an engineering geological characterisation of tropical clays - GBV

183
Black clays; plasticity index/ linear shrinkage
60
n = 36
Plasticity index PI (%)
55
50
45
40
35
PI = 1,89LS
PI = 1,87LS
less than 0,50m depth
0,50m depth and
greater
Linear (less than
0,50m depth)
Linear (0,50m depth
and greater)
30
20 25 30
Linear shrinkage LS (%)
Figure 10.1. Correlation of laboratory measured values of plasticity index and linear
shrinkage of black clays.
From the diagram, the plasticity index of black clays could be approximately estimated by the
relationship
and
PI = 1,87*LS (10.2)
PI = 1,89*LS (10.3)
for soil depths of less than 0,50m and those of depths of 0,50m and greater, respectively. The
two relationships derived for the two depth intervals are practically identical, and this serves
to support and confirm an earlier suggestion that the black clays of the study area are
homogeneous in their engineering character. On the average therefore, the plasticity index of
the whole soil thickness of black clays across the study area could be estimated from the
average relationship of
PI = 1,88*LS (10.4)
Calculated values of plasticity index, PILS, as estimated from laboratory measured values of
linear shrinkage, LS, using the above new relationship in Equation (10.4) are presented in
Table (10.1), both for depths of less than 0,50m and those of 0,50m and greater. A
comparison of the so calculated values of plasticity index (PILS) with plasticity index values
measured in the laboratory (PI meas.) reveals a generally close agreement between the two sets
of data. The maximum and minimum values as well as the median, mode and mean values
obtained for the two sets of data are identical at each one of the two depth intervals. This
implies that Equation (10.4) above is a better and preferred estimator for the plasticity index
of black clays than the British standard relationship of Equation (10.1). In addition, values of