an engineering geological characterisation of tropical clays - GBV

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Chapter 11
Conclusions
This study has highlighted the presence of extensive reactive (potentially shrinking and
swelling) and/ or expansive black cotton soils across the project area. Swelling capabilities
(free swell values) of 100-170% (mean: 130%) and swelling pressures of 49-104 kPa (mean:
77 kPa) are exhibited by the soils.
The study has also highlighted the influence of environmental factors such as rainfall, solar
radiation, vegetation cover and drainage on the nature and behaviour of the soils in situ. Some
sites in areas of black clays exhibited only a fraction of anticipated shrinkage phenomena due
to limited change in soil moisture conditions; whereas at other sites, extremely strong
shrinkage cracking up to substantial depths and in excess of that anticipated based on soil
index properties, has been observed and recorded.
A range of soil index and engineering properties as well as chemical/ mineralogical
composition consistent with the geological diversity of the project area and its surroundings,
has been obtained and recorded.
The variability of index properties and chemical/ mineralogical composition of black cotton
soils is generally uniform with depth and across the study area, implying occurrence of
generally homogeneous black clays.
Most soil index properties could be characterised and/ or estimated with a high degree of
approximation through correlation with results of Atterberg limits and other index tests
performed on disturbed and fractioned samples.
Characterisation of engineering behaviour of soils (shear strength, consolidation-settlement,
swelling pressure & percentage swelling) is better accomplished by direct measurements on
undisturbed core samples rather than through correlation with results of indirect index tests
performed on disturbed and part samples which harbour destroyed soil fabric and structure.
The field and laboratory investigations and studies of soils of the project area have provided a
vast data base on soil index and engineering properties as well as chemical/ mineralogical
composition. This data will provide the basis for on-going and future research on the
characterisation of expansive and reactive black cotton soils, especially as regards their
negative environmental, engineering geological/ civil engineering and social-economic
implications.
There has been a progressive depletion in the proportionate amounts of soluble bases (MgO,
CaO, Na2O, K2O) and a corresponding increase in the proportions of iron and aluminium
compounds (Fe2O3 and Al2O3) contained in the red soils with time. This is evidenced by
comparing results of chemical analyses for the red soils obtained in this study with previous
chemical analyses results for the same soils (Dumbleton, 1967; Sherwood, 1967). The
depletion is most probably attributed to leaching of the soluble bases from the red soils,
facilitated by the relatively good drainage conditions of the soils and high rainfall received
(760-1000 mm per year), thereby leaving the soils enriched in iron oxide and haematite, as
well as aluminium in the form of clay minerals.