Environmental Engineering Reference
In-Depth Information
For soils, the reference is mainly to the acidity of the soluble particles. Corrosion of iron or
steel embedded in soil, in the presence of moisture, increases with soil acidity.
Identification
Suspension:
If a specimen of clay is mixed with pure water to form a paste and then dis-
persed in pure water, particles generally smaller than about 1
µ
m (10
−3
mm) will remain in
suspension almost indefinitely and are considered as colloidal.
Electron microscope
: Used to view particles down to about 1
m (10
−6
mm). The crystal
shape is used to identify the clay type (see, e.g., Osipov and Sokolov, 1978).
X-ray diffraction
: Used to identify particles down to about 10
−8
µ
10
−7
mm). In
the diffractometer a powdered mineral sample mounted on a glass slide is rotated at a
fixed angular rate in an X-ray beam. A pick-up device, such as a Geiger tube, rotates about
the same axis, detecting the diffracted beams. The impulse is transmitted and recorded on
a strip chart. Expressions are available relating the wavelength of radiation and the angle
of rotation
mm (1 Å
to
d
, where
d
is the spacing of a particular set of crystal planes on the basis of
which the mineral is identified (Walhstrom, 1973).
Differential Thermal Analyzer
: It measures the temperatures and magnitudes of exother-
mic and endothermic changes occurring in a sample when it is heated at a uniform rate.
Measurements made by thermocouples embedded in the specimen and changes occurring
in the specimen during heating are recorded on a strip chart. The curve forms are charac-
teristic for various clay types. See Gillott (1968), Grim (1962), Leonards (1962), and Lambe
and Whitman (1969).
θ
5.3.4
Organic Materials
Origin and Formation
Origin
Organic matter is derived primarily from decayed plant life and occasionally from animal
organisms.
Formation
Topsoil
is formed as plant life dies and becomes fixed with the surficial soils. The layer's
thickness and characteristics are a function of climate and drainage; the latter is related to
slope and soil type. Well-drained granular soils above the water table, poor in minerals
other than quartz, develop very thin topsoil layers, even in humid climates. Thick topsoil
layers develop in mineral-rich soils and humid climates, particularly where the soil is cool.
Soil temperatures above 30
°
C destroy humus because of bacterial activity, whereas humus
accumulates below 20
C (Mohr et al., 1972). This phenomenon is evident in tropical coun-
tries where the topsoil is usually thin, except where drainage is poor.
Rootmat
forms in marshy regions (
Section 7.4.5)
and is a thick accumulation of living and
dead marsh growth, as illustrated in
Figure 5.12.
Peat
is fibrous material with a sponge-like structure, composed almost entirely of
dead organic matter, which can form to extensive thickness. Organic silts and clays
form in lakes and estuarine environments, where they can attain a thickness of 25 m or
more.
°
Occurrence
Although surface deposits during formation, organic layers can be found deeply buried
by alluvium as shown in
Figure 5.13
,
by beach deposits (see
Figure 7.46),
colluvium
