Development of peat land and types of peat - Geotechnics of Organic Soils and Peat - page 20

Table 2.5 Qualifying terms and symbols for peat soils.

Organic von Post scale Qualifying terms Symbol component

Peat Pt

H1-H3 Fibric/fibrous f

H4-H6 Hemic/moderately decomposed h

H7-H10 Sapric/amorphous a

Table 2.6 USDA classification of peat.

Type of peat Fibre content von Post scale

Fibric peat Over 66% H4 or less

Hemic peat 33-66% H5 or H6

Sapric peat Less than 33% H7

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 or non-woody) or coarse (woody).
The organic fibre content is also referred to as the fabric of organic soil. A fibre
is defined as > 0.15mm in diameter. An appreciation of the constituent matter and its
attributes, such as orientation, aids in the constitutive modelling of this soil type for
basic understanding of its mechanical behaviour (Molenkamp, 1994).
Fibric peat is mostly undecomposed and typically tan to light reddish brown in
colour. Hemic peat is intermediate between fibric and sapric peat in degree of decom-
position or humification, organic content and bulk density, and typically dark reddish
brown in colour. Sapric peat, on the other hand, is generally of darker colour than
the above two types of peat, and the most decomposed/humified. It generally has the
highest organic content and bulk density of the three types of peat.
Decomposition or humification involves the loss of organic matter either as gas
or in solution, the disappearance of physical structure and change in chemical state.
As previously described, the breakdown of plant remains is brought about by soil
microflora, bacteria and fungi, which are responsible for aerobic decay. Therefore
the end products of humification are carbon dioxide and water, the process being
essentially biochemical oxidation. Immersion in water reduces the oxygen supply enor-
mously, which in turn reduces aerobic microbial activity and encourages anaerobic
decay, which is much less rapid. This results in the accumulation of partially decayed
plant material as peat. Peat degree of decomposition or humification is usually assessed
by means of the von Post scale. This will be further described in details in Chapter 3.
Another three-way division of peat based on the von Post scale is described as
follows (see also Table 2.7):

• Fibrous peat is low humified and has a distinct plant structure. It is brown to

brownish yellow in colour. If a sample is squeezed in the hand, it gives brown
to colourless, cloudy to clear water, but without any peat matter. The material