Introduction - Geotechnics of Organic Soils and Peat - page 7

Figure 1.9 Colluvial soil deposit ( Source : Parent Materials Mode of Deposition in the Yukon Territory:
Colluvial, sis.agr.gc.ca, Agriculture and Agri-Food Canada ©. Reproduced with the permis-
sion of the Minister of Agriculture and Agri-Food Canada, 2014).
1.2.7 Colluvial soils
Colluvial soils or colluviums are soils transported down slopes by the action of gravity.
There are two types of down slope movement: slow and rapid. Both occur only on or
near sloping ground (Figure 1.9).
Slow movement, typically of the order of millimetres per year, is called creep.
It occurs because of gravity-induced shear stresses, frost action and other processes.
Creep typically extends to a depth of 0.3-3m, with the greatest displacement occurring
at the surface. Such slowmovement may at first appear to be inconsequential. But with
time they can produce significant distortions in structures founded on such soils.
Rapid movements are those created by dramatic events such as landsides or mud-
flow. Although this rapid movement can occur in any type of soil, the product is always
considered to be colluvial soil.
1.2.8 Organic soils and peat
Technically any material that contains carbon is called 'organic'. However, engineers
and geologists use a narrower definition when applying the term to soils. An organic
soil is one that contains a significant amount of organic material recently derived
from plant remains. This implies that it needs to be 'fresh' and still in the process of
decomposition, thus retaining a distinctive texture, colour and odour (Figure 1.10).
The most chemically active colloidal fraction of organic materials is humus.
Some soils contain carbon, but not from recently derived plant material. Thus they
are not considered organic in this context. For example, some sand contains calcium
carbonate (calcite), which arrived as a chemical precipitate.