an engineering geological characterisation of tropical clays - GBV

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Summary (Abstract)
An engineering geological characterisation of tropical clays. Case study: Clay soils of Nairobi, Kenya
Simon Wanyonyi Otando, M.Sc.
The purpose of this project was to study and evaluate soils engineering geological characteristics with a view to determining their form of distribution and
spatial variation across the study area. It also aimed at assessing the suitability, capacity and/ or limitations of the soils to support low/ light constructed
engineering structures, as well as recommending best suited and economical soil improvement/stabilisation techniques where necessary. The study and
investigation was therefore centered on determining/ developing statistical models to describe and predict the distribution of soils engineering properties.
Results of statistical analyses served to determine the degree of homogeneity or uniformity of the soils. It also aimed at developing/ preparing small-scale
and site specific engineering geological maps to provide spatially continouos ground information, soil data and depth variations across the study area. The
study also sought to present results of soil classifications and correlations between soil index and/ or engineering properties. A detailed chemical/
mineralogical analysis and evaluation of the soils and underlying bedrock was also carried out to investigate and establish the genesis of the soils.
The current study area lies mainly to the south of the fast growing Nairobi city, Kenya. It is covered by topographic sheet 148/4 of Nairobi from Survey of
Kenya, bounded by latitudes 1°15´S and 1°24´S to the north and south; and longitudes 36°45´E and 36°58´E to the west and east ; respectively, and
occupies an area of about 367 km². The area is covered mainly by varying thickness of black cotton soils/ black clays. Red soils are limited to the northwestern
part of the area. Underlying the soils are Tertiary volcanics which inturn overly a Basement system of folded Precambrian metamorphic rocks. The
research and study was centered on black clays with red soils included for comparison purposes.
The present area was chosen because it is mainly located on the outskirts of Nairobi city which are projected for future expansion of the town, as regards
construction of new social-economic and industrial/residential facilities. There is also a shiftimg trend in building tradition from timber and iron-sheet
structures on strip/ pad footings towards masonry construction of low buildings on slab foundations using dimension stone and bricks, hence the need for an
engineering properties characterisation of the soils. Current field observations also show the black clays to be potentially expansive/ reactive and exhibit
ground instabilities, failures and movements; with serious environmental, engineering and social-economic implications.
The study incorporated suitable field procedures, sampling criteria and laboratory chemical/ mineralogical and engineering soils testing methods. The field
study methods included cone penetration sounding, soil augering, in-situ description of soils and underlying rock, tests for possible carbonate content, as
well as undisturbed and disturbed soil sampling. This study phase had the objectives of providing information on the nature and distribution of expansive/
reactive black cotton soils as well as red soils of the study area. It also served to acquire site specific information on environmental characteristics
(groundwater conditions, climate, vegetation) which impact on in situ soil behaviour, investigate and model depth variation of soils; as well as compare in
situ soil properties/behaviour and derived parameters with those predicted and/ or derived from laboratory studies.
Laboratory studies performed on soil samples collected from the field included index tests of natural moisture content, Atterberg/plasticity limits, swelling
capability/free swell and linear shrinkage, as well as grain size analysis. Direct shear strength tests, Oedometer consolidation/ compressibilty tests,
percentage swelling and swelling pressure tests were also carried out by performing the tests on undisturbed samples. Laboratory chemical/ mineralogical
analyses and/ or studies employed the use of X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques.
The field and laboratory investigations/ studies of soils of the project area have provided a range of soil index and engineering properties as well as
chemical and mineralogical composition. The results of study show the distribution and spatial variation of index properties and chemical/ mineralogical
composition of black clays to be generally uniform across the study area, implying a generally homogeneous occurrence and engineering character of the
clays. Results of chemical/mineralogical analyses show the black clays to be composed mainly of smectites (montmorillonite) as well as having been
largely genetically derived from the gradual conversion of lacustrine deposits of volcanic ash, colluvium, alluvium and soft Pleistocene materials previously
deposited in a basin-like lake and swampy environment during a pluvial period. Contributions by leached components from areas of red soils as well as
eroded materials from surrounding areas are to a lesser extent.
Index properties of red soils point to a comparatively more stable and reliable engineering character of the soils. Results of chemical/ mineralogical analyses
show these soils to be mainly kaolinite in composition and polygenetic in origin. The soils must have derived/developed and formed under humid conditions
by weathering/alteration of underlying Nairobi trachytes/ volcanic tuff and volcanic ash. A limited contribution was derived from eroded and watertransported
detrital materials.
The index properties and grain sizes have been used to classify black clays into very high to extremely high plasticity clays and/ or silty clays exhibiting
medium to very high levels of activity as well as high swelling capabilities/ potential expansiveness on wetting from a dry condition. This engineering
behaviour could be accounted for by the high content of the more expansive clay minerals of smectites (90% and over) in the black clays. Likewise, the red
soils have been classified as medium to high plasticity clayey silts and silty clays with sand showing medium activity levels and generally low swelling
capabilities/ potential expansiveness when allowed free access to water. These characteristics have been attributed to the high content (80% and over) of the
rather less expansive clay mineral, kaolinite, in the red soils.
Results of laboratory shear strength investigations showed the black clays to be limited in strength and have relatively low angles of shear resistance
averaging about 18°. The red soils were found to be relatively more stable, with angles of shear resistance of 28° to 29°. On the other hand, results of
laboratory oedometer consolidation tests show the red soils to exhibit medium to very high compressibility and medium to high rates of consolidationsettlement
on normal external loading. This could be attributed to their loose friable nature and relatively higher porosity and permeability which facilitate
faster drainage on external loading. These soils would therefore be expected to undergo rapid consolidation – settlements, especially during the construction
stage, without posing long-term instability problems to constructed structures. However, the black clays would exhibit slightly lower compressibility and
low rates of consolidation-settlement due to their compact/cohesive nature as well as relatively lower porosity and permeability which tend to limit
drainage. In practice therefore, settlement and instability of structures located on these clays would be expected to persist beyond the construction stage.
Results of statistical analysis and correlations show that most soil index properties could be characterised and/ or estimated with a high degree of
approximation from results of Atterberg limits and other index tests performed on disturbed and fractioned samples. New relationships have also been
derived and developed to estimate and predict percentage swelling and swelling pressure characteristics of black clays in situ under various loading
conditions. However, in situ characterisation of engineering behaviour of soils as regards shear strength, consolidation-settlement, swelling pressure and
percentage swelling was shown to be better accomplished by direct measurements on undisturbed core samples rather than through correlation with results
of index tests performed on disturbed/ part samples which harbour destroyed soil fabric and structure.
The vast data base of soil index and engineering properties as well as chemical/ mineralogical composition obtained from field and laboratory
investigations/ studies of soils of the project area, together with prepared engineering geological/ geotechnical maps would be readily useful and applicable
by engineering geologists, geo-engineers and planners of future-intended construction projects in the area. The results of the investigations together with
related engineering geological assessments would assist especially at the stages of planning, designing and construction of projected facilities; and also
subsequent maintenance after completion. The results will also serve to complement on-going and future research on the characterisation of expansive and
reactive black cotton soils, especially as regards their negative environmental, engineering and social-economic implications.