an engineering geological characterisation of tropical clays - GBV

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