Environmental Engineering Reference
In-Depth Information
Figure 3.40.
Diagram showing the lateritic weathering profile of Selby (1982) and some of the processes
involved in its development (by permission of Oxford University Press).
Aleva, 1990). Laterites have been developed on the full range of common igneous, meta-
morphic and sedimentary rocks (Gidigasu, 1976).
There are several theories about the formation of laterites (Bardossy and Aleva, 1990;
McFarlane, 1976) but most include the influence of a fluctuating water table to allow
solution and transfer of soluble silica, iron and aluminium ions, resulting in iron and alu-
minium oxides accumulating in the upper part of the profile (Figure 3.40).
Lateritic profiles may be much shallower than described above e.g. in the Ranger Mine
area in northern Australia and in many other exposures in Australia and south-east Asia
they are less than 5 m thick. Some of these shallow laterite profiles are of detrital origin
i.e. they comprise ferricrete and/or alcrete gravels which have clearly been eroded from an
earlier laterite weathering profile and redeposited. These “reworked laterites” show varying
degrees of re-cementation and may or may not be underlain by mottled and pallid zones.
Figure 10.3 shows some common features of lateritic profiles in valley situations in
northern and western Australia.
3.11.2
Properties of lateritic soils
The most abundant soils in the mottled zone are usually clays, sandy clays or gravelly
sandy clays, which behave as soils of medium to high plasticity, but which usually plot
below the “A” line in the Casagrande classification chart. Strictly speaking they are there-
fore classified as silts according to the Unified Soil Classification. This behaviour is a
result of the presence of allophane, kaolin, gibbsite, bauxite and often halloysite.
Other clays have been found in some laterites, for example Gordon (1984) records
montmorillonite and illite in profiles developed over dolerite bedrock at Worsley, Western
Australia. However this would not be common. Gordon and Smith (1984a,b) describe the
results of field and laboratory tests on these and other laterite soils in that area.
As indicated on Figure 3.40
in situ
laterite profiles are often highly permeable. Many of
the structural features which cause the high permeability are near-vertical. In the upper
