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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Chapter 9 Implications <strong>of</strong> index/ <strong>engineering</strong> properties <strong>of</strong> black <strong>clays</strong> <strong>an</strong>d red soils<br />
on construction practice<br />
9.1 Atterberg limits <strong>an</strong>d other index parameters<br />
The spatial distribution <strong>of</strong> values <strong>of</strong> Atterberg limits for black <strong>clays</strong> across the study area at<br />
soil depths <strong>of</strong> less th<strong>an</strong> 0,50m <strong>an</strong>d above 0,50m is presented in the last chapter (Figs. 8.3-<br />
8.6). The distribution also serves to be indicative <strong>of</strong> the possible variation <strong>of</strong> other import<strong>an</strong>t<br />
<strong>engineering</strong> soil properties across the study area. This is evidenced by plotting on the same<br />
axes results <strong>of</strong> Atterberg limits against those <strong>of</strong> free swell <strong>an</strong>d linear shrinkage (see next<br />
chapter), all <strong>of</strong> which demonstrate generally similar variation trends in their spatial<br />
distribution across the area. This points to the possibility that the Atterberg limits <strong>an</strong>d other<br />
index properties are interrelated, so that parameters <strong>of</strong> one property could be readily estimated<br />
from measured values <strong>of</strong> the other, <strong>an</strong>d vice versa. In this study, a correlation <strong>of</strong> results <strong>of</strong><br />
Atterberg limits with those <strong>of</strong> other index properties has been attempted, <strong>an</strong>d the forms <strong>an</strong>d<br />
strengths <strong>of</strong> interrelationship determined <strong>an</strong>d discussed in the next chapter.<br />
The results <strong>of</strong> Atterberg limits could also be usefully applied in the selection <strong>of</strong> soils for use<br />
as compacted fill in various types <strong>of</strong> earthworks construction. This is because <strong>clays</strong> <strong>of</strong> high<br />
plasticity are also usually associated with properties <strong>of</strong> relatively low permeability <strong>an</strong>d a<br />
tendency to consolidate over longer periods <strong>of</strong> time under load th<strong>an</strong> <strong>clays</strong> <strong>of</strong> low plasticity<br />
(Head, 1984). As a result, the high plasticity black <strong>clays</strong> involved in this study would be more<br />
difficult to compact, <strong>an</strong>d are therefore unsuitable for use as fill material; while the relatively<br />
low plasticity red soils could be preferred for the purpose.<br />
The liquid <strong>an</strong>d plastic limits also serve to indicate <strong>an</strong>d describe the consistency state <strong>of</strong> clay<br />
soil involved, i.e. the condition <strong>of</strong> the clay soils in their natural state would be dependent upon<br />
<strong>an</strong>d described by their natural moisture content in relation to these limits, as expressed by the<br />
liquidity index, LI, <strong>an</strong>d relative consistency, Cr, (Table 7.2). The black <strong>clays</strong> could be<br />
described as having a generally firm consistency; while the red soils have firm to stiff<br />
consistency. (However, swampy areas <strong>of</strong> black <strong>clays</strong> <strong>an</strong>d forested areas <strong>of</strong> red soils were<br />
found to show generally s<strong>of</strong>t upper thicknesses <strong>of</strong> soils). As a result, red soils are expected to<br />
be comparatively more stable with respect to supporting light <strong>engineering</strong> structures th<strong>an</strong><br />
black <strong>clays</strong>. This is because the natural moisture content <strong>of</strong> soils as related to the plastic <strong>an</strong>d<br />
liquid limits through the liquidity index (LI) <strong>an</strong>d relative consistency (Cr) is also usually<br />
related to, <strong>an</strong>d serves to be indicative <strong>of</strong> the shear strength <strong>an</strong>d compressibility <strong>of</strong> soils in situ<br />
(Nelson <strong>an</strong>d Miller, 1992). A correlation <strong>of</strong> LI <strong>an</strong>d Cr values with shear strength <strong>an</strong>d<br />
consolidation parameters <strong>of</strong> soils involved in this study is presented <strong>an</strong>d discussed in<br />
respective sections <strong>of</strong> laboratory shear strength <strong>an</strong>d consolidation tests.<br />
Black <strong>clays</strong> have been found to harbour potentially high swelling (over 100% free swell) <strong>an</strong>d<br />
shrinkage (21-29% linear shrinkage) capabilities th<strong>an</strong> red soils (less th<strong>an</strong> 50% free swell;<br />
about 10% linear shrinkage). As a result, volumetric ch<strong>an</strong>ges <strong>an</strong>d/ or exp<strong>an</strong>sive movements in<br />
soils that usually accomp<strong>an</strong>y alternating seasons <strong>of</strong> wet <strong>an</strong>d dry months are likely to have a<br />
comparatively more destabilising effect on light <strong>engineering</strong> structures constructed on black<br />
<strong>clays</strong> th<strong>an</strong> those on red soils.