Page 23 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
2 Igneous Rocks, Ultimate Sources for Soils

2.1 GEOLOGY AND THE GEOLOGICAL CYCLE

2.1.1 Knowing the Materials
Regardless of the field of specialization, an engineer should know and
understand the materials with which he or she will be working. Naturally
occurring soils and rocks are complex, and an appreciation of their particular
properties can be critical to the success of a project. Formal course work in
geology therefore is a prerequisite to the study of geotechnical engineering. The
purpose of these introductory chapters is to serve as a reminder and to add
emphasis to points that are most relevant in engineering. Students also are
urged to re-read their basic text in physical geology. Large or complicated
projects usually utilize the services of one or more professional engineering
geologists.

2.1.2 Igneous Rocks as Primordial Sources for Soils

As shown in Fig. 2.1, the originating sources for soils are igneous rocks, that is,
rocks that have solidified from molten material, or magma. Although most
igneous activity occurred in past geological eras, active volcanoes are evidence
that such activity continues, being concentrated along weaker zones of the
Earth’s crust at plate margins.

2.1.3 Weathering, Saprolite, and Residual Soils
The conversion of rock to soil involves physical and chemical processes of
weathering. Generally, the farther a rock is removed from its nascent envi-
ronment, the more vulnerable it is to changes in the environment. Thus, a rock
that has formed under extremes of heat and pressure will be relatively immune

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