an engineering geological characterisation of tropical clays - GBV

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