an engineering geological characterisation of tropical clays - GBV

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On the other hand, laboratory measured plasticity values (PImeas.) compare exactly and are
practically the same as calculated values (PILS) derived using the new relationship (PI =
1,88*LS) developed and established in this study for the black clays. The relationship
between the calculated and measured plasticity indices is practically identical at both depths
of less than 0,50m i.e.
PImeas. = 1,00PILS (10.6)
and those of 0,50m and greater, i.e.
PImeas. = 1,01PILS (10.7)
so that the black clays must be uniform and homogeneous in their engineering character
across the study area. An average relationship of
PImeas. = 1,005PILS (10.8)
could be adopted for the two sets of plasticity indices.
The red soils involved in this study were investigated and analysed in a similar way.
Laboratory determined values of plasticity index are presented alongside those derived by
calculation using the British Standard relationship in Table (10.2). A comparison of the
minimum and maximum values as well as the median, mode and mean values of the two sets
of data reveals a slight overestimation of the calculated plasticity indices with respect to the
laboratory measured values, and this most probably due to reasons already stated above as for
black clays. A covariance of 0,76 and moderately strong correlation of 0,54 was also recorded
for the two sets of data.
A correlation of laboratory measured linear shrinkage (LS) and plasticity index (PI) yielded
new relationships for estimating the latter parameter from the former for the red soils (Fig.
10.3), i.e.
PI = 1,77LS (10.9)
for the sampling area Rd1 and
PI = 1,90LS (10.10)
for the sampling area Rd2, respectively. An average relationship of
PI = 1,83LS (10.11)
was derived and established for the red soils. Plasticity index values (PILS) derived by
calculation from laboratory linear shrinkage (LS) values using this new relationship are also
included in Table (10.2) for comparison purposes. The so calculated and laboratory measured
(PImeas.) plasticity index values are perfectly in close agreement and/ or practically identical in
terms of their maximum and minimum values as well as median and mean values. A
covariance of 0,65 and moderately strong correlation of 0,54 were registered for the two sets
of data.