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