Civil Engineering Reference
In-Depth Information
Fig. 11.7
Void ratio to effective pressure curves.
If the sample is recompressed after the initial cycle of compression and expansion, the e-p curve for
the whole operation is similar to the curves shown in Fig.
11.7b
; the recompression curve is flatter than
the original compression curve, primary compression being made up of (i) a reversible part and (ii) an
irreversible part. Once the consolidation pressure is extended beyond the original consolidation pressure
value (the preconsolidation pressure), the e-p curve follows the trend of the original compression curve.
All types of soil, whether sand, silt or clay, have the form of compression curves illustrated in Fig.
11.7.
The curves shown can be produced quite quickly in the laboratory for teaching purposes, using a dry sand
sample, but consolidation problems are mainly concerned with clays and the oedometer is therefore only
used to test these types of soil.
11.3.3 Volumetric change
The volume change per unit of original volume constitutes the volumetric change. If a mass of soil of
volume V
1
is compressed to a volume V
2
, the assumption is made that the change in volume has been
caused by a reduction in the volume of the voids.
V
−
V
1
2
Volumetric change
=
V
1
(
1
+ − +
+
e
)
(
1
e
)
1
2
=
1
e
1
e
−
+
e
e
1
2
=
1
1
where
e
1
=
void ratio at p
1
e
2
=
void ratio at p
2
.
The slope of the e-p curve is given the symbol 'a', then:
e
−
−
e
1
2
a
=
p
m /kN
2
p
1
2
i.e.
de
dp
a
=
