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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124<br />
cα = δHs/Ho (7.46)<br />
where δHs (mm) = ch<strong>an</strong>ge in specimen height over one log-cycle <strong>of</strong> time<br />
Ho (mm) = initial height <strong>of</strong> specimen<br />
cα = a dimensionless number<br />
R<strong>an</strong>ges <strong>of</strong> values <strong>of</strong> cα determined for black <strong>clays</strong> <strong>an</strong>d red soils in this study are presented <strong>an</strong>d<br />
compared with typical values for other <strong>clays</strong> in Table (7.17).<br />
7.4.4 Procedure<br />
Oedometer consolidation tests were carried out in this study according to British St<strong>an</strong>dard (BS<br />
1377: 1975, Test (17)). The tests were carried out on undisturbed specimens <strong>of</strong> the soils<br />
previously collected from the project area. Use was made <strong>of</strong> a fixed-ring type <strong>of</strong> consolidation<br />
cell consisting <strong>of</strong> a mould <strong>an</strong>d/ or steel cutting ring (for rigidly supporting the soil specimen),<br />
upper <strong>an</strong>d lower drainage surfaces, a loading cap <strong>an</strong>d a provision for adding <strong>an</strong>d/ or<br />
containing distilled water to keep <strong>an</strong>d maintain the soil specimen saturated. The ring was<br />
capable <strong>of</strong> accommodating specimens <strong>of</strong> 14 mm thickness <strong>an</strong>d 71,40 mm diameter. According<br />
to British St<strong>an</strong>dard (BS 1377: 1975), the height to diameter ratio <strong>of</strong> specimens is usually<br />
about 1: 4 to 1:5. The small thickness <strong>of</strong> test specimens used served to ensure that testing<br />
times were not excessively long; <strong>an</strong>d the tests could also be easily extended to long-term tests<br />
to provide secondary compression characteristics <strong>of</strong> soils. The set-up <strong>of</strong> apparatus as used in<br />
this study is presented in Plate (7.8).<br />
A st<strong>an</strong>dard sequence <strong>of</strong> vertical loads r<strong>an</strong>ging from 6 to 2400 kPa was applied to the laterally<br />
confined specimen contained in the consolidation ring; with 6-12 kPa as extended r<strong>an</strong>ge for<br />
very s<strong>of</strong>t soils; 25-800 kPa as normal r<strong>an</strong>ge; <strong>an</strong>d 1600-2400 kPa as extended r<strong>an</strong>ge for stiff to<br />
hard <strong>an</strong>d/ or overconsolidated soils. The actual initial incremental load used for the black<br />
<strong>clays</strong> depended on the amount <strong>of</strong> swelling pressure exhibited by the soils. The resulting<br />
vertical compression (mm) under each load was observed <strong>an</strong>d recorded over a 24 hour period<br />
<strong>of</strong> time. The laterally confining ring had the purpose <strong>of</strong> permitting vertical deformation <strong>of</strong> the<br />
soil specimen, without lateral deformation, so that one-dimensional consolidation<br />
characteristics <strong>of</strong> soils could be investigated.<br />
At the end <strong>of</strong> the last incremental loading stage, the specimen was unloaded in a sequence <strong>of</strong><br />
decrements <strong>an</strong>d allowed to swell in about half the number <strong>of</strong> stages as were applied during<br />
consolidation. Compression readings were plotted cumulatively against time, during both<br />
loading (consolidation) <strong>an</strong>d unloading (swelling) stages (Figs. 7.16 & 7.17). Compression/<br />
time curves obtained for loading stages were used for derivation <strong>of</strong> one-dimensional<br />
consolidation parameters.<br />
Subsequent calculations involving consolidation test data made use <strong>of</strong> <strong>an</strong> average value <strong>of</strong><br />
specific gravity <strong>of</strong> soil particles <strong>of</strong> 2,72 for red soils <strong>an</strong>d 2,70 for black <strong>clays</strong>; assuming<br />
inorg<strong>an</strong>ic <strong>clays</strong> for the two types <strong>of</strong> soil.