Faculty Profiles - NODA, Toshihiro

Papers - NODA, Toshihiro

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Elastoplastic finite element simulation of domino fault formation associated with tilting of highly structured ground Reviewed International journal

Yamada, E., Noda, T. and Asaoka, A.

SOILS AND FOUNDATIONS Vol. 64 ( 5 ) 2024.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Soils and Foundations

Ground deformation on the Earth's surface layer is strongly affected by the nonlinearity of geomaterials. However, the formation process of such deformation has yet to be described in a unified manner based on mechanics. The present study focuses on the normal faults in a submarine ground with highly developed soil skeleton structures and attempts to reproduce the process of normal fault formation associated with the tilting of a horizontally deposited submarine ground using an elastoplastic finite element simulation. The simulation was conducted using the soil–water coupled finite deformation analysis code GEOASIA, which incorporates an elastoplastic constitutive equation of the soil skeleton based on the modified Cam-clay model and the soil skeleton structure concept. The key findings are as follows: 1) Normal faults are formed from the ground surface to depth as shear bands, where shear strain is localized while exhibiting softening behavior with plastic volume compression. 2) Multiple normal faults are almost equally spaced and parallel to each other, with the inter-fault blocks rotating backward. The morphology of normal faults formed by the tilting of the ground shows domino-style characteristics. 3) The degree of the soil skeleton structure influences the formation of normal faults. This study demonstrates that elastoplastic geomechanics can explain the formation process of ground deformation, which has usually been interpreted from the perspectives of geomorphology and geology.

DOI： 10.1016/j.sandf.2024.101475

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Scopus

Soil-water-air coupled finite deformation analysis considering trapped air and continuous air phases Reviewed International journal

Yoshikawa, T. and Noda, T.

SOILS AND FOUNDATIONS Vol. 64 ( 5 ) 2024.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Soils and Foundations

Since the soil–water characteristic model relates the matric suction and the water content, it cannot describe changes in the water content when the suction is zero and constant, i.e., all the pore air is trapped air. To reasonably describe changes in the water content due to air entrapment and the compressibility of trapped air, this paper presents a deformation analysis method based on the mixture theory for a four-phase mixture consisting of the soil skeleton, capillary water, trapped air, and continuous air, in which the pore air phase is divided into trapped air and continuous air phases. Specifically, considering the mass conservation equation and the equation of motion for each phase of trapped air and continuous air, and considering the mass exchange between the trapped air and continuous air phases, governing equations were derived for the initial and boundary value problems of the four-phase mixture in a finite deformation field using a rate-type equation of motion. Two examples are provided to validate the new method. Firstly, experiments and analyses of soil water retention tests were conducted under multiple drying-wetting cycles. A comparison shows that, even if hysteresis is not considered in the relationship between the effective degree of saturation and suction, the new method can successfully describe the gradual decrease in the degree of saturation at a suction of 0 kPa with multiple drying-wetting cycles, indicating that the pore air gradually becomes trapped in the pore water, by modelling the mass exchange between the trapped air and continuous air phases. Secondly, analyses of an unexhausted and undrained triaxial compression test under zero suction were conducted, comparing the new and previous soil–water-air coupling methods. The results show that the new method, unlike the previous method, can successfully simulate the experimental result. This is because the new method is able to describe the compressibility of trapped air as the change in the capillary water degree of saturation, which is a novel state variable defined as the ratio of the volume of capillary water to the total volume of capillary water and trapped air. The new method contributes to the simplification of the soil–water characteristic model and enables evaluations of the soil deformation behavior due to the compressibility of trapped air, such as a countermeasure against liquefaction caused by unsaturation.

DOI： 10.1016/j.sandf.2024.101480

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Tsunami hazard evaluation of river embankment structures incorporating their vulnerability to seismic strong motion Reviewed International journal

Imai, K., Nakai, K., Hirai, T., Noda, T., Arai, N., Iwama, S., Iwase, H. and Baba, T

EARTHQUAKE SPECTRA Vol. 40 ( 3 ) page： 2008 - 2037 2024.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Earthquake Spectra

Development of coastal areas in Japan for various land uses since the 1960s has contributed to industrial upgrades and improved the efficiency of transportation networks. However, there are concerns about the vulnerability of developments on alluvial plains and reclaimed lands to geological events, like ground subsidence due to liquefaction during large earthquakes. Realistic assessment of earthquake and tsunami hazards and evaluation of possible countermeasures require accurate estimation of the amount of subsidence that can be expected from liquefaction at coastal and riverside sites supporting various structures. In this study, to evaluate the amount a river embankment structure might be expected to settle as a result of strong motion from an assumed Nankai Trough great earthquake, we conducted a numerical simulation using the soil–water coupled finite deformation analysis code GEOASIA. We then investigated the effect of the estimated embankment subsidence on tsunami inundation, which was simulated by using nonlinear shallow-water equations and a grid spacing as fine as 3.3 m. The influence of urban structures on the inundated area was taken into account by using a structure-embedded elevation model (SEM). The results showed that subsidence of river embankments and the collapse of parapet walls on top of them would increase both the depth and area of inundation caused by a tsunami triggered by a Nankai Trough scenario earthquake. Our findings underscore the importance of evaluating not only earthquake resistance but also vulnerability of coastal and riverside structures to strong motion in tsunami hazard analyses. Furthermore, the importance of tsunami inundation analysis using a SEM for predicting the behavior of tsunami flotsam in urban areas was demonstrated.

DOI： 10.1177/87552930241237815

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Super/sub-loading surface model for constitutive equation of friction Reviewed International journal

Toyoda, T., Yasuike, R. and Noda, T.

TRIBOLOGY INTERNATIONAL Vol. 191 page： 109080 2024.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Tribology International

In this study, an elasto–plastic constitutive model of friction incorporating the superloading surface concept was formulated as an extension of the subloading friction model. The proposed model regards the state in which the static friction coefficient is larger than the kinetic friction coefficient as the “bulkiness” of structure and the smooth transition of friction was expressed by the evolution rule of structure.The positive definiteness of the plastic multiplier, the loading condition, and a condition equation for material parameters were clearly derived. The capability of the proposed model in accordance with the subloading-friction model was confirmed in the stick–slip simulation of a mass-spring system.

DOI： 10.1016/j.triboint.2023.109080

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Comprehension of Seismic-Induced Groundwater Level Rise in Unsaturated Sandy Layer Based on Soil-Water-Air Coupled Finite Deformation Analysis Reviewed International journal

Yoshikawa, T. and Noda, T.

WATER Vol. 16 ( 3 ) 2024.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Water (Switzerland)

Immense liquefaction damage was observed in the 2011 off the Pacific coast of Tohoku Earthquake. It was reported that, in Chiba Prefecture, Japan, the main shock oozed muddy water from the sandy ground and the aftershock which occurred 29 min after the main shock intensified the water spouting; thus, the aftershock expanded the liquefaction damage in the sandy ground. For comprehending such a phenomenon, using a soil–water–air coupled elastoplastic finite deformation analysis code, a rise in groundwater level induced by main shock is demonstrated, which may increase the potential of liquefaction damage during the aftershock. The authors wish to emphasize that these results cannot be obtained without soil–water–air coupled elastoplastic finite deformation analysis. This is because the rise in groundwater level is caused by the negative dilatancy behavior (plastic volume compression) of the saturated soil layer which supplies water to the upper unsaturated soil layer, and it is necessary to precisely calculate the settlement of ground and the amount of water drainage/absorption to investigate the groundwater level rise. This study provides insight into the mechanism of ground liquefaction during a series of earthquakes.

DOI： 10.3390/w16030452

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Unconditional convergence of theoretical solutions to u-p formulation Reviewed International journal

Toyoda T. and Noda T.

Soils and Foundations Vol. 64 ( 1 ) page： 101421 2024.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Soils and Foundations

Dynamic soil-water coupling analyses, based on the u-p formulation, are inapplicable to highly permeable soils, causing numerical instability. In this study, it is demonstrated that theoretical solutions to the u-p formulation itself certainly exhibit unconditional convergence regardless of the permeability coefficient. This suggests that the instability is only numerical and can be observed in a temporally discretized system. Firstly, the linearized governing equation for the u-p formulation was proven to be reduced to a damped wave equation under a one-dimensional condition, similar to the Full formulation. Secondly, theoretical solutions for the u-p formulation were derived and their unconditional convergence was confirmed. Then, the essential characteristics of the u-p theoretical solutions, that is, the underestimation of permeability, overestimation of compression wave celerity, and occurrence of negative pore water pressure against positive load application, were described and compared with theoretical solutions for the Full formulation.

DOI： 10.1016/j.sandf.2023.101421

Scopus

Severe and nonuniform liquefaction damage of reclaimed ground contributed by interference between body waves and stratigraphic irregularity-induced surface waves Reviewed International journal

Nakai, K., Noda, T. and Asaoka, A.

EARTHQUAKE SPECTRA Vol. 40 ( 1 ) page： 287 - 313 2024.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Earthquake Spectra

The 2011 off the Pacific Coast of Tohoku earthquake caused extensive liquefaction damage to reclaimed land along the Tokyo Bay coast, even though it was approximately 400 km from the epicenter. The characteristics of the liquefaction damage include the fact that liquefaction occurred in soils with a high percentage of fine particles and that the distribution of liquefied and nonliquefied areas was nonuniform. The factors contributing to such nonuniform liquefaction damage included the heterogeneity of the ground materials and their depositional conditions, and the effects of the long earthquake duration. Although these points are certainly valid as reasons for the occurrence of severe liquefaction damage, they do not fully explain the mechanisms of the liquefaction of the fine-grained soils, or the localized extent of the liquefaction. To elucidate the severe and nonuniform damage, seismic response analyses of a multi-layered ground were conducted focusing on the stratigraphic irregularities in the ground beneath Urayasu city. The results showed that the thicker and softer sedimentary layers amplify the slightly long-period component of the seismic motion and increase the shaking at the ground surface. Moreover, the wave propagation in the ground became very complicated owing to the focal effect caused by the refractions and reflections of body waves at the stratum boundary, surface wave excitation at the base of the slope, and amplified interference between body and surface waves. This complex wave propagation contributed to nonuniform surface ground shaking and severe liquefaction damage. In addition, surface waves, which consist primarily of slightly long-period components, can propagate far and wide; as such, they triggered extensive damage owing to delayed shaking phenomena that continue even after the earthquake. The analysis results suggested that multidimensional elasto-plastic seismic response analyses considering stratigraphic irregularities are important for detailed seismic evaluation.

DOI： 10.1177/87552930231220005

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Experimental study on sand anisotropy using hollow cylinder apparatus Reviewed International journal

Yamada, S., Okada, M., Nakano, M. and Noda, T.

SOILS AND FOUNDATIONS Vol. 63 ( 3 ) 2023.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Soils and Foundations

This study investigated sand anisotropy experimentally using a hollow cylinder apparatus. The effect of the initial anisotropy on the shear behavior of sand was illustrated by conducting experiments on specimens with bedding planes and systematically varying the density, principal stress direction, and intermediate principal stress. The change in induced anisotropy during shearing was experimentally captured by re-shearing the specimens subjected to prior shear history. The experimental results revealed the following: (a) Anisotropy in sand, whether initial anisotropy developing during specimen preparation or induced anisotropy developing due to shear history, causes pseudo-density changes in the mechanical behavior, in which sand of the same density behaves as if it has a different density depending on the direction of shear. (b) The changes in induced anisotropy, due to shearing in the same direction as that of the prior shear, make the soil behave similarly to dense sand, whereas shearing in a direction perpendicular to the prior shear makes the soil behave similarly to loose sand. (c) The larger the prior shear, the more pronounced the pseudo-density changes that appear in the subsequent behavior. Moreover, the significance of induced anisotropy in liquefaction and compaction phenomena was experimentally demonstrated through single and double swing cyclic shear tests. The results obtained from the study will be useful for validating models that incorporate induced anisotropy.

DOI： 10.1016/j.sandf.2023.101326

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Finite-Deformation Cam-Clay Model Considering Elastic Dilatancy and Soil Skeleton Structure Reviewed International journal

Yamada Shotaro and Noda Toshihiro

INTERNATIONAL JOURNAL OF GEOMECHANICS Vol. 23 ( 2 ) 2023.2

Language：English Publishing type：Research paper (scientific journal)

Elastoplastic constitutive equations for soils often employ the nonlinear rate-type isotropic Hooke’s law, which is based on theadditive decomposition of strain rate and considers the pressure dependency in the elastic part. Hooke’s law has independent isotropic anddeviatoric components, and therefore, it cannot express elastic dilatancy as confirmed in experiments. In this model, although the elastic vol-ume change is path-independent, the shear strain exhibits a significant residual when subjected to an effective stress cycle. This study pro-poses a rate-type elastic constitutive equation for soil applicable to elastoplastic constitutive equations based onfinite deformation theoryusing an objective stress rate and additive decomposition of stretching; these equations cover these shortcomings of the rate-type Hooke’slaw. The proposed rate-type elasticity constitutive equation is based on the hyperelastic body in the infinitesimal deformation theory proposedby Einav and Puzrin or Houlsby et al. and has confining pressure dependence, path independence of elastic volume change, and elastic di-latancy. Further, the residual strain is considerably smaller than that of the rate-type Hooke’s law. Moreover, this study improves the elasto-plastic constitutive equation, SYS Cam-Clay model, based on the skeleton structure concept by introducing the proposed elastic constitutiveequation. The basic behavior of the elastoplastic constitutive equation for clay and sand is demonstrated to show that the newly acquiredelastic dilatancy improves the mean effective stress reduction behavior during shear stress unloading under undrained conditions and thestiffness recovery behavior during liquefaction, which is essential for describing cyclic mobility.

DOI： 10.1061/IJGNAI.GMENG-7117

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Verification of a Macroelement Method with Water Absorption and Discharge Functions in Quasi-Static Problems Reviewed International journal

JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING Vol. 148 ( 7 ) 2022.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ASCE

When simulating the vertical drain method using a soil–water coupled finite-element analysis, we the macroelement method can be adopted to provide the water absorption function of drains to individual elements approximately. A previous study added the discharge function of vertical drains to the technique so that the occurrence of well resistance can be considered in accordance with its mechanism. This paper verified the approximation accuracy of this new macroelement method in quasi-static problems. The macroelement method was derived under the assumption that the axisymmetric pore water pressure distribution is equal Barron’s solution, but the first numerical example demonstrated that the technique has high approximation accuracy even for nonaxisymmetric problems by appropriately converting the permeability of a drain. The second calculation example included a number of drains and applied the vacuum consolidation method to clayey ground containing a middle sand layer. In this calculation, there was remarkable well resistance, and pore water flowed from the unimproved region to the improved region or between the effective areas of each drain. The calculation results indicated the superiority of the new macroelement method in terms of accurate approximations even for such a complex problem. Moreover, this study demonstrated the effectiveness of waterproof sealing, a countermeasure for sucking pore water from the middle sand layer, using the new macroelement method.

DOI： 10.1061/(ASCE)GT.1943-5606.0002803

Web of Science

Method to introduce the cementation effect into existing elastoplastic constitutive models for soils Reviewed International journal

Journal of Geotechnical and Geoenvironmental Engineering Vol. 148 ( 5 ) page： 04022013-1 - 04022013-14 2022.5

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：ASCE

This study proposes a new method to introduce the cementation effect into existing elastoplastic constitutive models for soils. The mechanical properties of cement-treated soil are evaluated via element tests and compared with those of naturally deposited clay. The similarities and differences between cement-treated soils and naturally deposited clays are studied, focusing on two states, the undisturbed and remolded states. The effective stress for cement-treated soils incorporating an internal state variable representing the cementation effect is newly defined to describe the mechanical properties of cement-treated soils. Moreover, by applying this extended effective stress to the super-subloading yield surface (SYS) Cam-clay model, which is an elastoplastic model for soils based on the skeleton structure concept, the scope of this constitutive model is extended to include cement-treated soils. The cementation effect introduced by the proposed method allows reproducing the mechanical behavior of the cement-treated soil. Finally, a brittle behavior not described at the element level can be obtained, leading to a soil–water coupled finite deformation analysis incorporating the proposed constitutive model.

DOI： https://doi.org/10.1061/(ASCE)GT.1943-5606.0002727

Proposal of a simple evaluation method for evaluating the seismic aftershock resistance of a spherical gas holder inclined by the main earthquake Reviewed

Mio KOBAYASHI, Hideyuki TAZUKE, Kentaro NAKAI and Toshihiro NODA

Vol. 77 ( 2 ) page： I_543 - I_550 2022.2

Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Japan Society of Civil Engineers

It is desirable for spherical gas holders, which require a high level of safety, to be able to retain their functions during aftershocks even when the foundation is subjected to unequal settlement due to a largescale earthquake. For this purpose, it is necessary to develop a simple method to evaluate the seismic resistance of inclined gas holders under the assumption of maximum aftershocks, which will contribute to the decision on whether or not to use the holders after a large-scale earthquake. The analysis to evaluate each element of the holder must take into account the material and geometrical nonlinear response mechanism of the supporting structure and nonlinear dampers, and it is not realistic to conduct detailed evaluation by 3D analysis in a short period of time after an earthquake. Therefore, in this paper, the simple evaluation method using a single mass-spring analysis that reflects the nonlinear spring characteristics from the pushover analysis is proposed. This method shows good agreement with the dynamic response of a 3D framework model of an actual gas holder until the maximum response value, and after that, the response value is calculated on the safe side, which means that the aftershock seismic resistance of the holder can be evaluated.

DOI： https://doi.org/10.2208/jscejam.77.2_I_543

Combined-loading elastoplastic constitutive model for a unified description of the mechanical behavior of the soil skeleton Reviewed International journal

Yamada S., Noda T., Nakano M., Asaoka A.

Computers and Geotechnics Vol. 141 2022.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Computers and Geotechnics

This paper proposes a combined-loading elastoplastic constitutive model (CL model) for a unified description of the mechanical behavior of soil materials, including clay, sand, and intermediate soil. This mathematical model combines the super-subloading yield surface Cam-clay model and the non-associated Drucker–Prager (DP) model. The first model introduces the skeleton soil structure concept into the Cam-clay model based on critical-state soil mechanics. It describes the mechanical behavior of naturally deposited clay more accurately than the existing model does. The non-associated DP model considers the frictional properties of granular materials and is widely used to describe the undrained shear behavior, particularly that of sandy soil. The newly proposed model is one in which these two independently existing models simultaneously or independently work in loading states according to the loading criterion, which can be derived as a natural result from plastic multipliers. This framework allows for a unified description of a wide range of soil materials, including naturally deposited clayey soils and sandy soils.

DOI： 10.1016/j.compgeo.2021.104521

Scopus

Effects of strong ground motion with identical response spectra and different duration on pile support mechanism and seismic resistance of spherical gas holders on soft ground International journal

Applied Sciences (Switzerland) Vol. 11 ( 23 ) 2021.12

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)

Safety measures are required for spherical gas holders to prevent them from malfunctioning even after a large earthquake. In this study, considering the strong nonlinearity of the ground and damage to the pile during an earthquake, a three-dimensional seismic response analysis of the holder-pile-ground interaction system was conducted for an actual gas holder on the soft ground consisting of alternating layers of sand and clay. In the analysis, the seismic response of the structure to strong ground motions of different durations with the same acceleration response spectrum was verified. The results show that the piles were relatively effective in controlling the settlement when the duration of the earthquake motion was long. This is because the axial force acting on the pile increased due to the redistribution of the holder load caused by the lowering of the effective confining pressure of the sand and clay layers during the earthquake, which increased the bearing capacity of the pile. In contrast, when the duration of the seismic motion was short, the piles had little effect on the reduction in the settlement because the maximum acceleration was higher than that in the former case, and the piles immediately lost their support function.

DOI： 10.3390/app112311152

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Numerical simulation based heuristic investigation of inertia-induced phenomena and theoretical solution based verification by the damped wave equation for the dynamic deformation of saturated soil based on the u–w–p governing equation Reviewed International journal

Toyoda, T. and Noda, T.

Soils and Foundations Vol. 61 ( 2 ) page： 352 - 370 2021.4

Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Soils and Foundations

The theoretical and numerical solutions for the one-dimensional dynamic deformation of saturated ground were obtained using the u–w–p governing equation. The numerical analysis was performed based on the finite element method, and the occurrence of superficially peculiar phenomena, i.e., S-shaped settlement–time relation, non-harmonic oscillation in the settlement/pressure–time relation in highly permeable soil, and inconsistency in the vertical load and immediate pressure was identified heuristically. The theoretical solution of the damped wave equation was derived from the original governing equation expressed as the superposition of several modes with different eigenvalues, and this solution could successfully explain the phenomena observed heuristically. It was noted that the phenomena were induced by the presence of inertia, because the static solution and u–p analysis did not exhibit such phenomena. The verification of the analysis was conducted owing to the correspondence of the theoretical solutions of the damped wave equation and the u–w–p analysis results.

DOI： 10.1016/j.sandf.2020.12.007

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Importance of considering unsaturated triaxial tests including ceramic disk as initial and boundary value problems Reviewed International journal

Yoshikawa, T., Noda, T. and Kodaka, T.

Soils and Foundations 2021.3

Language：English Publishing type：Research paper (scientific journal)

Considering unsaturated triaxial tests including a ceramic disk as initial and boundary value problems, a series of suction change, isotropic consolidation, and drained shearing processes was simulated using a soil-water-air coupled elastoplastic finite deformation analysis code. As a result, it was demonstrated that the delayed behavior of a wetting-induced collapse during the suction change and isotropic consolidation processes, as well as water-absorption behavior during the subsequent drained shearing process, were attributed not to the characteristics of the soil specimen, but to the permeability of the disk. Thus, it is important to take the ceramic disk into account in unsaturated triaxial tests regarded as initial and boundary value problems in order to understand the temporal change behavior of unsaturated soil specimens. Otherwise, there is a risk that the behavior appearing as the solution of an initial and boundary value problem may be modeled as a constitutive relation.

DOI： 10.1016/j.sandf.2020.12.009

Scopus

LIQUEFACTION DAMAGE SUPPRESSION EFFECTS OF EXISTING SMALL SCALE STRUCTURE RECEIVING ECCENTRIC LOAD USING FLOATING GRID TYPE IMPROVEMENT Reviewed

Vol. 76 ( 2 ) page： I_279 - I_288 2021.2

Language：Japanese Publishing type：Research paper (scientific journal)

all over the world. If the existing small-scale structures such as houses are tilted due to liquefaction, they suffer more serious damage than they are subsided. In this paper, the effect of countermeasure against tilting damage due to liquefaction using floating grid-type improvement was investigated by 2D model tests under 1g condition using eccentric model structure, and non-linear FEM analyses. From the results, it was found that the floating grid-type improvement can significantly suppress the settlement damage of structure by suppressing the lateral flow of the liquefied ground even if the dead load of structure is eccentric. It was also found that lateral flow of the liquefied soil hardly occurs when the structure inclines due to liquefaction, and the effect of countermeasure against tilting damage due to liquefaction using floating grid-type improvement was not as high as the effect of suppressing subsidence damage. Therefore, it was found that desirable to use a floating grid-type improvement and drainage method to prevent tilting damage of structure.

DOI： https://doi.org/10.2208/jscejam.76.2_I_279

PROGRESSIVE FAILURE OF UNSATURATED FILL SLOPE CAUSED BY CUMULATIVE DAMAGE UNDER SEEPAGE SURFACE Reviewed International journal

Murao Hidehiko, Nakai Kentaro, Yoshikawa Takahiro, Noda Toshihiro

INTERNATIONAL JOURNAL OF GEOMATE Vol. 20 ( 78 ) page： 1 - 8 2021.2

Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：International Journal of GEOMATE

Seepage surface position of fill slope can fluctuate owing to rainfall and groundwater supply from behind the slope. Therefore, in this study, shaking-table tests of an unsaturated fill slope were conducted to examine the influence of the seepage surface position on the fill slope stability by comparing with previous results in the fully saturated condition. An acceleration amplification factor was used to evaluate the stability. If the embankment was in an unsaturated condition, a slip plane was formed along the seepage surface. With ongoing increase of input acceleration, the slip progressed to behind the slope. Comparison of the unsaturated and saturated results showed that although the failure mechanism in which the slip surface was formed by the reduction in rigidity was common, the input acceleration at failure and deformation mode were different. When the slip surface was formed and failure occurred, the input acceleration in the unsaturated fill slope was more massive, and the deformation at the same excitation stage was small. These results imply that the unsaturated fill slope has a higher earthquake resistance than the saturated fill slope. Moreover, it is shown that the progress of plastic deformation inside the embankment and the formation process of the slip plane can be estimated from the change in the acceleration amplification factor regardless of seepage surface position.

DOI： 10.21660/2020.78.j2036

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Triaxial test on water absorption compression of unsaturated soil and its soil-water-air coupled elastoplastic finite deformation analysis Reviewed International journal

Yoshikawa Takahiro, Noda Toshihiro

SOILS AND FOUNDATIONS Vol. 60 ( 5 ) page： 1151 - 1170 2020.10

Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Soils and Foundations

Triaxial tests and numerical simulations were conducted to determine the water absorption compression of unsaturated soil. In the triaxial tests, unsaturated silt specimens under the same initial condition were prepared and the processes of different suction reduction, isotropic consolidation, and exhausted-drained shearing were applied to the specimens. The numerical simulations used a soil-water-air coupled finite deformation analysis code with a newly incorporated elastoplastic constitutive model. The degree of saturation was introduced to the internal state variables of the yield function of the SYS Cam-clay model and a soil-water characteristic model with void ratio dependency. Then, the triaxial test was treated as an initial and boundary value problem, and the series of processes in the experiments were simulated using a single set of material constants and initial values. The new findings are as follows:
1)The loading criterion and the classification of loading conditions considering change in saturation degree can be specified in the elastoplastic constitutive model for unsaturated soil.
2)Water absorption compression behavior was observed in the experiments not only under suction reduction but also under constant suction condition. This water absorption compression under constant suction condition was explained as a saturation-increase behavior due to volume compression using the soil-water characteristic model with void ratio dependency.
3)By considering the triaxial test as an initial and boundary value problem, the temporal change in the volume and the amount of water absorption/drainage were able to be reproduced.

DOI： 10.1016/j.sandf.2020.06.010

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Seismic damage of soft clay layer directly under the river levee that becomes prominent by L2 earthquake International journal

Noda, T. and Nakai, K.

Proc. of the 17th World Conference on Earthquake Engineering (17WCEE) page： 4b-0040 2020.9

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Papers - NODA, Toshihiro