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