Development of peat land and types of peat - Geotechnics of Organic Soils and Peat - page 17

Table 2.3 Organic content ranges (ASTM D4427).

Basic soil type Description Organic content (%)

Clay or silt or sand Slightly organic 2-20

Organic soil - 25-75

Peat - > 75

Table 2.4 Organic soil classification based on organic content ranges ( after Jarrett, 1995).

Basic soil type Description Symbol Organic content (%)

Clay or silt or sand Slightly organic O 2-20

Organic soil - O 25-75

Peat - Pt

> 75

Slightly organic silts or clays will most probably appear as inorganic fined-grained
soils, probably black to dark brown in colour, with an organic odour and possibly
some visible organic remains. Their plasticity limits should be evaluated as for other
fine-grained inorganic soils. These soils would then be classified as silts or clays of low,
medium or high plasticity.
Peat, on the other hand, may well appear to be completely organic, contain recog-
nizable plant remains, have a low density and also be black to dark brown in colour.
Organic soils, however, are more difficult to sub-divide. Under the Unified Soil Classi-
fication System (UCS), organic soils are recognized as a separate soil entity and have a
major division called Highly Organic Soils (Pt), which refers to peat, muck and highly
organic soils.
Jarrett (1995) gives a classification for organic soils which purportedly can be
integrated with the UCS to bridge the gap between peat, as defined above, and purely
inorganic clays, and silts, and is shown in Table 2.4.
Hobbs (1986) illustrates the various classification of peat between countries as
follows:

1. Russian geotechnical engineers assume that peat is a soil containing more than
50% of particle weight of vegetable origin (organic matters), while peaty soil
contains 10-50% of particles of vegetable origin.
2. ASTM, D2607-69 assumes peat as a soil having organic matter of more than 75%,
as stated above.
3. Hobbs suggests that peat is a soil having organic matter more than 27.5%.

LPC (France), on the other hand, describes organic soils as those having greater
than 10% organic content (Figure 2.4).
Figure 2.5 is a comparison of various classification systems made by Andrejko
et al . (1983).
To avoid further confusion, we will generally term all organic soils (soils having
an organic content greater than 20%) as peat in this topic, meaning that the terms
peat and organic soils will be used interchangeably.