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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205<br />
cc = 0,0099(122-LL)<br />
for black <strong>clays</strong>, <strong>an</strong>d<br />
cc = 0,0016(308-LL)<br />
for both black <strong>clays</strong> <strong>an</strong>d red soils.<br />
Correlation between so calculated compression indices <strong>an</strong>d laboratory measured compression<br />
indices were found to be strong (R = 0,85) for the black <strong>clays</strong> alone; <strong>an</strong>d moderate (R = 0,58)<br />
for combined black <strong>clays</strong> <strong>an</strong>d red soils.<br />
Correlations between swell indices, Cs, <strong>an</strong>d index properties (Atterberg limits, free swell) <strong>of</strong><br />
black <strong>clays</strong> <strong>an</strong>d red soils were found to be generally poor. The black <strong>clays</strong> exhibit signific<strong>an</strong>t<br />
swelling pressures <strong>of</strong> 49-104 kPa. However, strengths <strong>of</strong> correlation between swelling<br />
pressures <strong>an</strong>d index properties were found to be generally weak. The above findings serve to<br />
highlight uncertainities that may be encountered in attempting to assess <strong>an</strong>d predict exp<strong>an</strong>sion<br />
tendencies (on unloading in situ) as well as swelling pressures imposed by clay soils on their<br />
saturation, based on results <strong>of</strong> laboratory index tests carried out on disturbed <strong>an</strong>d fractioned<br />
soils.<br />
Swelling test results have facilitated derivation <strong>of</strong> two methods <strong>an</strong>d/ or relationships that serve<br />
to summarise the swelling characteristics <strong>of</strong> black <strong>clays</strong>. Both relationships are logarithmic in<br />
representing the variation <strong>of</strong> percentage swelling (S%) with extent <strong>of</strong> external loading, P%<br />
(i.e., load decrements, P kPa, expressed as a percentage <strong>of</strong> swelling pressure, SP kPa). The<br />
first relationship involves deriving percentage swelling by expressing amount <strong>of</strong> swelling (S<br />
mm) as a fraction <strong>of</strong> the ultimate swelling (Smax mm) which occurs under zero external<br />
loading conditions. This relationship exhibits a very strong correlation (R = 0,995), i.e.<br />
P = -22,3Ln(S) + 100,80, where<br />
S (%) = percentage swelling (S mm/Smax mm)<br />
P (%) = imposed loads (P kPa) expressed as a percentage <strong>of</strong> swelling pressure (SP kPa)<br />
Based on this first relationship, therefore, it has been shown that black <strong>clays</strong> would undergo<br />
high percentage swelling (S >75%) when externally loaded to less th<strong>an</strong> 5 kPa; <strong>an</strong>d low<br />
percentage swelling for loads <strong>of</strong> over 80 kPa. As a result, external loads <strong>of</strong> well over 100 kPa<br />
would ensure minimal potential destabilisation <strong>of</strong> constructed light <strong>engineering</strong> structures<br />
from swelling effects <strong>of</strong> the <strong>clays</strong> on wetting. For known swelling pressure (SP) <strong>an</strong>d ultimate<br />
amount <strong>of</strong> swelling (Smax) under zero loading, the relationship could be possibly used as a<br />
guide to estimate external loading <strong>of</strong> light <strong>engineering</strong> structures necessary to give a desired<br />
<strong>an</strong>d/ or permitted percentage swelling, as may be required by design computations. However,<br />
the relationship c<strong>an</strong>not be reliably used for estimation <strong>of</strong> percentage swelling from known<br />
values <strong>of</strong> external loads, since the former is usually underestimated by <strong>an</strong> average amount <strong>of</strong><br />
S = 8%.<br />
The second relationship has percentage swelling derived by relating amount <strong>of</strong> swelling, S<br />
mm, to initial test specimen thickness, Ho mm. It is also closely logarithmic with a similarly<br />
very strong correlation (R = 0,98), i.e.<br />
P = -15,86Ln(S) + 29,84