an engineering geological characterisation of tropical clays - GBV

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where w (%) = moisture content of soil
S (%) = degree of saturation, i.e. volume of water contained in the void space between
soil particles expressed as a percentage of total voids
The volume change which occurs during consolidation takes place only in the voids so that a
change in height ∆H from an initial height Ho in the specimen corresponds to a change in
voids ratio ∆e from an initial value eo. By proportion therefore,
∆H/Ho = ∆e/(1+eo), so that
∆e = ((1+eo)/Ho) * ∆H, or
∆e = F∆H (7.37)
Where F (mm¯¹) = change in voids ratio per unit height, with respect to initial conditions of
test specimen
The change in height of specimen is used to give the voids ratio,e, at any stage of a
consolidation test through
e = eo – ∆e (7.38)
where eo = initial voids ratio of test specimen
7.4.3.3 Coefficient of compressibility (av), coefficient of volume compressibility (mv)
and compression index (cc)
These coefficients are derived from consolidation tests and are indicative of the
compressibility of soils. They serve to give estimates of the amount of settlement due to
primary consolidation.
The coefficient of compressibility, av, and the coefficient of volume compressibility, mv, are
usually calculated for each load increment while the compression index, cc, is derived from an
e/ log p curve obtained by plotting voids ratio, e, against applied pressure, p, (log scale), i.e.
(Fig. 7.15a). In practice, mv is applied to overconsolidated clays and cc to normally
consolidated ones. The swell index, cs, gives an indication of expansion of a soil on
unloading.
The coefficient of compressibility, av, is the change in voids ratio, δe, per unit pressure
change, during a given load increment, due to consolidation caused by that incremental
pressure δp, i.e.
av = (e2 – e1)/ δp
= - δe/ δp (m²/kN) (7.39)
where e1 and e2 = voids ratios at start and end of consolidation under the load increment
δp in kN/m²
The negative sign indicates decrease of e as p increases.