Engineering Geology

Chapter 5
Table 5.15. Strength of weathered (brown) and unweathered (blue)
London Clay (after Cripps and Taylor, 1981)
Parameter Brown Blue
t u (kPa) 100–175 120–250
c¢ (kPa) 0–31 35–252
j∞ 20–23 25–29
Tropical Soils
In humid tropical regions, weathering of rock is more intense and extends to greater depths
than in other parts of the world. Residual soils develop in place as a consequence of weathering,
primarily chemical weathering (Rahardjo et al., 2004). Consequently, climate (temperature
and rainfall), parent rock, water movement (drainage and topography), age and
vegetation cover are responsible for the development of the soil profile.
Ferruginous and aluminous clay soils are frequent products of weathering in tropical latitudes.
They are characterized by the presence of iron and aluminium oxides and hydroxides.
These compounds, especially those of iron, are responsible for the red, brown and yellow
colours of the soils. The soils may be fine grained, or they may contain nodules or concretions.
Concretions occur in the matrix where there are higher concentrations of oxides in the
soil. More extensive accumulations of oxides give rise to laterite.
Laterite is a residual ferruginous clay-like deposit that generally occurs below a hardened ferruginous
crust or hardpan (Charman, 1988). The ratios of silica (SiO 2 ) to sesquioxides (Fe 2 O 3 ,
Al 2 O 3 ) in laterites usually are less than 1.33, those ratios between 1.33 and 2.0 are indicative
of lateritic soils, and those greater than 2.0 are indicative of non-lateritic types. During drier
periods, the water table is lowered. The small amount of iron that has been mobilized in the
ferrous state by the groundwater is then oxidized, forming hematite, or goethite if hydrated.
The movement of the water table leads to the gradual accumulation of iron oxides at a given
horizon in the soil profile. A cemented layer of laterite is formed that may be a continuous or
honeycombed mass, or nodules may be formed, as in laterite gravel. Concretionary layers
often are developed near the surface in lowland areas because of the high water table.
Laterite hardens on exposure to air. Hardening may be due to a change in the hydration of
iron and aluminium oxides.
Laterite commonly contains all size fractions from clay to gravel and sometimes even larger
material (Fig. 5.6). Usually, at or near the surface, the liquid limit of laterite does not exceed
60% and the plasticity index is less than 30%. Consequently, laterite is of low to medium
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