an engineering geological characterisation of tropical clays - GBV

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10.3) for the black clays. The calculated values are shown to be strongly in close agreement
with laboratory determined plasticity indices (PImeasured) in terms of maximum (53-55%),
minimum (37-39%) and range (15-17%) values; as well as the median, mode and mean
values (47-48%). Standard deviation (2,85-3,09) and variance (8,13-9,53) are also in strong
agreement and of the same order of magnitude, signifying a similar form of distribution across
the study area. A covariance of 5,89-6,37 and a strong correlation of 0,70 was also registered
between the calculated and laboratory determined plasticity indices.
Table 10.4. Calculated and laboratory measured plasticity indices of red soils.
Profile Sample No. Wn (%) LL (%) PL (%)
PImeasured
(%) PI BS (%) PILL (%)
Rd1 -30cm 39 49 31 18 21 19
Red soils Rd1-100cm 24 51 31 21 23 21
Rd1-200cm 24 49 30 20 21 19
Rd1-400cm 25 48 30 18 20 18
Rd2-30cm 38 53 35 19 24 22
Rd2-100cm 26 57 34 22 27 25
Rd2-200cm 25 54 33 21 25 23
Rd2-400cm 26 53 34 19 24 22
max 22 27 25
Statistical min 18 20 18
analysis range 5 7 7
median 20 24 21
mode #NV 24 22
mean 20 23 21
standard dev. 1,62 2,29 2,48
variance 2,63 5,26 6,16
covariance 2,56 2,77
correlation 0,79 0,79
Calculated plasticity indices for red soils (Table 10.4) are also closely comparable to the
laboratory determined plasticity indices, with maximum and minimum values of 22-27% and
18-20%, as well as a range of 5-7%. Median, mode, and mean values of 20-24%, 22-24% and
20-23% were also recorded. The standard deviation and variance are of the same order of
magnitude while a covariance of 2,56-2,77 and strong correlation of 0,79 was recorded
between the calculated and measured sets of data.
The calculated and laboratory measured plasticity indices for both black clays and red soils
are in a very strong and near perfect agreement (Fig. 10.6), being related in the form of
and
PImeasured = 0,99PIBS, (10.16)
PImeasured = 0,99PILL (10.17)