Geoscience Reference
In-Depth Information
during the last 20,000 years. On the other hand, some buried peat may have been
developed during the inter-glacial period. Peat also has accumulated in postglacial
lakes and marshes, where they are interbedded with silts and muds.
2.2 DEFINITION OF PEAT AND ORGANIC SOILS
Peat commonly occurs as extremely soft, wet, unconsolidated superficial deposits,
normally as an integral part of wetland systems. It may also occur as strata beneath
other superficial deposits. The term
peat
describes a naturally occurring highly organic
substance derived primarily from plant materials. It is formed when organic (usually
plant) matter accumulates more quickly than it humifies (decays). This usually occurs
when organic matter is preserved below a high water table, as in swamps or wetlands.
Peat is predominantly made up of plant remains such as leaves and stems. It is
produced by the incomplete decomposition and disintegration of sedges, trees, mosses
and other plants growing in wet places and marshes where there is a lack of oxygen.
Therefore the colour of peat is usually dark brown or black, and it has a distinctive
odour (Craig, 1992). Since the main component is organic matter, peat is very spongy,
highly compressible and combustible. These characteristics also lend peat its own dis-
tinctive geotechnical properties compared with inorganic soils, such as clay and sandy
soils, which are made up only of soil particles (Deboucha
et al.
, 2008).
Commonly, peat is classified based on its fibre, organic and ash content. Decom-
position is the breakdown process of plant remains by the soil microflora, bacteria
and fungi via aerobic decay. In this procedure, as mentioned earlier, the peat's struc-
ture breaks down and its primary chemical composition changes. The end products of
the decomposition process are carbon dioxide and water. The degree of decomposition
varies throughout peat, since some plants (or parts of plants) are more resistant than
others. Also, the degree of decomposition depends on a combination of conditions,
such as the chemistry of the water supply, the temperature of the region, aeration and
the biochemical stability of the peat-forming plant (Lishtvan
et al
., 1985).
However, the cut-off value of the percentage of organic matter necessary to classify
a superficial deposit or soil as peat varies throughout the world, usually depending on
the purpose of classification. This cut-off value also serves to differentiate peat from
superficial deposits or soils with lesser amounts of organic content. The terms
peat
and
organic soils
, used to describe soils with an organic content, were once synonymous,
but the latter is now used for superficial deposits or soils that contain organic matter.
As described in Chapter 1, soils with an organic content greater than 20% are
generally termed organic soils. The precise definition of peat, however, varies between
of the disciplines of soil science and engineering, as well as between countries. Soil sci-
entists define peat as soil with an organic content greater than 35%. To a geotechnical
engineer, all soils with an organic content greater than 20% are known as organic
soil. 'Peat' is an organic soil with organic content of more than 75%. The engi-
neering definition is essentially based on the mechanical properties of the soil. It is
generally recognized that when a soil possess organic content greater than 20%, the
mechanical criteria of conventional mineral soil (silt and clay) can no longer be gen-
erally applied. Table 2.3 shows the ASTM (D4427) classification of peat samples by
laboratory testing.
