Environmental Engineering Reference
In-Depth Information
- Potential for slope instability?
(c) Landslide debris:
-Variability in composition and properties - laterally and vertically?
- Boulders?
- Large voids?
- Gaping or infilled cracks?
- High compressibility?
-Timber, rotting or preserved?
- High permeability?
- High water tables - wet conditions?
-Old slide surfaces of low strength at the base, or at other levels in the deposit?
- Potential for renewed sliding movements?
3.11
LATERITES AND LATERITIC WEATHERING PROFILES
Townsend et al. (1982) provide the following definition of laterite:
“Laterite refers to varied reddish highly weathered soils that have concentrated oxides of
iron and aluminium and may contain quartz and kaolinite. Laterite may have hardened
either partially or extensively into pisolitic, gravel like, or rock-like masses; it may have
cemented other materials into rock-like aggregates or it may be relatively soft but with the
property of self-hardening after exposure”.
3.11.1
Composition, thicknesses and origin of lateritic weathering profiles
The following features of lateritic weathering profiles, some of which are shown also on
Figure 3.40
, are based mainly on Selby (1982).
Thickness (m)
Description
0 to 2
Soil zone, often sandy and sometimes containing nodules or concretions; this
may be eroded away.
2 to 10
Ferricrete or alcrete crust of reddish or brown hardened or slightly hardened
material, with vermiform (or vermicular) structures (i.e. having tube-like
cavities 20-30 mm in diameter) which may be filled with kaolin; less
cemented horizons may be pisolithic (i.e. formed by pea-sized grains of red
brown oxides).
1 to 10
Mottled zone; white clayey “kaolinitic” material with patches of yellowish
iron and aluminium sesquioxides.
Up to 60 but often
25
Pallid zone; bleached kaolinitic material; the distinction between the mottled
and pallid zone is not always apparent and they can be reversed; silicified
zone which may be hardened (i.e. silcrete on Figure 3.40).
1 to 60
Weathered rock showing original rock structures.
The “crust” materials are termed ferricrete when they consist mainly of iron oxides. The
crust can be either gravel or rock ranging from very weak to very strong, often requiring
blasting for its excavation. The very weak rock materials often become stronger when
exposed to the weather.
Laterites are believed to have been formed under tropical or sub-tropical climatic con-
ditions and most laterites of Tertiary Age and younger occur in tropical or sub-tropical
areas on both sides of the equator. Lateritic profiles are found also within rocks of
Mesozoic and Palaeozoic Ages, mostly at higher latitudes, where their presence indicates
ancient tropical conditions which can be explained by continental drift (Bardossy and
