42
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
0
200
400
600
800
1000
0
200 400 600 800 1000
p' (kPa)
q (kPa)
CSL 1.33
0.868
Ko line
True creep or
cyclic loading with
constant p'
True creep
Cyclic loading
with constant p'
Fig. 17. Effective stress paths followed in creep-cyclic interaction stress-
path triaxial tests on TVS specimens: Rimoy and Jardine 2011
Figure 17 sets out the effective stress paths followed by
Rimoy and Jardine 2011, indicating the pause points at which
drained creep straining was observed for 2 to 4 day durations
under constant stresses - either in an undisturbed ‘true’ state or in
combination with low-level drained cyclic loading.
0.00%
0.02%
0.04%
0.06%
0.08%
0.10%
0.12%
0.14%
0.16%
0.18%
0.20%
0
1000
2000
3000
4000
5000
6000
minutes
Shear strain invariant (% )
Creep, p' = 600kPa
Creep, p' = 400kPa
Creep, p' = 200kPa
Fig. 18. Shear strain invariant-time trends followed in ‘true creep’ stages
of stress-path triaxial tests on TVS specimens: Rimoy and Jardine 2011
Figures 18 and 19 show the volumetric and shear strain
invariant responses observed during ‘true’ creep at three p΄
levels, showing stable and consistent trends. While the invariant
shear strain increased monotonically with time and p΄ level, the
volumetric trends reversed when ε
s
exceeded ≈ 0.015% after
several hours and diverged strongly from the initially near K
0
pattern, where dε
a
/dε
vol
= 1 and dε
s
/dε
vol
= 2/3 for zero radial
strains. Monotonically continuing shear distortion led to sharp
rotation of strain increment directions, eventually establishing a
steady trend for dε
s
/dε
vol
≈ -1.
This interesting kinematic yielding trend, which was not
apparent in the shorter duration creep tests investigated by
Kuwano 1999, can be seen as the (stationary) effective stress
point engaging a kinematic yield surface that is moving with
respect to time or strain rate. Given the final strain increment
direction, it appears that the Y
2
‘bubble’ has moved rightwards
with time and the fixed effective stress point has engaged its
leftward limit. Under strain-controlled K
0
conditions any radial
dilation has to be suppressed, leading to radial effective stresses
and increases in K
0.
Bowman and Soga (2005) noted similar
features in independent experiments, speculating that this feature
might play a significant role in pile capacity growth with age.
Rimoy and Jardine 2012 also explored interactions between
creep and low-level cyclic loading. Figure 20 plots the ε
s
- t
trends from tests where the deviator stresses q were varied by
one cycle per minute (as in the Dunkerque pile tests) while
keeping p΄ constant. The cycling commenced as soon as the
stress path arrived at the desired p΄ level with (half peak-to-
trough) amplitudes q
cyc
equal to 5, 10 and 15% of p΄. The cyclic
tests showed augmented rates of permanent strain development,
which in the q
cyc
= 0.15p΄ test doubled those seen in the ‘true
creep’ experiment. Other experiments showed that prior drained
ageing (creep) or overconsolidation slow permanent strain
development.
-0.04
0.00
0.04
0.08
0.12
0.16
0.20
0
1000
2000
3000
4000
5000
6000
minutes
V olum etric strains (% )
Creep, p' = 600kPa
Creep, p' = 400kPa
Creep, p' = 200kPa
Fig. 19. Volume strain-time trends followed in ‘true creep’ stages of
stress-path triaxial tests on TVS specimens: Rimoy and Jardine 2011
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0
1000
2000
3000
4000
5000
6000
Cycles
qcyc, 0.05p' = 30kPa
qcyc, 0.025p' = 15kPa
qcyc, 0.015p' = 10kPa
ε
cyc axial
- ε
creep
(% )
Fig. 20. Shear strain invariant-time trends from cyclic stress-path tests on
TVS specimens conducted at 1cycle/minute: Rimoy and Jardine 2011
More complex interactions are revealed by plotting ε
s
against
ε
vol
in Fig. 21. It can be seen that cyclic loading retards the shift
from contractive-to-dilative volumetric response. The time-
dependent Y
2
point is pushed forward in terms of both creep
duration and shear strain developed. Low-level cyclic loading
does not simply accelerate creep. It also holds back and probably
expands the time-dependent kinematic Y
2
surface. It is
interesting that low-level cycling enhances pile capacity growth,
suggesting that the delayed dilation mechanism may be playing a
more complex role than had been appreciated in pile axial
capacity growth with time. The laboratory tests provide critical
data against which new time-dependent and kinematic yielding
models may be tested.
