an engineering geological characterisation of tropical clays - GBV

125
Plate 7.8. Oedometer consolidation testing in this study.
The British Standard consolidation testing procedure adopted in this study is very similar in
principle to the one-dimensional consolidation test specified by the ASTM Designation
D2435. However, the ASTM standard incorporates either a fixed-ring type or floating-ring
type of consolidation cell, with minimum specimen dimensions of 50 mm diameter and 12,5
mm thickness; and a minimum height to diameter ratio of 1:2,5. In addition, a copper disc is
also used for the deformation calibration of the apparatus, while an initial seating load of 2,5-
5,0 kPa is usually applied to the soil specimen depending on the softness of the soil. In
practice, a standard loading sequence of pressures consist of 5, 12,5; 25, 50, 100, 200, 400,
800 kPa, even though smaller increments could be used on very soft soils (BS 1377: 1975;
ASTM D2435-70).
7.4.5 Analysis of consolidation test data
A graphical analysis of consolidation test data obtained for black clays made use of
conventional log-time method. However, both log-time and square-root-time methods were
employed in the graphical analysis of test data for the more silty red clay soils, whose early
portion of log- time/ settlement curves depart from the conventional shape as derived from
theoretical relationships (Fig. 7.17). Determination of the do point corresponding to 0%
primary consolidation was therefore not possible. The departure from conventional shape
could be attributed to the relatively much higher permeability (k = 7,57E-11 to 7,33E-9) of
the red soils which caused rapid settlement immediately after initial loading, so that the initial