HDS26-01

Room:203

Time:May 26 17:15-17:40

Numerical and Centrifuge Modelling of Submarine Landslides
KOBAYASHI, Takaaki1∗ ; SOGA, Kenichi2 ; TAKAHASHI, Hidenori1 ; SASSA, Shinji11

Soil Dynamics Group, Port and Airport Research Institute, 2 Department of Engineering, University of Cambridge

Submarine landslides can cause a significant damage to offshore structures. Their mechanism is not well understood due to thedifficulty of direct observation of the phenomenon. In engineering perspective, assessing the risk of submarine landslides is necessary for further development of offshore areas. Estimating the extent and the impact of submarine debris flows is particularly
important when designing subsea facilities. In the field of geotechnical engineering, physical and numerical modelling are often
used to tackle these problems. The attempt here is to introduce recent research activities in physical and numerical modelling ofsubmarine landslides.
One of the major discussions in numerical modelling of submarine landslides is how one can model the change in mechanicalproperties of the material during an entire flow event. Submarine landslides typically originate from collapse of seabed sediment.
The flow process involves a phase transition of the material from solid to semi-fluid by shearing, mixing, and entraining ambient
water. In the conventional equivalent fluid method based on Non-Newtonian fluid models, the change in material properties is neglected by assuming constant rheological parameters throughout the flow event. Often the rheological parameters are calibratedusing the evidence of previous flow events. Such parameters may not represent the real physics involved in submarine debris
flows.
The work by the author presented here aims to develop a new modelling framework for submarine landslides based on observations from conventional soil tests. Our modelling strategy is to extend the critical state concept in soil mechanics to a rangeof higher water content to account for the solid-fluid phase transition. The depth-averaged momentum conservation is solved
by a numerical scheme known as the Material Point Method together with the developed constitutive model. Simulations areperformed against evidence of a previous flow event. The capability of the numerical model is highlighted in a comparison withthe conventional Bingham fluid model.
Another way to look into the mechanism of submarine landslides is physical modelling. Full-scale testing, however, is almost
impossible for a submarine landslide event. In such situation, physical modelling can be a powerful tool to observe particular
aspects of the phenomena. Physical models are usually constructed at much smaller scale than the prototype. Due to this, one has
to carefully consider the scaling laws to extrapolate the observation in the model to the prototype. Among various scaling laws,that regarding to stress and strain is most important in geotechnical problems because of the non-linear, stress-strain behaviourof soil. One way to satisfy this scaling law is to increase the gravity, which can be achieved by applying centrifugal acceleration
to the model.
The geotechnical centrifuge at the Port and Airport Research Institute (Japan) has a 10 m-beam with 1.6m x 1.6m platforms
at both sides. A soil model is loaded at one side while a counter weight is loaded at the other side. The acceleration is appliedby rotating the beam and it can be increased up to about 100 g. This means that the model can be scaled up by 100 times insize. Here, an example of the centrifuge modelling is presented. The aim of the tests is to observe the earthquake-induced slope
failure and the transition to the gravity-flow. An underwater slope constructed in a model container was subjected to shaking
while spinning the centrifuge. Several tests were performed with different sand/clay ratios and various slope angles. The results
indicate the sand/clay ratio significantly influences the pore pressure dissipation during shaking and thus changes the flowability
afterwards. Furthermore, the effects of the viscosity of the pore fluid and the osmotic pressure applied to the slope from the bed
were examined. The detailed results will be presented at the oral/poster session.
Keywords: submarine landslies, centrifuge modelling, Material Point Method

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HDS26-02

Room:203

Time:May 26 17:40-17:55

Seismic wave simulation for terrestrial and submarine landslide sources in and around theKii peninsula, southwest Japan
NAKAMURA, Takeshi1∗ ; TAKENAKA, Hiroshi2 ; OKAMOTO, Taro3 ; KANEDA, Yoshiyuki4
1

Japan Agency for Marine-Earth Science and Technology, 2 Okayama University, 3 Tokyo Institute of Technology, 4 Nagoya
UniversityWe deployed permanent seafloor stations in water depths of 1,900-4,400 m near the Nankai trough of southwest Japan in 2010.
We observed seismic signals in the broadband seismometer of the seafloor station at a terrestrial landslide event caused by atyphoon passing over the Kii peninsula on September 4, 2011 (Nakamura et al., 2014). We simulated seismic waveforms forthe event with the finite-difference method by using a three-dimensional structure model in and around the Nankai trough. From
our simulations, we reproduced the features of the observed waveforms in terms of the arrival times of the main phases and thewaveform as a whole.
We also simulated seismic waveforms at the seafloor stations for a submarine landslide event assuming the epicenter to belocated in the middle of the large slump between the source area of the Tonankai and Nankai earthquakes. We assumed thesource time function with a duration time of more than 50 s referring to the analysis results of the 2011 terrestrial landslide event
presented by Yamada et al. (2013). Our simulation results show the prominent propagation of Rayleigh waves in the verticalcomponent because of the shallow source location of the submarine landslide near the seafloor and the effective developmentof the waves. We also find the significant variations of the Rayleigh wave propagation depending on the presence of a seawaterlayer. This is because the phase and group velocities and the dispersion of Rayleigh waves can be affected by the thickness ofa seawater layer. In our simulation, the maximum amplitude of the vertical component for the structure model with a seawaterlayer is four times greater than that for the non-seawater case at the seafloor stations. Our results indicate that a seawater layer
should be correctly incorporated into source analyses such as the size of the submarine landslide and the mechanism analysis
when we use waveform data observed at seafloor stations.

Keywords: submarine landslide, wave propagation, seafloor observation, Tonankai area, DONET

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HDS26-P01

Room:Convention Hall

Time:May 26 18:15-19:30

Slope failure observed off the eastern coast of Amami-Oshima Island, Central Ryukyu
Islands
MATSUMOTO, Takeshi1∗1Faculty of Science, University of the Ryukyus

Amami-Oshima Island is located along the Ryukyu Arc around 28.5N. Unlike the islands south Tokunoshima Island, no active
across-arc faults are identified on and around this island. However, a large-scale (about 600km2) slope failure towards theRyukyu (Nanseishoto) Trench is identified according to the topographic map based on the 500m-gridded topographic data by
Japan Coast Guard. Precise topography and shallow sub-surface structure were observed by 3.5kHz sub-bottom profiler (SBP)
through the two cruises by the Training Ship NAGASAKI-MARU owned by Nagasaki University in 2013 and 2014. Since thisarea is characterised by a potential methane hydrate reservoir, the relationship between the potential hydrate and the slope failureis also to be discussed. The slope failure was surveyed by total 6 SBP lines with the distance of 30 nm. Another slope failureis also identified along the northern slope of the deep sea valley NE offing of Amami-Oshima. This failure was surveyed by 4SBP lines with the distance of 15 nm. The former site was characterised by rough topography consisting of many mounds with
200-300m in relative height and 2-4km in radius, whereas the latter site by smaller-scale blocks.

Keywords: Amami-Oshima, Ryukyu Islands, Slope Failure

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