Environmental Engineering Reference
In-Depth Information
higher than 200 kPa and a corresponding drop
of the shear resistance down to a few kilopascals.
The apparent friction angle at the final stage (given
by the arctangent of the ratio between the mobi-
lised shear resistance and the total normal stress)
was about 3°. evident crushing was observed in all
specimens under the applied testing conditions.
in the second type of tests conducted with
drainages open at the specimen top, shear resist-
ance initially increased gradually up to a maximum
value, corresponding to a peak friction angle of
about 44°. The linear increase in shear indicates
that shearing is likely to occur under drained condi-
tions. after peak, the shear resistance temporarily
dropped to a value corresponding to an apparent
instantaneous friction angle of 18° and thereafter
increased up to a stable value, yielding a friction
angle at large displacements of roughly 34°. The
drop in shear strength (below the dynamic residual
friction angle) is likely to be a consequence of local
excess pore-water pressures within the shear zone,
which can temporarily build-up due to the high
crushability of the material.
higher than the analogous test by Boldini
et al
.
(2009). in the same hydraulic conditions and at
higher loading rate, an incipient liquefaction was
again observed.
Figure 2 reports time-histories of total normal
stress, shear resistance, pore pressure and shear
displacement,
δ
s
, after the consolidation phase.
The test was stopped at the onset of liquefaction
in order to measure void ratio. Before this stage,
as expected, pore pressure initially increases and
then gradually decreases down to around
-
30 kPa.
correspondingly a decrease in the effective normal
stress followed by an inversion, due to material
displacement reaches 15 mm, in correspondence
with the minimum pore pressure, the sign of pore
pressure increments inverts even though it cannot
be appreciated in Figure 2 due to the plot scales.
imminent liquefaction can be argued from the
drop of shear resistance.
The instantaneous friction angle during the
dilative phase (i.e. when pore pressures are nega-
(39°) obtained in the past testing campaign.
The void ratio was accurately measured in the
shear zone. its thickness,
h
s
, was estimated to be
26 mm. This value was calculated adopting the ratio
between
h
s
and the specimen height,
h
, observed
in other ring shear tests on crushable materials
(coop
et al
. 2004). The specimen was ideally sub-
divided into three layers (upper layer, shear zone,
lower layer) which were separately recovered. The
material of each layer was weighted after oven-
drying at 105°c for 25 h and void ratio,
e
, was
calculated as
4
neW TesTs
The new campaign of lsRsT tests carried out at
the DPRi consists of 6 tests on fully saturated spec-
imens subjected to different shearing and drainage
conditions. only four tests reached the incipient
liquefaction stage or large displacement condi-
tions stage, which are of interest for the analysis of
instability mechanisms. specimens were prepared
and consolidated according to the same procedure
described by Boldini
et al
. (2009) and reported in
the previous section. conditions adopted in the
different tests are reported in Table 1 together with
values of the void ratio determined at different
stages of each test.
e
VV
V
−
s
(1)
=
s
The solid volume
V
s
derives from the dry weight
and the solid matrix density, whilst
V
was calcu-
lated from
h
,
h
s
and from the diameter of the inner
and outer rings. in the shear zone a void ratio
equal to 0.39, greatly lower than that obtained on
the whole, was measured (
Table 1
).
4.1
Stress controlled test
Test #2 was carried out in stress controlled condi-
tions. Test #2 was performed by applying a shear
torque with a loading rate of 200 Pa/s, four times
Table 1.
Test conditions and void ratio measured at different stages of the test.
Test
control type
Rate
Drainages
e
0
i
e
c
e
f
notes
#2
stress
200 Pa/s
closed
0.79
0.69
0.69 (0.39)
e
f
at
δ
s
= 0.015 m
#4
displacement
5 mm/s
open
0.79
0.69
0.57 (0.36)
e
f
at
δ
s
= 20 m
#5
displacement
100 mm/s
open
0.79
0.71
0.52 (0.36)
e
f
at
δ
s
= 1.6 m
#6
displacement
5 mm/s
open
0.83
0.72
0.59 (0.25)
e
f
at
δ
s
= 20 m
* the bracketed value refers to the shear zone.
