an engineering geological characterisation of tropical clays - GBV

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Table 10.2. Calculated and laboratory measured values of plasticity index of red soils.
Soil type Depth (m) Sample No. LS (%) PI meas. (%) PIBS (%) PILS (%)
0,30 Rd1-30cm 10 18 21 18
Red soils 1,00 Rd1-100cm 11 21 23 20
2,00 Rd1-200cm 11 20 23 20
4,00 Rd1-400cm 11 18 23 20
0,30 Rd2-30cm 10 19 21 18
1,00 Rd2-100cm 11 22 23 20
2,00 Rd2-200cm 11 21 23 20
4,00 Rd2-400cm 11 19 23 20
max 11 22 23 20
Statistical min 10 18 21 18
analysis range 1 5 2 2
median 11 20 23 20
mode 11 #NV 23 20
mean 11 20 23 20
std 0,46 1,62 0,99 0,85
var 0,21 2,63 0,97 0,72
cov 0,76 0,65
correlation 0,54 0,54
as obtained for the black clays in this study at depths of less than 0,50m as well as those at
0,50m and greater, are given in Table (10.1).
Calculated values of plasticity index, PI(BS), as estimated from laboratory determined linear
shrinkage values using the British Standard relationship (BS 1377: 1967), i.e.
PI = 2,13 * LS ( derivation based on British soils) (10.1)
are also included in the table for the two depth intervals. The calculated plasticity index
values are observed to be overestimated with respect to the laboratory determined values for
both the two depth intervals. A comparison of the respective maximum and minimum values
as well as the median, mode and mean values of calculated and measured plasticity indices
further indicates and confirms an overestimation of the former with respect to the latter (Table
10.1). The overestimation could be attributed to the generally higher organic content of
tropical black clays which tends to inhibit and/ or suppress their plasticity to some degree.
The coefficient of variation (covariance) with respect to the calculated and measured values
are 0,57 and 3,23 for the two depth intervals, respectively. The standard deviation and
variance for the calculated and measured plasticity values are in close agreement and more or
less of the same order of magnitude in each case, implying a similar form and/ or mode of
distribution and variation across the study area.
A correlation of laboratory measured values of plasticity index and linear shrinkage is
presented diagrammatically in Fig. (10.1).