an engineering geological characterisation of tropical clays - GBV

202
The black clays and red soils generally exhibit low total carbon contents of less than 2,5%.
The black clays show 0,50-1,48% total carbon (mean: 1,17%), 0,22-1,19% organic carbon
(mean: 0,93%) and 0,04-0,74% inorganic carbon (mean: 0,24%); while the red soils exhibit
0,45-2,47% total carbon (mean: 1,12%), 0,35-1,90 organic carbon (mean: 0,85%) and 0,10-
0,57% inorganic carbon (mean: 0,27%). The black clays also show a general decrease of total
carbon and organic carbon, and therefore, organic matter content of the soils with depth; the
variation being best described and represented by a polynomial (R = 0,56) and exponential (R
= 0,85) relationship, respectively. The inorganic carbon component generally increases with
depth through in situ conversion of underlying volcanic tuffs to secondary limestone; and this
variation tends to fit a polynomial distribution (R = 0,63). On the other hand, the red soils
show decreasing total carbon as well as organic and inorganic carbon contents with depth, the
three variations closely approximating a potential relationship with fairly strong correlations
of R = 0,94. The decrease of the organic carbon is in reflection of decreasing organic matter
content of the soils with depth; while decreasing inorganic carbon could be attributed to low
pH conditions and leaching of carbonates and other soluble components of Mg, Ca, K and Na
from the soils during rainfall.
Very soft and sensitive clays were also encountered in the present area. They occur in terms
of black clays in peaty/ swampy environs and/ or those of impended drainage; as well as red
soils in densely forested and uncultivated zones. The soils generally exhibit a very soft to firm
consistency, with vane shear strength values of less than 100 kPa. The undrained shear
strength ranges from 30,94 – 41,50 kPa for black clays; and 10,92 – 87,36 kPa for red soils,
and generally increases with increased soil depths. The variation of vane shear strength with
soil depth closely fits a logarithmic relationship with a strong correlation (R = 0,94 for black
clays; and R = 0,97 – 1,0 for red soils). The variation of vane shear strength with natural
moisture content of soils is best described by a polynomial relationship with a very strong
correlation (R = 1.0 for black clays and R = 0,91 - 0,99 for red soils). Variation of vane shear
strength could also be predicted from bulk densities of soils through a polynomial relationship
with a very strong correlation (R = 0,97 for black clays; R = 0,99-1,0 for red soils).
A soils classification based on index properties and grain sizes places the black clays into very
high to extremely high plasticity clays, silty clays and/ or silty clays with sand, as well as
clayey silts; all of which harbour medium to very high levels of activity (normal to highly
active) so that they exhibit high swelling capabilities and potential expansiveness on wetting
from a dry condition. This engineering behaviour has been explained by the high content of
the more expansive clay minerals, i.e. smectites (90% and over) in the clay fraction. On the
other hand, the red soils are medium to high plasticity clayey silts and silty clays with sand;
having at most only medium activity levels (normal active) and therefore exhibiting generally
low swelling capabilities and potential expansiveness when allowed free access to water.
These characteristics have been attributed to the high content (80% and over) of the rather less
expansive clay mineral, kaolinite, in the clay fraction of the soils.
The distribution and variation of index properties of black clays across the study area at
depths of less than 0,50m and those at 0,50m and greater, were found to be generally similar,
comparable and in good agreement, implying a generally homogeneous engineering character
of the soils. Index properties of red soils point to a comparatively more stable and reliable
engineering character of the soils.
A new relationship has been derived, developed and established in this study for the
estimation of the plasticity index (PI) of black clays from laboratory determined linear
shrinkage (LS) of the soils, i.e.