an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
8<br />
<strong>an</strong>d soil behaviour, based on results <strong>of</strong> field <strong>an</strong>d laboratory investigations. The investigations would<br />
be specific in seeking to establish <strong>an</strong>d/ or model moisture variation with depth, as well as providing<br />
details on depth <strong>of</strong> cracking, soil reactivity in terms <strong>of</strong> shrink-swell potential, active depth; <strong>an</strong>d<br />
other essential variables necessary in the assessment <strong>an</strong>d/ or computation <strong>of</strong> potential ground<br />
surface movement.<br />
In conclusion, this study sought to investigate <strong>an</strong>d present a background on the nature <strong>of</strong> exp<strong>an</strong>sive<br />
<strong>an</strong>d/ or reactive soils <strong>of</strong> the study area, by following suitable field procedures <strong>an</strong>d sampling criteria<br />
as well as laboratory chemical/ mineralogical <strong>an</strong>d <strong>engineering</strong> soils testing methods. It also sought<br />
to present results <strong>of</strong> soil classifications <strong>an</strong>d correlations between soil index <strong>an</strong>d/ or <strong>engineering</strong><br />
properties. Results <strong>of</strong> statistical <strong>an</strong>alyses involving hypothesis testing seeking to determine the<br />
degree <strong>of</strong> homogeneity or uniformity <strong>of</strong> the soils found, have also been presented. The undertaking<br />
<strong>of</strong> this project by the author was also <strong>an</strong> initiative aimed at providing <strong>an</strong> approach <strong>of</strong> study that<br />
would serve as a basis <strong>an</strong>d/ or framework for comprehensive <strong>an</strong>d detailed <strong>engineering</strong> <strong>geological</strong><br />
investigation <strong>an</strong>d assessment <strong>of</strong> exp<strong>an</strong>sive <strong>an</strong>d reactive soils in lithologically similar areas around<br />
Nairobi, <strong>an</strong>d Kenya at large.<br />
1.5 Physiography<br />
The <strong>geological</strong> history <strong>of</strong> Nairobi, including the current study area, has been dominated by volc<strong>an</strong>ic<br />
activity since Miocene times. These areas are currently underlain by a series <strong>of</strong> volc<strong>an</strong>ic rocks as a<br />
result <strong>of</strong> successive lava flows that originated from centres <strong>an</strong>d fissures on the high eastern fl<strong>an</strong>k <strong>of</strong><br />
the Rift region to the west (Gregory, 1921; Saggerson, 1991; Fig. 3.1; Plates 1.7 (a) & (b)).<br />
Plates 1.7 (a) & (b) View <strong>of</strong> the Great Rift Valley from its eastern fl<strong>an</strong>k, to the west <strong>of</strong> Nairobi area.<br />
The flow <strong>of</strong> lava was generally eastwards, onto a warped <strong>an</strong>d partly dissected pre-Miocene erosion<br />
surface below which were older crystalline metamorphic rocks. A few isolated occurrences <strong>of</strong> rocks<br />
<strong>of</strong> Precambri<strong>an</strong> <strong>an</strong>d Quaternary age are also found.<br />
The geomorphological evolution <strong>of</strong> Nairobi <strong>an</strong>d the study area has therefore been mainly controlled<br />
by the volc<strong>an</strong>ic activity that accomp<strong>an</strong>ied the Rift Valley formation. The physiography <strong>of</strong> these<br />
areas is consequent upon the volc<strong>an</strong>ic rocks found, <strong>an</strong>d the tectonic movements which have affected<br />
them (Saggerson, 1991; Sikes, 1926).<br />
The study area is a part <strong>of</strong> the lava plains (Plate 1.8 (a) & (b))that are bordered to the north-west <strong>an</strong>d<br />
west by the Kikuyu highl<strong>an</strong>ds, <strong>an</strong> extension <strong>of</strong> the high ground <strong>of</strong> the eastern fl<strong>an</strong>k <strong>of</strong> the Rift<br />
Valley; <strong>an</strong>d to the south-west by the Ngong hills. The Kikuyu highl<strong>an</strong>ds are characterised by a steep<br />
downward slope in <strong>an</strong> easterly direction. The drainage is consequent, follows the slope <strong>an</strong>d<br />
comprises <strong>of</strong> a number <strong>of</strong> streams giving rise to parallel ridges, some <strong>of</strong> which get broader as the<br />
streams converge. The streams are still down-cutting giving rise to ridges with convex to uniform