an engineering geological characterisation of tropical clays - GBV

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Fig. 7.15a. Log-pressure/ voids ratio (e/ log p)
curve .
Fig. 7.15b. Determination of coefficient of
secondary compression cα.
The coefficient of volume compressibility, mv, is the change in volume of soil test specimen
per unit volume, per unit pressure change, as a result of consolidation due to that pressure
change. It is a modulus of volume change and serves to indicate the compressibility per unit
thickness of soil under test, i.e.
mv = av/(1+e1) = 1/(1+e1) * (- δe/ δp) m²/kN, or
mv = 1000/(1+e1) * (- δe/ δp) m²/MN (7.40)
where e1 = voids ratio at start of load increment δp (kN/m²)
Typical values of mv for a number of types of clay are given in Table (7.13), alongside values
of mv obtained for black clays and red soils of this study, for comparison purposes.
The compression index, cc, is a dimensionless number. It refers to the slope of the linear
section of a consolidation curve of voids ratio, e, plotted against loading pressure, p (log
scale). It is therefore the change in voids ratio for one log cycle of pressure change on the
curve whose linear section is represented by the equation
e = eo – cclog10 (po+ δp)/po (7.41)
where eo and po refer to initial conditions of voids ratio and pressure.
Skempton (1944), found cc to be related to liquid limit, LL, of clays to a reasonable degree of
approximation through
cc = 0,009 (LL-10%) (7.42)
for undisturbed clay, and
cc = 0,007(LL-10%) (7.43)
for remoulded clay.