an engineering geological characterisation of tropical clays - GBV

95
where
7.1.4.4 Results
V = volume of settled solid particles
Vo = 10ml, i.e. original volume of dry soil
Results of free swell test (FS %) obtained from the present study are given in Table (7.2), and
are reported to the nearest whole number or 1%. Classification of soils based on swelling
capabilitiy is summarised in Table (7.3). Black clays have high swelling capabilities, being
characterised by free swell values of 100% and over. The red soils with free swell values of
less than 50% (i.e. 15-20%) are classified as low swelling capability clays. Potential influence
and/ or effects of these swelling capabilities on light constructed structures are discussed in
the next chapter.
7.1.5 Linear shrinkage
7.1.5.1 Scope and procedure
Linear shrinkage tests were carried out in this study according to British Standard testing
procedure (BS 1377: 1975, Test 5). The tests usually serve to determine the percentage linear
shrinkage of clays as well as soils of low plasticity such as silts (Head, 1984). In practice, the
results of the tests serve to point to possible volumetric changes soils would undergo in situ
during a change of seasons from rainy wet months to hot dry ones. In this study, the tests were
performed on both the highly plastic black clays as well as the low plasticity silty red soils;
and involved determination of linear or one-dimensional change in length of a semicylindrical
bar sample of clay soil when dried out, starting from near the liquid limit.
The apparatus and procedure are as summarised in Plate (7.4) in which a suitable amount of
selected soil, previously dried (105 °C) and conveniently prepared, was mixed with distilled
water to produce a homogeneous paste around its liquid limit. The paste, initially filled into a
semi-cylindrical standard metal (brass) mould of 140 mm length and 25 mm diameter, was
dried (105 °C) and the average length (Ld) of dried test specimen determined by measuring its
top and lower surface lengths. Linear shrinkage, LS, was calculated as a percentage of the
original length of the specimen, i.e.
LS (%) = ((Lo-Ld)/Lo)*100 (7.9)
where Lo = original length (140 mm) of semi-cylindrical bar sample of soil at about its liquid
limit
Ld = average length of dried specimen
Results of linear shrinkage tests obtained for the soils in this study are presented in Table
(7.2), and are reported to the nearest 1%. Black clays exhibit LS values of 21-29%, while red
soils show relatively lower values of 10-11%.