Geoscience Reference
In-Depth Information
6
CREEP AND CONSOLIDATION
A
TIME
-
DEPENDENCY
Time-rate effects are observed in clay and peat, referred to by creep, relaxation
and consolidation. Creep is the strain increment under sustained constant stress.
Relaxation is the stress reduction under sustained constant strain. Consolidation in
soft soil is the dissipation of excess pore pressures with time, involving
groundwater flow (drainage) and deformation. Ageing is used to express that soil
gains strength with creep/relaxation. In sand these time-rate effects are less
significant. If in saturated and laterally confined soil the volume reduction by creep
occurs, outflow of pore water is required. The corresponding hydraulic gradients
are quite small (in the so-called secondary phase, after primary consolidation).
However, if drainage is restricted, for example in the centre of a thick soft layer,
creep may cause increase of pore pressure.
A general uniaxial stress-strain relation, following the concept of (5.10), allows
these time-rate phenomena to be visualised as shown in Fig 6.1, referred to by the
abc-model (after Den Haan).
24
t
C
t
C
t
C
t
C
t
C
t
D
t
D
t
D
t
D
t
D
'
A
'
A
'
A
'
A
'
B
'
B
'
B
'
B
'
C
'
C
'
C
'
C
ln
ln
ln
ln
'
'
'
'
ln
t
ln
t
ln
t
ln
t
ln
t
'
'
A
A
A
A
A
A
A
A
a
a
a
a
B
B
B
B
B
B
B
B
'
=
'
C
'
=
'
C
b
b
b
b
consolidation
consolidation
F
F
F
C
C
C
C
C
C
C
C
C
C
C
C
C
relaxation
cree
p
relaxation
cree
p
E'
E'
E'
c
c
c
c
c
C
C
C
D
D
D
D
E
E
D
D
D
D
D
D
D
D
D
'
C
'
C
'
C
'
C
'
C
D
D
D
creep
creep
creep
t
C
t
C
t
C
C
C
C
=
=
C
C
relaxation
relaxation
relaxation
F
F
F
c/b
c/b
c/b
t
F
t
F
t
F
ln
t
ln
t
ln
t
'
F
'
F
'
F
'
C
'
C
'
C
(a) (b)
Figure 6.1 Model for uniaxial deformation
24
In 1972, Garlanger defined a similar relation:
e
=
e
0
-
C
r
log(
'
B
/
'
A
)-
C
c
log(
'
/
'
B
)-
C
log((
t
+
t
i
)/
t
i
), where
e
is the voids ratio,
'
B
the preconsolidation pressure,
C
r
is the
recompression index,
C
c
the inelastic-compression index,
C
the creep index (secondary
compression),
t
the elapsed time and
t
i
a reference time (intrinsic time).
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