Page 111 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
106 Geotechnical Engineering

The next weathering stage is represented by calcite, or calcium carbonate. Thus,
a soil that contains calcite still is in an early stage of weathering.

The presence of quartz and loss of feldspars represents an intermediate stage of
weathering. Quartz, although highly resistant to weathering in sand and silt sizes,
is more aggressively attacked in the fine fraction because of surface energy
considerations.

A more advanced intermediate stage includes a weathering sequence mica-
illite-smectite that reflects a progressive leaching of K, or potassium. These clay
mineral groups are discussed in more detail in Chapter 6. They dominate soil
profiles in most of the U.S. Smectite is expansive clay. Smectite clays occur in
soil profiles in the central and south-central U.S., and in soils formed in basic
(volcanic) rocks of the far west.

More intense weathering is characterized by a sequence smectite-kaolinite-gibbsite
that shows progressive removal of silica, or SiO 2 , from the mineral structures.
Kaolinite represents a more advanced stage of weathering than smectite. Kaolinite
dominates soils in warm climates with an annual rainfall exceeding about 100 cm
(40 in.), such as in the southeastern U.S. and in subtropical areas. This means that
these areas have little expansive clay except for that which may be brought in by
wind or water.

Kaolinite is named for a mountain in China and is used in China and elsewhere to
make porcelain. Weathering to gibbsite requires tropical or subtropical climates
with high rainfall. Gibbsite deposits are sources for aluminum ore.

Significance of a Red Soil Color
Smectite clays usually contain iron as part of their somewhat casual crystalline
structure, and weathering to well-crystallized kaolinite ejects the iron, which oxi-
dizes to give soil a characteristic red color. Tropical soils therefore generally are
rust-red, which also identifies them as being nonexpansive.

5.3.2 Mathematical Expression for Soil Profile
The natural factors influencing soils were formulated by a Russian soil scientist,
Dokuchaev, in 1898. His formula was modified by Hans Jenny in 1941 to an
algebraic notation:

s ¼ fðcl,o,r,p,tÞ ð5:1Þ
This formula is shorthand for soil (s) as a function of climate (cl), organisms (o),
topography (r), parent material (p), and time (t). The formula means that if any
one of the factors changes, the soil will be different.

The significance of climate ‘‘cl’’ already has been discussed.

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