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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
The behavior in the case of medium slope with stress ratio of
0.4 was more complicated. The strain started to increase with a
similar rate to those in the other cases, but it slowed down when
the change of volumetric water content was between around
7 % and 18 %. And then, it yielded, followed by a quick
deformation with high strain rate.
The reason for the slowing down of strain rate observed in
the case of medium slope could be explained with the behaviors
of suction. The authors developed a miniature suction sensor to
measure the suction in the specimen (Figure 9). A tiny metal
pipe (
1.5 mm x 4 mm) with small hole is wrapped with a
micro-porous membrane. One end of the pipe is connected to a
water pressure transducer via a plastic tube, while the other end
is closed. The membrane allows water to pass through, but
prevent air to pass by capillary effect. Thus, the pipe with the
membrane works just like a miniature ceramic cup. Properties
of a similar membrane are studied by Nishimura et. al. (2011).
Due to its small size, the miniature suction sensor can be
installed with minimum disturbance to deformation and water
seepage in the specimen as seen in Figure 9.
Figure 10 shows relationships between the suction and the
change of volumetric water content measured in another
specimen during water infiltration process. The suction
decreases with fast rate until the change of volumetric water
content reaches around 4.5 %. Then, the decreasing rate of
suction slowed down.
Micro porous
membrane
Metal pipe 15x4mm
Figure 9 Miniature suction sensor with microporous membrane.
0
5
1 0
1 5
2 0
0
1
2
3
4
5
6
7
8
Shear strain(%)
C h a n g e o f V o l u m e t r i c w a t e r c o n t e n t ( % )
0
5
1 0
1 5
2 0
0
5
1 0
1 5
2 0
2 5
3 0
3 5
4 0
4 5
5 0
I n t ia l V W C = 1 1 %
N o r m a l s t r e s s = 6 0 k p a
S t r e s s r a t io = 0 .4
D r = 7 0 %
S uc tion (kP a)
C h a n g e o f v o lu m e t r ic w a t e r c o n t e n t ( %)
Failure
Initiation
Suction
inflextion
Figure 10 Relations among volumetric water content, suction, and
shear strain.
The strain rate of the specimen also slowed down at the same
change of volumetric water content. Thus, this behavior can be
explained with the suction-water characteristics curve (SWCC)
of the soil. Finally, the specimen yielded at higher change of
volumetric water content, corresponding to lower suction.
This observation suggests that a tentative slowing down of
slope deformation does not always mean stabilization of slope.
4 CONCLUSIONS
A slope failure test with an artificial rainfall was conducted on a
natural slope. The tilt sensors installed into the slope showed
tilting rates between 0.1 and 0.5 degree / hour. These values of
tilting rate could be used as criteria of early warning for slope
disasters.
The deformation proceeded when the water content was high
corresponding to rainfall, while it is less progressive when the
water content is low. Similar behaviours were also observed in
model test in laboratory, where an unsaturated soil layer was
sheared under constant shear stress with cyclic water infiltration
and drainage. It seems that there is a unique relation between
the deformation and the water content, which is independent of
the time history of the artificial rainfall. These results suggest a
possibility of conbined monitoring of tilting angle and water
contents for more precise comprehension of the status of slopes.
The results of direct shear tests on unsaturated soil under
constant shear stress and constant water injection rate suggest
that there are three patterns of deformation and failure processes
corresponding to the slope angle. In a case of medium slope, the
strain rate may slow down due to the SWCC of the soil even
though water is injected with a constant rate. However, it does
not always mean that the slope is getting stable.
5 ACKNOWLEDGEMENTS
These researches are supported by Grants-in-Aid for Scientific
Research of Japan Society for the Promotion of Science (JSPS),
Joint Research Projects/Seminars by JSPS, and International
Cooperate Project of Chinese Ministry of Science and
Technology.
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