Geoscience Reference
In-Depth Information
these components and their specific nature determine the physical and geotechnical
properties of these soils (Edil, 2003).
The mineral component is similar to that of inorganic soils consisting mainly of
clay minerals, but non-clay minerals are also encountered. Clay minerals consist of
small particles, typically less than 0.002 mm in diameter. The mineral constituent is
generally incombustible and ash-forming, while the organic matter is generally com-
bustible carbonaceous matter. Soil organic matter has been defined (Russell, 1952) as
'…a whole series of products which range from undecayed plant and animal tissues
through ephemeral products of decomposition to fairly stable amorphous brown to
black material bearing no trace of the anatomical structure of the material from which
it was derived; and it is this latter material that is normally defined as the soil humus'.
Additionally, soil organic matter also contains products of microbial synthesis. In sum-
mary, soil organic matter includes: (1) fresh plant and animal residues (decomposable),
(2) humus (resistant) and (3) inert forms of nearly elemental carbon (charcoal, coal or
graphite). Carbon is the chief element of soil organic matter that is readily measured
quantitatively by combustion (C is determined as CO
2
emitted). The organic carbon
content may be used to estimate total organic matter by multiplying the figure for
organic carbon by a factor of 1.724 or somewhat higher figures up to 2.
Alternatively, ignition of the soil at high temperature (450 °C to 550 °C) to achieve
destruction of organic matter can be used to determine organic content directly. The
loss-on-ignition method gives quantitative oxidation of organic matter, but because it
may decompose some of the inorganic constituents of the soil it may give a number in
excess of the actual organic content (Edil, 2003). Procedures for obtaining the organic
content of peat by the loss on ignition method will be described later in this chapter.
The organic matter, when extracted, can be fractionated into components, pri-
marily those characteristic of plant tissues and those based on humus. The first group
(nonhumic matter) includes fats, waxes, oils, resins, water-soluble polysaccharides,
hemicellulose, cellulose and protein. The second group includes the humus fraction
consisting of basically humic and fulvic acids and humin and exists in both solid and
liquid phases (Huttunen
et al
., 1996).
3.3 BOTANICAL ORIGIN AND FIBRE CONTENT
Botanical terms are used in describing peat especially for horticultural purposes, but
such terms also have relevance to engineering because of the texture they imply.
Sphag-
num peat
designates a material with predominantly sphagnum moss (
>
60-75%).
Sedge peat
has one or more species of sedge (plants that are grass-like in appearance).
Woody peat
is indicates dominance of woody pieces. Other terms include
taxodium
peat, reed peat
and combinations of botanical terms. Of course, for highly decomposed
peat with low fibre content (
<
33%), it would be difficult to identify the botanical
origin.
The structure of peat and organic soils is an arrangement of primary and secondary
elements that make up the soil. On a qualitative basis, however, organic ground can be
described in terms of its fibre content and type. Fibre content is determined typically
from dry weight of fibres retained on a #100 sieve (
>
0.15mm opening size) as a
percentage of oven-dried mass (ASTM Standard D 1997). Fibres may be fine (woody
