Environmental Engineering Reference
In-Depth Information
tested material revealed the presence of an inten-
sively crushed inner shear zone about 20 mm thick
and an outer shear zone characterised by a less
severe crushing ( Fig. 6 ) . The total thickness of the
shear zone was estimated to be about 30 mm.
since a significant grain crushing was expected
during such a prolonged shearing, an attempt to
measure the void ratio directly within the shear
zone was performed after the test #4. Miniaturised
samplers, 30 mm in height and about 20 mm in
diameter, were driven into the shear zone ( Fig. 7 )
and immediately weighted. successively, they were
oven-dried at 105°c in order to determine their
water content. The inner shear zone and the outer
shear zones can be clearly individuated on the
extruded oven-dried sample ( Fig. 7 ) .
The void ratio was calculated adopting the
following relationship
Figure 7. Miniaturised sampler driven into the shear
zone after test #4 (left) and material extruded from the
sampler after oven-drying (right).
250
2
Normal stress
200
1.6
Shear
displacement
A
150
1.2
Pore water
pressure
V
V
C
w
s
(2)
e
=
100
0.8
50
0.4
where V w and V s are the water and solid volumes,
respectively. eq. (1) is not recommended in this
case because the specimen is irregular and its vol-
ume is considerably lower than that of the sampler.
a value of 0.36 was determined for the whole shear
zone; it could be slightly overestimated because
some swelling (about 4 mm) occurred when the
apparatus was opened at the end of the test.
B
0
0
Shear resistance
951
955 59
963
967
Elapsed time (s)
Figure 8. Time-histories of normal stress, shear resistance,
pore pressure and shear displacement recorded in test #5.
on the specimen of test #6 the void ratio was
determined using the procedure described in sec-
tion 4.1 (miniaturised samplers were not used);
the obtained value of 0.25 ( Table 1 ) is different
from that found in test #4 and it is deemed to be
unlikely low.
in the last test (test #5) a much higher shear dis-
placement rate was adopted, in order to investigate
the influence of movement rate on liquefaction
triggering. Figure 8 shows the time-histories of
normal stress, shear resistance, pore water pressure
and shear displacement, while the effective and
total stress paths are plotted in Figure 9.
after an initial increment up to 141 kPa (point a),
shear resistance rapidly drops to 38 kPa (point B).
This is due to the contemporary build-up of pore
pressure, which increases until point c, where it
attains the maximum value of 110 kPa. The mobi-
lised friction angle, calculated from a regression of
the data comprised between c and the end of the
test, is equal to 39° ( Fig. 9 ) , the same obtained in
test #4.
This response can be interpreted as a phenom-
enon of limited liquefaction, related to the local
Figure 6. sub-horizontal section of the specimen after
test #4. X- and Z-axes are directed diametrally (outward)
and tangentially. small arrows indicate the step marking
the passage between the inner and outer shear zone.
 
 
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