an engineering geological characterisation of tropical clays - GBV

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The same relationship is represented by a straight-line graph by plotting S% on a logarithmic
scale (Fig. 7.36b).
The above relationship could be used as a guide to estimate the amount of external loading (P
kPa) necessary to produce an allowed amount of swelling (S mm) or percentage swelling
(S%) of the clays, as may be required for engineering design purposes, if the swelling
pressure (SP), as well as ultimate swelling (Smax) under zero loading, are already known. For
instance, limited swelling of the clays ( i.e. S/ Smax < 0,01or S% < 1) would only be realised
in practice with external loads (P ) close to the swelling pressure (SP), i.e. P% (or P/SP *100)
is close or equal to 100. This could also be confirmed by the above established relationship
by substituting for S/Smax = 0,01 (or S% = 1), i.e.
P% = -22,3Ln(1) + 100,8 = 0 + 100,8, or
P% = (P/SP)*100 = 100,8
On the other hand, zero external loading conditions would be expected to allow for maximum
swelling of the clays, i.e. S% = 100, in normal practice. However, substituting P% (i.e. P = 0
kPa) in the above relationship gives an underestimated value of S% = 92, i.e.
0 = -22,3Ln(S) + 100,8 or Ln(S) = 100,8/22,3
so that S = e^4,52 = 92%
The underestimation of percentage swelling from known loading pressures could be explained
by the fact that, test specimens of clays underwent some amount of plastic deformation (or
consolidation) when initially loaded to their swelling pressure values, and could not therefore
recover their original sizes on complete unloading. In this case, the extent of plastic
deformation is equivalent to S% = 8 (i.e., 100-92), on the average. A suggested remedy of this
shortcoming would be to determine the ultimate amount of swelling (Smax) under zero
loading on separate test specimens from those used in the swelling test. Cumulative swelling
values obtained during the test would then be expressed as a percentage of this separately
determined maximun swelling value of Smax.
Future works on swelling tests involving larger clay sample sizes and data could yield more
closely approximating logarithmic relationships with relatively stronger correlations.