an engineering geological characterisation of tropical clays - GBV

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9.2 Grain size distribution
The results of particle size analysis obtained in this study have been used to complement those
of Atterberg limits to give a complete description, and thereby reflect the probable physical
and/ or engineering behaviour of the clay soils. This is illustrated in Tables (7.1) and (7.4), as
well as Figure (7.2) where the colloidal activity and probable expansive behaviour of the soils
have been calculated and/ or inferred from measured values of Atterberg limits (plasticity
index), free swell and percentage clay fraction. The black clays and red soils both show high
contents of the fine fraction (silt and clay sizes) of over 70%. The higher plasticity and
potentially expansive behaviour exhibited by the black clays could be accounted for in terms
of clay mineral composition, i.e. a higher content of smectites (over 90%) which usually
exhibit high swelling capabilities on wetting. On the contrary, the red soils are predominated
by the rather less expansive kaolinite (over 80%) which usually exhibits limited activity and
swelling capabilities on wetting.
In addition, the results could be used in conjuction with those of Atterberg limits and free
swell (swelling capability) when considering the soils for use as fill material in various civil
and construction works. This is because specific civil engineering works such as
embankments, earth dams and/ or sub-base materials for roads and airfield runways; all
require the soils to meet certain grading specifications to provide a mechanically stable
foundation (Nelson and Miller, 1992). The results of particle size analysis would also serve to
assess and indicate the probable feasibility and/ or effectiveness of dynamic compaction
technique when used to improve poor ground conditions in projected sites. In the present
study for instance, the black clays and red soils have been classified into soils of high and low
activity and/ or swelling capability/ expansiveness, respectively; on the basis of grain sizes,
Atterberg limits and free swell (Tables 7.1 and 7.4). As a result, the black clays with probable
high swelling capability/ expansive behaviour would generally be difficult to compact and
thus unsuitable for use as fill material; while the relatively low swelling capability/ expansive
red soils would be preferred.
Results of particle size analysis, and especially the clay fraction and/ or fine fraction, have
been correlated with other soil parameters and index properties (see next chapter). This had
the purpose of exploring and establishing possible forms of interrelationships that could aid
in approximately estimating and/ or inferring for important soils` physical/ mechanical and/ or
engineering behaviour relevant to projection/ planning and construction of light structures.
The results of the analysis have also aided in classification of the black soils as being mainly
silty clays and/ or silty clays with sand, while varieties classified as clay and clayey silt also
occur but to a lesser extent. The red soils fall in the classes of clayey silts for depths of up to
1,0m; and silty clays with sand, for depths greater than 1,0m (Table 7.7). The classification
implies that the fine fraction (silt and clay sizes) predominates both types of clays, and would
most likely be the significant component in influencing and/ or controlling the engineering
properties/ behaviour of these soils.
9.3 Shear strength parameters
The red soils are characterised by larger shear angles (mean φ´: 28,5°) than the black clays
(mean φ´: 18°); and this implies that the former are relatively more stable and have a larger
capacity in supporting light engineering structures than the latter.