology exists in many areas of the world and was studied by Terzaghi early in his career (1913) (see Section 2.2.5).