an engineering geological characterisation of tropical clays - GBV

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