Page 27 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
22 Geotechnical Engineering

fallen off a cliff reflects the composition of the rocks in the cliff. Rock frag-
ments making up rockfalls are released as a result of physical weathering by
cyclical heating and cooling, wetting and drying, and freezing and thawing.
The loose rock is then transported by gravity and a little bouncing to the bottom
of the cliff.

2.2.2 Making New Minerals

Chemical weathering involves modification or destruction of the original min-
erals. Thus, if the rockfall of the preceding paragraph has been modified by
chemical weathering, then eroded, transported, and redeposited as a sediment,
the composition of the sediment will reflect its entire history, including
composition of the parent rock, the extent to which the minerals have been
modified by weathering, the transporting agent, and the mode of deposition.
Because of the importance of sedimentary soils for engineering, the various
possibilities are discussed in the next chapter.

Chemical weathering creates a new suite of soft, hydrated minerals called
clay minerals. As the name implies, the fine-grained portion of soil that is
clay mostly consists of these special clay minerals. Clay mineral particles are
ionically active and carry a pattern of electrical charges that contribute to
a distinctive and often bad behavior for engineering. When dried, a soil contain-
ing clay minerals will become a hard solid that can support a skyscraper, but
the same soil when wet may be as soft as a bowl of gelatin. Clay soils are
commonly at the base of landslides and are problem soils for foundations and
other uses.

2.2.3 Conflicting Definitions of Soil
The geological term for soil is regolith, which is from the Greek for blanket-
stone. Regolith signifies the mantle of loose, incoherent rock material at or
near the Earth’s surface, regardless of whether that material has been eroded
and redeposited, or has been left at its place of origin from weathering. Thus,
both residual soils and sediments are considered part of the regolith.

Engineers regard soil as unconsolidated surficial material that does not
require blasting for removal. The engineer’s soil therefore is the approximate
equivalent of the geologist’s regolith, except that with the development of
ever-heavier earth-moving equipment the engineer’s definition of soil changes,
and what in the days of horse-drawn scrapers was rock now is soil. An exact
definition based on physical properties therefore is important for cost esti-
mates and bid documents, and has been central to more than a few lawsuits,
as excavating costs are multiplied roughly by a factor or 10 if blasting is
required. Generally the engineering definition of soil now relates to the ease
of removal by a ripping tooth pulled behind the largest readily available
bulldozer.

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