an engineering geological characterisation of tropical clays - GBV

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standard deviation and variance derived for the calculated and measured plasticity values
(Table 10.1) are comparable and of the same order of magnitude at each of the two depth
intervals; and this further pointing to the possible general uniform character and/ or
homogeneity of the soils across the study area. A covariance of 0,51 and 2,85 is also
registered for the two sets of data at depths of less than 0,50m and those of 0,50m and greater,
respectively.
A correlation of calculated and laboratory measured plasticity indices is presented in Fig.
(10.2).
Black clays; measured/ calculated plasticity indices
Measured plasticity index PImeas. (%)
60
55
50
45
40
35
n = 36
PImeas = 1,01PILS
(> 0,50m depth)
PImeas. = 0,89PIBS.
(> 0,50m depth)
PImeas. = 1,00PILS
(< 0,50m depth)
PImeas. = 0,88PIBS
(< 0,50m depth)
PI (BS) vs PI (meas.), <
0,50m depth
PI (BS) vs PI (meas.),
0,50m depth and greater
PI (LS) vs PI (meas.), <
0,50m depth
PI (LS) vs PI (meas.),
0,50m depth and greater
30
40 45 50 55 60 65
Calculated plasticity index PILS (%)
Figure 10.2. A correlation of calculated and laboratory measured plasticity indices of black
clays.
It is observed that actual laboratory determined/ measured values of plasticity index (PImeas.)
would usually be underestimated with respect to calculated plasticity values (PIBS) obtained
using the British Standard relationship (PI = 2,13*LS). However, the calculated and measured
plasticity indices are practically related in the same way for depths of less than 0,50m (PImeas.
= 0,88PIBS) and those of 0,50m and greater (PImeas. = 0,89PIBS), implying homogeneous black
clays across the study area. An average relationship of
PImeas. = 0,885PIBS (10.5)
could therefore be safely adopted for the whole thickness of black clays across the study area.