03-07-2012, 01:05 PM
SOIL TESTING
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SOIL TESTING AND PROPERTIES
In the course of the site investigation and
materials survey for the proposed road between
Biu and Damboa (Fig. I), 86 subgrade samples of
black cotton soils on the Biu Upland and Biu
Plain were subjected to soil classification tests,
including particle size distribution down to the
63 urn sieve size and nine samples were subjected
to compaction and California bearing ratio
(CBR) tests. In addition, the free-swell test was
carried out to determine the expansive potential
of the clays.
Test results are summarized in Table 1 and
indicated that the plain could be subdivided into
three sections, proceeding north-eastwards, with
medium-high, medium and low-medium potential
for expansion on the basis of criteria suggested
by Holtz & Gibbs (1956).
COMPARISON OF PROPERTIES
Regarding particle size distribution, there is a
significant difference between the two groups of
black cotton soils in the proportion of silt.to clay,
the ratios for the residual clays (range l.l(r2.61)
being much greater than for the lacustrine clays
(0.324.58). Results from other sources confirm
this difference. Lyon Associates Inc. (1971)
obtained ratios in the range of 0.29-0.65 for four
samples from the Chad Basin and a value of 1.13
for a black clay developed over basalt in the West
Cameroun. Grainger’s work (1951) gives a value
of 0.30 for a sample from the Maiduguri-
Gamboru road. Ola (1978), however, who carried
out a detailed investigation on a sample of lacustrine
clay from km 112.6 on the same road, has
rhis difference is silt:clay ratios probably helps
produced results indicating a higher silt:clay ratio
greatly to explain in general the differences in
plasticity indicated by the Casagrande chart plots
of 1.04.
CONCLUSIONS
The main difference between the residual and
lacustrine black cotton soils is in their silt:clay
ratios. The lower clay content of the residual
clays results in generally lower plasticity,
although both groups show expansive properties.
Mineralogical analysis confirms montmorillonite
as the predominating mineral in both groups with
kaolinite and quartz present in smaller quantities.
Quartz is present in a relatively greater proportion
to kaolinite in the residual clays.