an engineering geological characterisation of tropical clays - GBV

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convex-upwards portion of the primary curve was passed before any readings could be taken.
In this case, the do point was estimated from the square-root-time/ settlement curve and then
transferred to the log-time curve for the conventional analysis to be carried out. According to
Head (1988), cv values in the range of 10-100 m²/year are indicative of quite rapid settlements
which would not be expected to cause long-term instability problems.
Some of the red soils classified as clayey silts to silt types gave rise to log-time/ settlement
curves which were concave upwards from the start implying relatively rapid draining soils.
Both the initial convex portion and point of inflexion (Figs. 7.11 & 7.13) were passed before
the first recorded reading was made so that determination of do and d100 points corresponding
to 0% and 100% primary consolidation, respectively, was difficult. The square-root-time
curve could also not be used for determination of the do point due to lack of the necessary
linear portion of the curve (Figs. 7.12 & 7.14). The value of cv was therefore estimated by
assuming the occurrence of the d50 point somewhere mid-way between points do and df on the
log-time curve (Fig. 7.13). A value of t50 was then estimated and used to calculate cv. A range
of t50 less than 0,10 minutes, implied
cv > (0,0256 * H'²)/t50 = 0,0256 * 14²/0,1 or
cv > 50 m²/year, implying very rapid drainage and settlement of the soil,
where H' (mm) = mean specimen height
d50 (mm) = compression reading for 50% primary consolidation
dc (mm) = corrected initial compression reading
df (mm) = final compression reading
t5o (min) = time for 50% primary consolidation
Use of a Rowe consolidation cell (Rowe,1966), capable of accommodating larger soil
specimens would be recommended for oedometer tests on the clayey silt and silty varieties of
red soils, so as to obtain a more definite value of cv for estimation of rates of settlement under
various loads.
7.4.6 Results of consolidation tests
Results of log-time settlement curves obtained for black clays and red soils in this study are
illustrated in Figures (7.16) and (7.17), respectively. The red soil specimen is shown to
undergo settlements of more than 6mm over a loading range of up to 2400 kPa. Settlements
undergone by black clay specimen are less than 3mm over the same loading range. Results in
this study therefore show the red soils to be relatively more compressible than the black clays.
In addition, unloading curves for the black clays are more inclined than those of the red soils
which are more or less flat. This is indicative of the black clays having relatively more swelling
capabilities and/ or expansion tendencies on unloading than the red soils.