Actes du colloque - Volume 2 - page 733

1612
Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
is representative of natural fill deposition. The procedures for
each method are briefly illustrated in Figure 1.
Figure 1. Sample reconstitution methods: (a) moist tamping; and (b) dry
deposition.
2.2 Control of Initial State and Initial Shear
Specimens prepared by either method was brought to saturation
through CO2 flushing and then de-aired water flushing to
achieve a B-value higher than 0.98. The stress state in terms of
the effective normal stress nc’ and the initial static shear stress
ratio  was attained through anisotropic consolidation such that
 nc’ = q s / 2 + 3c’ (1)
 = qs / 2nc’ (2)
where qs is the initial deviatoric stress and 3c’ is the minor
effective consolidation pressure. Undrained cyclic loading, in
terms of cyclic stress ratio CSRn,
CSRn = qcyc / 2 nc’ (3)
where qcyc is the cyclic deviatoric stress, was then applied at a
0.01 Hz. Such a low frequency allowed more stable input and
output signals and hence ensured the soil loading responses to
be precisely represented and confidently examined.
2.3 Test Program
The cyclic triaxial test program is listed in Table 2. Loose sands
with post-consolidation density Drc at 20% and 35% were tested
at various combinations of  nc’ and  values. Test at each
combination was repeated for MT and DD methods. Such step-
wise varied test program allowed the examination of: 1) the
fabric effect under otherwise identical conditions; and 2) the
inter-relations of the fabric effect with both initial state and
initial shear.
Table 2. Cyclic triaxial test program.
Reconstitution
Method
Drc
(%)
nc’
(kPa)
3c’
(kPa)
qs
(kPa) 
MT DD 20 100 100 0 0
MT DD 20 100 90 20 0.1
MT DD 20 100 75 50 0.25
MT DD 20 100 60 80 0.4
MT DD 20 500 500 0 0
MT DD 20 500 450 100 0.1
MT DD 20 500 375 250 0.25
MT - 20 500 300 400 0.4
MT DD 35 100 100 0 0
- DD 35 100 75 50 0.25
MT DD 35 100 60 80 0.4
MT DD 35 500 500 0 0
MT DD 35 500 450 100 0.1
MT DD 35 500 375 250 0.25
MT DD 35 500 300 400 0.4
MT DD 50 100 100 0 0
1st layer
Dump in moist sand at
5% water content
Tamping until
desired density Under-compaction
method:
1st = Dr,target – 2%
2nd = D r,target – 1%
3rd = Dr,target
4th = Dr,target + 1%
5th = Dr,target + 2%
Opening to avoid
PWP buildup
(a) Moist Tamping (MT) method
Densify by uniform tapping
around periphery
Dead load on top
Deposit at zero fall height
Oven-dried sand
(b) Dry Deposition (DD) method
Raise funnel continuously 3 CYCLIC FAILURE BEHAVIOR
The cyclic loading responses of sand possessing a certain type
of fabric have been found to be distinctive under different initial
state and shear conditions (Yang & Sze 2011a & b). The test
results from this study further reveal that sands possessing
different initial fabric would exhibit differentiating failure
patterns under otherwise identical conditions. This can be
exemplified by the following four cases.
3.1 Runaway Deformation (MT) vs Limited Deformation (DD)
Very loose sand (Drc = 20%) is found to exhibit runaway
deformation when subjected to cyclic loading regardless of its
initial state and shear stress conditions. This complete collapse
phenomenon is exhibited consistently only in MT specimen.
When initial shear is absent, DD specimen shows similar
behavior. Yet, when shear stress is present, a distinctive failure
pattern – limited deformation prevails with this type of fabric,
even  is small. This is demonstrated in Figure 2.
Runaway deformation is featured by its sudden large abrupt
strain gain associated with sudden build-up of excess pore water
pressure (PWP) directly to  3c’ resulting in a total loss of shear
strength and stiffness of sand. On the contrary, limited
deformation is featured with its sudden but limited or controlled
axial strain and PWP gain resulting in partial collapse of soil
structure. This partial collapse is followed by a rapid plastic
strain accumulation at a strain rate over 1 % per loading cycle.
-50
0
50
100
150
200
250
0 2 4 6 8 10 12 Deviatoric stress (kPa)
Axial strain (%)
MT
DD
14
0
100
200
300
400
500
0 20 40 60 80
Excess PWP (kPa)
Loading cycles
MT DD
100
Figure 2. Runaway deformation (MT) & limited deformation (DD):
[D rc=20%, nc’=500kPa,  =0.1, CSRn =0.11]