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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13<br />
marked daily r<strong>an</strong>ge <strong>of</strong> relative humidity is not uncommon, with the air in the early mornings at/ or<br />
very close to saturation. The afternoons are characterised by a relative humidity <strong>of</strong> about 50%,<br />
although this may sometimes be as low as 10% on clear sunny days especially in February <strong>an</strong>d<br />
March (Kenya Meteorological Department, 2001). Table 1.2 shows a summary <strong>of</strong> me<strong>an</strong> relative<br />
humidity values (%) for each month in a year, both in the mornings (9:00 am) <strong>an</strong>d in the afternoons<br />
(3:00 pm).<br />
Nairobi <strong>an</strong>d its surroundings receive a considerable amount <strong>of</strong> sunshine, averaged at about 7 hours<br />
<strong>of</strong> bright sunshine per day, throughout the year. The quality <strong>of</strong> ultra-Violet radiation has also been<br />
shown to be very high (Kenya Meteorological Department, 2001). The early mornings are <strong>of</strong>ten<br />
cloudy. There is therefore 30% more sunshine in the afternoon th<strong>an</strong> in the morning, with westerly<br />
exposures receiving more insolation th<strong>an</strong> those facing east. There is also considerably more<br />
sunshine during the 6 months that the sun is in the southern hemisphere, th<strong>an</strong> when it is in the north.<br />
However, days with no sunshine at all occasionally occur; <strong>an</strong>d this especially during the rainy<br />
season (mid-March to May), <strong>an</strong>d/ or in the months <strong>of</strong> June, July <strong>an</strong>d August. Average number <strong>of</strong><br />
hours per day <strong>of</strong> bright sunshine for each month <strong>of</strong> the year are summarised in Table 1.2 (Kenya<br />
Meteorological Department, 2001).<br />
1.7 Method <strong>of</strong> survey<br />
The criteria used in demarcating the current project area <strong>an</strong>d selection <strong>of</strong> sampling sites were in<br />
such a way as to accomplish the following:<br />
(i)<br />
(ii)<br />
(iii)<br />
(iv)<br />
(v)<br />
Include areas <strong>of</strong> future residential, industrial <strong>an</strong>d civil <strong>engineering</strong> structural<br />
developments<br />
Include areas currently subjected to exp<strong>an</strong>sive ground movements, pronounced soil<br />
reactivity (strong shrinkage cracking <strong>an</strong>d potentially high exp<strong>an</strong>sive/ swelling character)<br />
<strong>an</strong>d with deteriorating <strong>an</strong>d/ or damaged <strong>engineering</strong> structures<br />
Include a r<strong>an</strong>ge <strong>of</strong> terrain, vegetation <strong>an</strong>d site development conditions<br />
Cover fluvial <strong>an</strong>d swampy soil deposits<br />
Ensure site accessibility during the whole project period<br />
In <strong>an</strong>y case, the project area was selected after consulting with relev<strong>an</strong>t government ministries<br />
(Ministry <strong>of</strong> Environment <strong>an</strong>d Natural Resources, Ministry <strong>of</strong> Public works), local councils <strong>an</strong>d<br />
authorities (Nairobi City Council, Kenya Airports Authority), as well as private comp<strong>an</strong>ies <strong>an</strong>d<br />
l<strong>an</strong>downers.<br />
A desk study phase was undertaken before work in the field commenced. This phase involved the<br />
study <strong>an</strong>d interpretation <strong>of</strong> aerial photographs under a stereoscope (Allum, 1966; Colwell, 1983;<br />
Verstappen, 1980), by drawing boundary lines round the various shade tones, vegetation <strong>an</strong>d relief<br />
patterns (steep slopes, valleys, plains, rocky outcrops). These features were later investigated during<br />
the field study phase by following pl<strong>an</strong>ned field pr<strong>of</strong>iles <strong>an</strong>d road traverses. The patterns which<br />
were found to be related to soil had their boundaries marked/ traced on the photos. The information<br />
was then tr<strong>an</strong>sferred from the photos to the 1:25 000/ 50,000 base maps on return from the field.<br />
The aerial photographs covering the present study area <strong>an</strong>d its environs were provided by the<br />
Survey <strong>of</strong> Kenya, <strong>an</strong>d were printed in 1990.<br />
Observations made during the field survey showed that soil boundaries could easily be delineated<br />
on the aerial photos in areas <strong>of</strong> natural vegetation, perennial crops <strong>an</strong>d wetl<strong>an</strong>ds. Varying shade<br />
tones on the photos were found to correspond to different vegetation types <strong>an</strong>d patterns which in<br />
turn served to reflect soil ch<strong>an</strong>ges across the study area in terms <strong>of</strong> differing texture, depths as well