Applied geomorphology in tropical regions
587
objective (Ollier and Pain, 1996). Seismic refraction profiling gives good results for
obtaining regolith depth but only barely defines some of these zones (Kesel, 1976). The
hammer seismic technique is the most adequate for calculating regolith thicknesses less
than 30 m and has been used to distinguish regolith from colluvium in slopes, in order to
know their evolution (Mills, 1990). The electric method allows the differentiation of layers
with varying electrical resistivity and their thickness within the regolith. Both land and
airborne electromagnetic devices permit the obtaining of three-dimensional maps of the
regolith by the means of measuring the vertical conductivity distribution (Street and
Anderson, 1993). Ground-penetrating radar techniques (GPR) also allow three-dimensional mapping up to depths of about 40 m (Mellet, 1990).
3. Laterites for construction
Laterite is a rock of very diverse uses and is resistant to erosion. It has been used in the
construction of temples in Cambodia, in the building of the San Marcos Spanish Castle in
Saint Augustine, Florida, and in the construction of many routes in tropical zones, such as
the resistant roads of Gabon (Persons, 1970).
As far as engineering is concerned, it is worth differentiating between laterite, which is
an irreversibly hardened rock, and lateritic soil (latosol), which softens when moistened
and hardens when dried (Persons, 1984). Lateritic soils are easily extracted with axes and
pickaxes in quarries. Bricks obtained are dried in the open air and need at least 2 years for
hardening (McFarlane, 1976). Hardened laterite bricks have been employed for wall
construction, revetments, small dams, sewers, channels, and pavements. Its nice texture
and resistance make this rock an attractive material in architecture and engineering
(Persons, 1970, 1984). Its use as a construction material has declined in most tropical
countries, however, and only persists in those where laterite is the only existing resistant
rock. Numerous remains of ancient civilisations have been preserved in which springs,
breakwaters, moats, sewers, and so forth, which were constructed of such materials, have
functioned for hundreds of years (Persons, 1970).
Laterite is also used in the construction of road and airport subgrades. In the streets and
radial roads of Lagos, Nigeria laterite is commonly used due to its resistance and proximity
to quarries. In Brazil and different countries of the Guinea Gulf the construction technique
is as follows (Figure 22.2): oblong blocks of laterite are placed on an excavated base and
the voids are filled with crushed laterite, whose sizes vary between 0.1 and 10 mm
(Persons, 1970, 1984). Afterwards, it is compacted and a 5-cm layer of crushed laterite is
added. In the humid tropics it is necessary for the road subgrade to have good drainage in
order to prevent deformation in the bearing surface. Some lateritic soils are sensitive to
moisture and lose strength with an increase in humidity. For this reason, it is necessary to
ventilate the lateritic soil for enough time before its completion.
Dams made with rock fills and constructed with ground and compacted laterites present
great resistance. This is in part because laterite particles are able to cement together and so
produce an impermeable surface. On the other hand, where no laterite is available, dams
are filled with diverse rocks that are still permeable after compaction and need a later
impermeabilisation (Persons, 1970, 1984).
587
objective (Ollier and Pain, 1996). Seismic refraction profiling gives good results for
obtaining regolith depth but only barely defines some of these zones (Kesel, 1976). The
hammer seismic technique is the most adequate for calculating regolith thicknesses less
than 30 m and has been used to distinguish regolith from colluvium in slopes, in order to
know their evolution (Mills, 1990). The electric method allows the differentiation of layers
with varying electrical resistivity and their thickness within the regolith. Both land and
airborne electromagnetic devices permit the obtaining of three-dimensional maps of the
regolith by the means of measuring the vertical conductivity distribution (Street and
Anderson, 1993). Ground-penetrating radar techniques (GPR) also allow three-dimensional mapping up to depths of about 40 m (Mellet, 1990).
3. Laterites for construction
Laterite is a rock of very diverse uses and is resistant to erosion. It has been used in the
construction of temples in Cambodia, in the building of the San Marcos Spanish Castle in
Saint Augustine, Florida, and in the construction of many routes in tropical zones, such as
the resistant roads of Gabon (Persons, 1970).
As far as engineering is concerned, it is worth differentiating between laterite, which is
an irreversibly hardened rock, and lateritic soil (latosol), which softens when moistened
and hardens when dried (Persons, 1984). Lateritic soils are easily extracted with axes and
pickaxes in quarries. Bricks obtained are dried in the open air and need at least 2 years for
hardening (McFarlane, 1976). Hardened laterite bricks have been employed for wall
construction, revetments, small dams, sewers, channels, and pavements. Its nice texture
and resistance make this rock an attractive material in architecture and engineering
(Persons, 1970, 1984). Its use as a construction material has declined in most tropical
countries, however, and only persists in those where laterite is the only existing resistant
rock. Numerous remains of ancient civilisations have been preserved in which springs,
breakwaters, moats, sewers, and so forth, which were constructed of such materials, have
functioned for hundreds of years (Persons, 1970).
Laterite is also used in the construction of road and airport subgrades. In the streets and
radial roads of Lagos, Nigeria laterite is commonly used due to its resistance and proximity
to quarries. In Brazil and different countries of the Guinea Gulf the construction technique
is as follows (Figure 22.2): oblong blocks of laterite are placed on an excavated base and
the voids are filled with crushed laterite, whose sizes vary between 0.1 and 10 mm
(Persons, 1970, 1984). Afterwards, it is compacted and a 5-cm layer of crushed laterite is
added. In the humid tropics it is necessary for the road subgrade to have good drainage in
order to prevent deformation in the bearing surface. Some lateritic soils are sensitive to
moisture and lose strength with an increase in humidity. For this reason, it is necessary to
ventilate the lateritic soil for enough time before its completion.
Dams made with rock fills and constructed with ground and compacted laterites present
great resistance. This is in part because laterite particles are able to cement together and so
produce an impermeable surface. On the other hand, where no laterite is available, dams
are filled with diverse rocks that are still permeable after compaction and need a later
impermeabilisation (Persons, 1970, 1984).