D06

Effect of non-plastic fines content on the initiation and movement of rainfall-induced
landslides in flume tests〇Chao HUANG, Gonghui WANG
Introduction
Landslide denotes “the movement of a massof rock, debris or earth down a slope”.
Landslides affect the flanks of mountainousand volcanic edifices on earth, which locallyconstitute an important natural hazard on Earth,causing great loss of properties and casualties.
For original slope, some landslides with long
traveling distances tend to occur on the material
rich in fine soil particles like granitic soil
generated by weathered coarse-grained granite,such as the 2014 Hiroshima landslides
triggered by rainstorm, Japan.
Debris avalanches are a kind of largevolume and high-speed landslides. A
hypothesis of dynamic fragmentation has been
proposed as one of the reasons for their highmobility. It means that the presence of finegrained material may lower the shear resistanceof the sliding debris. Some rock avalanche
deposits show “inverse grading”, with mean
particle sizes fining from the surface to the base.
Wenchuan earthquake induced a number ofcoseismic landslides and landslide dams, andsome dams composed of unconsolidated finedebris resulting from the fragmentation oflandslide materials during transportation.
Recent study revealed that the secondary
landsliding phenomena had been widely
triggered on these landslide deposits, and theinitiation and movement of these landslides aregreatly related to the involvement of finematerials. It is noted that these fine materials
resulting from mechanical fragmentation are
no plastic, because they did not suffer fromchemical weathering during the short period
after the earthquake. Nevertheless, the detailed

roles of fine materials had not been clarified.
Therefore, study will focus on examining theeffect of non-plastic fines content on theinitiation and movement of rainfall-induced
landslides through a series of flume tests.
Material and Method
Samples for the tests are mixture of silica
sand No. 7 with differing contents (0%, 10%,20%, 30%, 40% and 50%) of silica powder byweight. Before setting, the mixtures wereprepared to have an initial water content 10%.
A flume as descripted in Fig. 1 was employed,
which has transparent sides and is 180 cm long,
24 cm wide, and 20 cm high. During test, theflume was sloped to 20°. Tilting transducers areinstalled at mid-depth within the soil model tomonitor the possible inclination of the soil mass.
Cameras were used to monitor the entire test
process from one side of the flume and also
from the toe part. Artificial rainfall (100 mm/h)
was simulated by sprinklers mounted above thesoil model to trigger landsliding.

Fig. 1 Arrangement of the flume test. P1-P7: locations wheretilting transducers were installed.

ResultIn general, the entire process required 30–45
min for slope wetting, and few minutes forfailure. In the series tests on silica 7 and its
mixtures with 10%, 40%, 50% silica powder,
retrogressive sliding occurred in each test.

However, in the series tests on the mixture ofsilica 7 and 20%, 30% silica powder, flowslides
were initiated.
Results of tilting angle variation of the P 4 in
the test of silica 7 with dry density 1.2 g/cm3
was presented in the Fig. 2, where it is notedthat the soil mass began to title at the time of
1879 s, then the curve of tilting angle increased
gradually, and the sharply dropped to -30
degree at 1953 s, indicating the soil massrotated rapidly from 1953 s to 1986 s.
According to Fig. 2, we define the location(1879 s) where the tilting sensor began to rotate
as the initiation point of the landslide, and thelowest point of rapidly dropping curve as theend point of the landslide since tilting sensors
could detect the rotation of inner soil mass, thetime scale from initiation to end was the periodof entire landsliding progress.

then decreases with further increasing fines
contents. The results presented in Figs. 3 and 4,and also videos recorded by cameras indicatethat landsliding transforms from retrogressive
one to fluidized landsliding with collapsed
failure with the increasing of fines content,
while the landslide type transforms from debris
avalanche to retrogressive landslide withgreater percentage (>30%) fines content.

Fig. 2 Results of tilting angle variation of one certain soil mass

Fig. 4 Tilting rates of point P2 (showed in Figs. 1 and 3) in allpoint

flume tests against the silica powder contents

Fig. 3 shows the different periods oflandslides in the flume tests. As shown in Fig.
3, the colorful point presents the results of eachtilting transducers in the flume tests. It isnoticed that the curve firstly decreases withincreasing fine particles content until reaching
30%, then grows up with further increasing fineparticles content. The period of landslide
reaches to the lowest value when the silica
powder content is at 30%.
Fig. 4 shows the tilting rates of P 2 (showedin figures 1and 3) in all tests against the silica
powder contents. From this figure, it isconcluded that the peak tilting rate increases
with silica powder contents increase until thesilica powder content reaches to 20% and 30%,

Conclusion
Non-plastic fines content has a remarkableeffect on the initiation and movement of
rainfall-induced landslides in flume tests. With
the increasing of fines content, the landslide
type transforms from retrogressive landslide to
debris avalanche, while the landslide typetransforms from debris avalanche toretrogressive landslide with greater percentage(>30%) fines content.
Periods of landslides and tilting rates of innersoil mass vary with increasing non-plastic fines
content. Under the tested conditions, when thefines content reaches to 30%, the period oflandslide is the shortest and peak tilting rate of
inner soil mass is the greatest.

Fig. 3 Periods of landslide against the silica powder contents