Geoscience Reference
In-Depth Information
bogs) is generally acidic with pH values in the range 3 to 4. Fen peat, on the other hand,
is generally neutral or slightly alkaline. Bog peat is generally more fibrous compared
with fen peat. In the tropics, peat is generally acidic, with pH values in the range 3 to
4.5 (Mutalib
et al
., 1991).
The loss of organic matter and the accompanying changes in chemical state result
from the breakdown of cellulose within plant tissues, so that the detritus gradually
becomes increasingly fine until all trace of fibrous structure disappear. The peat then
has an amorphous granular appearance, the material consisting principally of organic
acids, which have a sponge-like fabric. The degree of humification varies throughout
the peat since some plants are more resistant than others, and certain parts of the
plants are more resistant than others. The change undergone as a result of increasing
humification is not uniform, since the fibres are reduced in size and strength in an
irregular manner as the quantity of totally humified peat increases. Generally, the
fresher the peat, the more fibrous material it contains, and as far as engineering is
concerned, the more fibrous the peat, the higher are the shear strength, void ratio and
water content.
Under normal field conditions, total degradation of the organic fraction under
water is limited due to the volatile acid toxicity and nutrient imbalance. The intro-
duction of nutrients by groundwater seepage may initiate or sustain decomposition
over long periods. The process is finally complete when only humus (nondegradable
residue) and microbial cells are left. Decomposition causes a decrease in solid volume,
i.e. compression.
From a physical point of view, the humification processes cause some changes,
including reduction of the total water content, increase in specific gravity, increase in
compaction, decrease in the pore space, and changes in colour towards dark brown
and black (Gunther, 1983).
As described above, the degree of decomposition or humification is usually assessed
by means of the von Post scale. von Post (1922) proposed a classification system
based on a number of factors, such as degree of humification, botanical composition,
water content, content of fine and coarse fibres and woody remnants. There are 10
degrees of humification (H1 to H10) in the von Post system, which are determined
based on the appearance of soil water that is extruded when the soil is squeezed in
hand. For geotechnical purposes, these 10 degrees of humification are often reduced to
three classes: fibric or fibrous (least decomposed), hemic or semi-fibrous (intermediate)
and sapric or amorphous (most decomposed), respectively (Magnan, 1980; ASTM
Standard D 5715).
Table 3.1 shows the von Post scale for assessing the degree of humification. To
perform the test, a sample of peat is squeezed in the hand. The colour and form of
fluid that is extruded between the fingers is observed together with the pressed residue
remaining in the hand after squeezing, with reference to the 10 point scale mentioned
above.
However, the von Post scale is adapted to pure peat containing little or no mineral
matter. Its use in organic soils with more than 20-25% mineral mater is difficult. As a
result, various coarser scales have been devised with 3-5 degrees of humification (see
also Chapter 2).
Figure 3.6 illustrates an example of an internal stratification of a tropical basin
peat dome based on the degree of humification.
