an engineering geological characterisation of tropical clays - GBV

209
cc = 0,0099(122-LL)
for black clays, and
cc = 0,0016(308-LL)
for both black clays and red soils, where
cc = a dimensionless number, and
LL (%) = liquid limit
Correlation between so calculated compression indices and laboratory measured compression
indices were found to be strong (R = 0,85) for the black clays alone; and moderate (R = 0,58)
for combined black clays and red soils. However, these derivations were based on a limited
amount of data on compression indices (n = 6 for black clays; n = 9 for black clays and red
soils) available in the current study. It would be useful that future works attempt the same
correlations and derivations based on a larger amount of data as regards laboratory results of
liquid limit tests performed on disturbed samples on one hand, and those of compression
indices obtained from oedometer tests on undisturbed specimens on the other. Possible
improvements in the strength of correlations and assessment of compressibility characteristics
of soils using index properties (Atterberg limits) would also be indicated. Investigations could
also be extended to other types of soil world-wide, to find out if the same and/ or similar
relationships hold.
All in all, it may be useful to initiate and establish a field monitoring program for the soils of
the Nairobi area, aimed at monitoring of expansive/ reactive soil movements and moisture
changes. The selected sites would involve soils of varying geographic and lithological
conditions. This would serve the purpose of providing a comprehensive picture of expansive
and/ or reactive soil behaviour, in terms of characterisation of site reactivity (shrink-swell
potential) and unsaturated moisture flow. The scheme my take the form of monitoring and
recording profiles of ground movements and moisture changes as well as providing general
data on soils with time (e.g. on a monthly basis), over a period of some years. A further
analysis in terms of environmental factors would be necessary and helpful.
According to Delaney & Allman (1998), useful instrumentation for a field monitoring
program may include use of surface and sub-surface pegs (surface and sub-surface levels),
neutron probes (soil volumetric moisture content), thermocouples (soil temperature), gypsum
blocks (matrix soil suction), piezometer (groundwater level, especially in alluvial soils),
automatic weather stations (rainfall, humidity, temperature, solar radiation, wind velocity/
direction) etc. Results may include data on design parameters for foundations on expansive
and/ or reactive soils (e.g. active depth, suction change, seasonal heave etc.), as well as
detailed information on field behaviour to compare with models which may be developed in
the laboratory to simulate and characterise possible field behaviour of the soils. The results
would also provide a basis for comparing and assessing the different methods of
characterising site reactivity and/ or shrink-swell behaviour.
The first of the two methods developed in this study to estimate swelling characteristics of
black clays [i.e. Eq. (7.49): P = -22,3Ln(S) + 100,8] was shown to be limited in predicting
percentage swelling that may be realised under selected structural loading conditions. It was
also explained that the underestimation of percentage swelling values was most probably a
result of some degree of plastic deformation (or consolidation of test specimens when initially