Page 104 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
5 The Soil Profile

5.1 SOIL PROFILES IN ENGINEERING

5.1.1 Occurrence
A soil profile designates near-surface soil layers that have been modified by
processes associated with weathering. Weathering not only changes some soil
minerals and removes others, it concentrates and relocates soil particles in charac-
teristic layers. In lay terms the layers have names like ‘‘topsoil,’’ ‘‘subsoil,’’ and
‘‘hardpan.’’ Many soil profiles contain concentrations of expansive clay minerals,
and soil profiles develop in rocks as well as in sediments. As sedimentary soils
normally have been recycled from earlier stages of weathering, they tend to be less
susceptible to further change.

The most important function of a soil profile is for agriculture. Soils that are
good for agriculture generally are poor for engineering and vice versa, making
the uses complementary instead of competitive. For example, black topsoil that
is excellent for plant growth has poor properties for engineering, and therefore is
normally stripped off from construction sites and saved for later spreading
and landscaping. In fact, the topsoil that is left in place and buried under fill
soil eventually can cause serious problems including foundation failures and
landslides.

5.1.2 Relation to Erosion
A soil profile represents a balance between weathering and erosion, weathering
developing the profile while erosion simultaneously strips away the upper part.
A steep eroded hillside may have little or no development of a soil profile while
nearby flat areas on material of the same age can have a deep and strongly
developed weathering profile.

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