an engineering geological characterisation of tropical clays - GBV

5
Plates 1.2 (a) & (b) Strong shrinkage cracks in black cotton soils (black clays) during dry months.
Examples of structural damage triggered off by ground movements associated with expansive and
reactive soils in the Nairobi area include cracked pavements as well as cracked and peeled-off
asphalt on tarmac roads giving way to formation of pot holes (Plates 1.3 (a) & (b)). The cause is
strong shrinkage cracks (Plates 1.2 (a) & (b)) that usually characterise black cotton soils during the
dry months. In light residential and low-rise buildings, the instabilities manifest themselves in the
form of cracked floors and walls and/ or tilting buildings (Plate 1.4). As part of an effort to offset
the huge costs the government has to incur every year in rehabilitating and maintaining damaged
roads, pavements and other infrastructure, the ministries of Environment and Public Works are
emphasising on soil research aimed at providing data and information relevant to producing suitable
foundation designs capable of withstanding the otherwise destructive effects of the reactive black
cotton soils found. Best suited and economical ground and/ or soil stabilisation techniques are also
to be sought. This study therefore forms a part of the collective efforts geared towards fulfilling
these goals.
Plate 1.3 (a) Peeling asphalt on tarmac road.
Plate 1.3 (b) Pothole formed on tarmac road.
Light and surface engineering structures are usually constructed on/ or within the upper layers of
soils (Galster, 1977; Maharaj, 1995; Plate 1.5). Because of the above stated potential instability
problems that would be posed to such structures in the current study area, a prior systematic and
sound mapping, characterisation and evaluation of the black cotton soils found together with the
underlying bedrock, just as was undertaken in this study, are an essential prerequisite. The results of
study and investigations so performed would be crucial to a sound engineering geological
assessment of the soils. These results would assist geo-engineers and city planners to anticipate in
advance potential limitations and engineering behaviour of the soils at sites intended for selected
projects. The possible alternatives at this stage would be for the geo-engineers and planners to use
the ground information and data so provided to recommend appropriate ground stabilisation/
improvement techniques aimed at improving the capacity of the soils to support increased loads