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