Page 25 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
20 Geotechnical Engineering

Figure 2.2
Weathering of
granite proceeds
faster along
fractures, leaving
an array of
rounded
outcrops and
boulders.

2.1.5 Sedimentary Soils

Residual soils are readily removed and redeposited through actions of wind,
moving water, or glacial ice, to become sedimentary soils or sediments. Engineers
frequently refer to these materials simply as ‘‘soils,’’ but the term ‘‘sedimentary
soils’’ is more descriptive and helps to flag major differences in properties and
in behavior from residual soils. The properties of sedimentary soils relate to the
source rock and to the transporting agent, whether it is gravity, water, wind, glacial
ice, or activities of scrapers, bulldozers, and pocket gophers. For example, if the
source rock is granite, granite disintegrates into sand having the same mineralogical
composition as the granite; then as transportation proceeds by wind or water, softer
minerals are worn away and concentrate harder minerals, in particular quartz.

2.1.6 Recycling of Sediments
Sedimentary soils readily can become eroded, transported, and redeposited as new
sediments that reflect the source material and the most recent mode of origin.
As an example, a glacial deposit that is a mixture of sand, silt, and clay can be a
source for alluvium where the sand, silt, and clay are segregated.

2.1.7 Sediments into Rocks

Sediments that are buried for long periods of time can succumb to pressures
from overlying sediments, and/or cementation by secondary minerals such as
carbonates. Sediments that are cemented become sedimentary rocks—shale,
sandstone, and limestone. Sedimentary rocks in turn can weather and support
weathering to residual soils that can re-enter the geological cycle.

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