an engineering geological characterisation of tropical clays - GBV

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There has also been a decrease in the proportions of clay fraction as well as values of
Atterberg limits and other index properties in the red soils with time. This has been evidenced
by comparing results of soil classification tests obtained for the soils in this study with similar
and earlier results obtained for the same soils (Sherwood, 1967). This decrease could be
explained by the increasing presence of free iron oxide in the soils, and which has the effect
of cementing the clay particles together, thereby causing a progressive decrease in both the
proportions of clay as well as values of Atterberg limits of the red soils with time.
Results of X-ray diffraction (XRD) analyses show the mineralogical composition of red soils
to be mainly kaolinite (80-81%) and haematie (15-16%). Accessories of quartz (1-3%),
titanium, i.e.ilmenite and/ or rutile (~ 2%) and manganese (~ 1%) also occur. The kaolinite is
depleted in Al2O3 and this is probably caused by low PH conditions imparted to the soils by
weak carbonic acids formed by processes of organic matter decomposition.
The red soils encountered in this study are polygenetic; and have been derived, developed and
formed under humid conditions, high temperatures and presence of carbon dioxide by
weathering and alteration of Nairobi trachytes, volcanic tuff and ash. This suggestion is
supported by comparing results of chemical analyses obtained for the red soils in this study
with similar results previously obtained for the underlying volcanic materials; and both of
which generally fall into close agreement. A limited contribution by eroded and watertransported
detrital materials is confirmed by results of scanning electron microscope which
reveal inclusions of sub-rounded to rounded quartz grains. Leaching effects in the red soils
and consequent removal of silica and soluble bases have caused the soils to exhibit lower
contents of SiO2, MgO, CaO, Na2O, K2O and correspondingly higher contents of Fe2O3 and
Al2O3 relative to the volcanic materials. This is evidenced by analysis results of scanning
electron microscope for the red soils which show Fe-rich crusts as part of the matrix while
quartz and feldspars are in limited availability and/or virtually absent.
On the other hand, results of X-ray diffraction and scanning electron microscope analyses
show the clay minerals composition of black clays to be mainly smectites and/ or
montmorillonite (over 90%) and some kaolinite (less than 10%). Occurrence of illite is
occasional and only in trace form. Quartz (4-9%) and accessories of K-feldspars (sanidine,
orthoclase, microcline), haematite and carbonates (calcite, dolomite, ankerite, siderite) also
occur.
Results of chemical analyses together with current field observations and evidences related to
the nature of occurrence of black clays show that these soils are not wholly developed in situ;
and are not derived from the underlying practically impermeable Nairobi and Kapiti
phonolites through weathering and alteration. Rather, the black clays have been derived from
the gradual conversion of volcanic ash as well as colluvium, alluvium and soft Pleistocene
materials, all previously occurring as lacustrine deposits in a basin-like lake and swampy
environment during a pluvial period. Further confirmation is provided by results of scanning
electron microscope which show presence of detrital components of biotite and sub-rounded
to rounded quartz grains as well as heavy minerals of Ti and Zn in the black clays, a result of
erosion processes and water transport of soil/rock materials from surrounding areas. The black
clays generally exhibit higher concentrations of soluble bases and silica than the red soils; and
this confirms the suggestion that leached components of soluble bases and silica derived from
areas of red soils occurring on higher grounds serve to further contribute to the formation of
black clays in the plains under alkaline and impeded drainage conditions.