an engineering geological characterisation of tropical clays - GBV

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