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