IIZUKA Atsushi | ![]() |
Resaarch Center for Urban Safety and Security | |
Professor | |
Civil Engineering / Architecutre and planning / Building Engineering |
Jun. 2020 Institution of Civil Engineers, ICE Publishing Awards 2020, Telford Premium, Extension of unsaturated soil mechanics and its applications
Aug. 2014 独立行政法人 日本学術振興会, 平成25年度日本学術振興会・特別研究員等審査会専門委員 (書面担当)表彰
Others
May 2008 地盤工学会, 平成19年度 地盤工学会技術開発賞, 土/水連成弾(粘)塑性有限要素・飽和地盤静的解析パッケージDACSARの開発と実用化
May 2008 地盤工学会, 平成19年度 地盤工学会 功労章
Jan. 2008 土木学会関西支部, 支部活動功労賞(土木学会関西支部創立80周年記念)
Jun. 2007 地盤工学会, Award by Japanese Society of Geotechnical Enginnering, 2007, 地盤工学会技術者交流特別セッションの企画と運営
Oct. 2006 神戸大学工学部, Excellent teacher, Faculty of Engineering, Kobe University, 2006, 平成17年度後期学部授業「連続体力学」
May 1990 土質工学会(地盤工学会), Award by Japanese Society of Geotechnical Enginnering, 1990, General Theory of shear bands formation by a non-coaxial Cam-clay model
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Although the soil, water and air coupling theory has been deductively derived from the three-phase mixture theory, assumptions and interpretations inherent in soil mechanics are also conveniently introduced in this theory. Mechanical properties of unsaturated soils are phenomenologically integrated into the constitutive equation for a deformation problem, with effective stress expressed as a combination of partial stresses acting on each phase of the mixture. The governing equations of two seepage problems describing the movements of pore water and pore air are formulated by a priori introduction of Darcy's law. Consequently, the coupling theory consists of a deformation problem and two seepage problems, in which the soil-water retention characteristics play the role of a mediator connecting these problems. If the theoretical structure is clarified in this way, an extension of this theory is easily made. In this paper, the extension of the theory to each of four examples and their performance is demonstrated. The features of settlement caused by absorption of soil water due to vegetation; dissolution, advection and diffusion of substances in groundwater; desaturation and state change due to vaporisation of dissolved gas from pore water; and the liquefaction potential of unsaturated soil are presented.
ICE Publishing, 01 Sep. 2019, Geotechnical Research, 6 (3), 156 - 176, English[Refereed]
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In the wide area of the eastern part of Tokyo, the ground level is less than mean sea level. This area is more vulnerable to disasters than other areas. If large flood damage such as storm surge should occur in this area, the disaster would be a long-term catastrophe. On the coast of Tokyo Bay, countermeasures have been taken by tide embankments and floodgates. However, considering the damage scale when it occurs, an analysis in this area is very important. In this area, ground settlement occurred and groundwater head dropped because groundwater excessively withdrew by the industrial purpose during the period of economic growth. Currently, the groundwater head recovers and the ground settlement has been subsided. However, due to the groundwater head fluctuation, pore water pressure distribution had been different from hydrostatic pressure distribution. Therefore, in the analysis in this area, it is necessary to consider past groundwater head fluctuation. In this research, the ground settlement and the distribution of pore water pressure are simulated from groundwater level fluctuation over the past 100 years. Then, we conducted the seismic analysis by input the distribution of effective stress calculating from the simulated ground water pressure. The sites analyzed in this research are Tokyo Sea Life Park at the mouth of Arakawa River.
Tech Reviews Ltd, 15 May 2019, Journal of Civil Engineering and Construction, 8 (2), 41 - 47, English[Refereed]
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The migration of contaminant through soil is usually modeled using the advection-dispersion equation and assumes that the porous media is stationary without introducing a constitutive equation to represent soil structure. Consequently, time-dependent deformation induced by soil consolidation or physical remediation is not considered, despite the need to consider these variables during planning for the remediation of contaminated ground, the prediction of contaminated groundwater movement, and the design of engineered landfills. This study focuses on the numerical modeling of solute transfer during consolidation as a first step to resolve some of these issues. We combine a coupling theory-based mass conservation law for soil-fluid-solute phases with finite element modeling to simulate solute transfer during deformation and groundwater convection. We also assessed the sensitivity of solute transfer to the initial boundary conditions. The modeling shows the migration of solute toward the ground surface as a result of ground settlement and the dissipation of excess pore water pressure. The form of solute transport is dependent on the ground conditions, including factors such as the loading schedule, contamination depth, and water content. The results indicate that an understanding of the interaction between coupling phases is essential in predicting solute transfer in ground deformation and could provide an appropriate approach to ground management for soil remediation.
WILEY, Jul. 2018, INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, 42 (10), 1172 - 1196, English[Refereed]
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Currently, finite element codes for small-strain have been widely applied in geotechnical engineering problems. However, the limitations of these codes are recognized, especially when the strain is large. To overthrow such limitations, finite-strain finite element code for porous media is needed. Therefore, this study aims to implement the finite element code for coupled solid-fluid at finite strain for static and dynamic problems. Firstly, the governing equation for porous media at finite-strain was introduced. Then, the mixed formulation of both phases with respect to the reference coordinates by deriving the governing equations with respect to the reference or current coordinates and by considering the solid and fluid as separate, was given. At present, the code is developed only for elastic regime as its preliminary step. In this study, the proposed code has been verified with the static and dynamic problem. And, the result of the finite-strain finite element code is then compared with the result of the analytical solution and small-strain finite element code within the small-strain regime. Thereafter, the proposed code was used to estimate the response of the earthen dam when subjected to a dynamic load. Finally, the results, discussion, and comparison between small- and finite-strain finite element analyses were presented.
GEOMATE International Society, 2018, International Journal of GEOMATE, 15 (51), 1 - 8, English[Refereed]
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In view of the need to utilize ocean space and to develop seabed resources, the assessment of the stability of deep seabed soil has emerged as an important challenge in the field of geomechanics. To study seabed stability, the strength and stiffness of the natural ground must be ascertained. Accordingly, it is necessary either to conduct laboratory testing on soil specimens sampled from the seabed or to estimate the strength and stiffness by in-situ tests. While in the future it may be reasonable to conduct in-situ tests to estimate the stiffness and strength of seabed soil, it will still be necessary to compare the physical properties measured by in-situ testing with those measured by laboratory testing in advance of these determinations. In short, soil specimens must be sampled from the actual deep seabed, and laboratory mechanical tests must be conducted on the sampled soil specimens. However, soil sampled from the ocean bottom is subject to effects that differ from those exerted on soil sampled from the earth. More specifically, the non-negligible effects of disturbance are expected with soil sampled from the ocean bottom. The effects of disturbance occur during the sampling process due to the vaporization of dissolved gases, as these soil specimens are under relatively higher pressure and contain pore water with a high amount of dissolved gases. Therefore, numerical simulations were conducted in the present study to investigate the effects of vaporized dissolved gases on the mechanical behavior of soil specimens during sampling and on the undrained shear strength as determined by laboratory tests. The analyses revealed that the combination of the decreasing effective stress caused by the sampling and factors such as overconsolidation and unsaturation is attributable to the difference between the soil strength ascertained by laboratory testing and the in-situ soil strength.
Japanese Geotechnical Society, 2018, Soils and Foundations, Vol.58 (No.3), pp.689 - 701, English[Refereed]
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Bentonite materials have been considered as a candidate material for engineered barriers in transuranium (TRU) waste disposal facilities due to their physical and chemical characteristics. Bentonite materials will be initially compacted and unsaturated. After closing the cavern, the bentonite material will become saturated through exposure to a percolating groundwater. It is therefore important to understand the behaviour of bentonite during saturation, which provides an initial condition of the disposal facilities. This paper explains a mathematical model which can express the saturation process. Subsequently, a series of numerical simulations were carried out. First, the swelling pressure of buffer and backfill was examined to verify the numerical model. It was confirmed that the simulated swelling pressure rises with water absorption, and swelling pressure test results from the literature are shown to be satisfactorily predicted by the numerical model, although these were slightly overestimated. Next, the mechanical behaviour of bentonite materials in a TRU waste disposal facility during the saturation process was examined using soil/ water/ air coupled finite-element method. Although it is difficult to examine the validity of analysis results because of the difficulty of an extremely long-term measurement, the qualitative behaviour of the saturation process could be inferred.
ICE Publishing, Oct. 2017, Environmental Geotechnics, 4 (5), 339 - 352, English[Refereed]
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The impermeability of bentonite is dependent on montmorillonite microstructures and varies with chemical alteration caused by related changes. The Kozeny-Carman equation is viewed as a rational hydraulic model for bentonite because it incorporates microstructure parameters such as specific surface area in addition to macrostructure parameters such as the void ratio. In the study reported here, a technique for measuring the specific surface area of compacted bentonite under constant volume conditions using X-ray diffraction was developed, and the hydraulic conductivity of various bentonite materials was evaluated based on the measurements thus obtained. The applicability of the Kozeny-Carman hydraulic model for evaluation of impermeability of bentonite was clarified. (C) 2017 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
JAPANESE GEOTECHNICAL SOC, Oct. 2017, SOILS AND FOUNDATIONS, 57 (5), 683 - 697, English[Refereed]
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Bentonite has remarkable swelling characteristics and low permeability that enhance the stability of the buffer materials in repositories for the geological disposal of radioactive waste. It is necessary to apply reliable numerical simulation techniques to assess the safety and mechanical stability of repositories over a long period of time. Having a constitutive model that can describe the mechanical behavior of bentonite is key to such numerical simulations. This paper proposes an elasto-plastic constitutive model to describe the changes in the mechanical properties of bentonite due to saturation in the progress of a repository becoming submerged under groundwater. In the proposed model, the swelling index is formulated as a function of the degree-of-saturation to express not only the swelling behavior, but also the dependency of the degree-of-saturation on the dilatancy characteristics. The montmorillonite content is used as a material parameter to determine the normal consolidation line. The experimental results of swelling volume and swelling pressure tests in previous literature are shown to have been satisfactorily predicted by the proposed model. (C) 2017 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
JAPANESE GEOTECHNICAL SOC, Feb. 2017, SOILS AND FOUNDATIONS, 57 (1), 80 - 91, English[Refereed]
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The B-value, measured after setting up a specimen for triaxial testing, is used for evaluating the degree of saturation of a specimen. However, the stress conditions used when measuring the B-value have not been standardized. Although some experimental approaches have been pursued, no detailed analyses have evaluated the effects of the measurement conditions on the B-value results. In this study, the B-values of nearly saturated specimens under various stress conditions were investigated, using a smart triaxial test apparatus capable of elaborate stress control, and comparisons among measurement results were conducted. Moreover, by applying Henry's law, which governs the dissolution of the gas phase into the liquid phase, to an existing constitutive model for unsaturated soil, a new soil model was proposed. This new model accurately represents the behaviors of pore air even under nearly saturated conditions. In addition, the effects of the stress conditions on the B-values were investigated through numerical simulations of B-value measurements. (C) 2016 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V.
JAPANESE GEOTECHNICAL SOC, Oct. 2016, SOILS AND FOUNDATIONS, 56 (5), 848 - 860, English[Refereed]
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Most onshore earth structures, such as earthen dams, embankment, and river levees, are constructed by compaction. Transportation geotechnics also includes construction of compacted earth structures, such as road and railway embankment. Optimum compaction can increase shear strength, while decreasing compressibility and permeability of earth structures. For this reason, compaction has been used for earthworks since the dawn of times. Generally, a series of lab compaction tests is conducted to obtain a compaction curve before constructing these structures. The shape of the compaction curve, which determines the maximum dry density at the optimum water content, is very important in the design and construction of structures. However, it is difficult to control compaction quality on the basis of lab tests alone, as soil materials are inhomogeneous. Furthermore, there are differences in the compaction methods used in lab tests and at construction sites. Therefore, it is necessary to explain the mechanisms of compaction in the framework of soil mechanics. In this study, compaction of earthworks is simulated in a similar way. The effect of compaction layer thickness and the number of compaction layers on the distribution of dry densities in an earth structure was investigated through soil/water/air coupled simulation. Consequently, it was found that multi-layered compaction can create a gap in the dry density between two layers and that this gap can be reduced by thinner layer compaction. On the other hand, though the average dry density in a compacted earth structure can be increased by increasing the number of compacted layers, some gap between compaction layers was inevitable. Through these findings, this study contributes to the performance design of compacted earth structures.
ELSEVIER SCIENCE BV, 2016, ADVANCES IN TRANSPORTATION GEOTECHNICS III, 143, 276 - 283, English[Refereed]
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In recent years, enormous damage has been caused by natural disasters. Disaster Medical Assistance Team (DMAT) play an important role in minimizing human suffering. However, since factual information is intermingled with disinformation in the event of a disaster, it is difficult to obtain correct information, such as where and to what extent medical care is required, in order to dispatch DMAT appropriately. In this study, using a victims estimation formula empirically derived from the cumulative death toll data with the passage of time in past seismic hazard, an optimization problem was addressed to minimize the number of deaths.
Consequently, it was found that early dispatch of DMAT would be effective, and DMAT should be dispatched according to the circumstances (number of DMAT, conditions of affected areas) in order to minimize the cumulative death toll. And, by setting the conditions of the affected area, it was possible to consider the way of reasonable DMAT dispatch. Thus, it has become possible to suggest the way of effective dispatch of finite DMAT, to use limited DMAT resources efficiently, and to help make decisions.
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Decision makers must decide the disaster correspondence by using uncertain information. When the decision maker makes this decision, they have the dilemma to prioritize which of “the rapidity of correspondence” and “the accuracy of information”. The result for disaster varies according to the difference in priority.
This study approximate the change of cumulative number of death with time lapse by using the Weibull distribution and proposed the Bayesian estimation model to predict the total number of deaths by utilizing the change of cumulative number of deaths. By considering the estimated total number of deaths with past earthquakes cases, it was possible to estimate the time period to make the disaster response decision. Therefore, this would be a judgment material to reduce the dilemma of decision makers.
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Due to its low permeability and excellent expansion characteristics, bentonite materials are considered to be the engineered barrier in Trans-Uranium (TRU) disposal facility. It is necessity to predict the mechanical behavior of bentonite using numerical simulation to evaluate the long-term performance of TRU disposal facility. In this paper, we simulated the saturation process of bentonite materials in TRU disposal facility from an initially unsaturated state. And we examined the mechanical behavior of bentonite buffer during the saturation process. This simulation result shows that Disposal waste is moved upward due to the difference of swelling pressure between buffer and backfill. And the facility existed in unsaturated state even under steady state conditions due to the entrapped air.
CRC PRESS-TAYLOR & FRANCIS GROUP, Sep. 2014, Computer Methods and Recent Advances in Geomechanics, 409 - 414, English[Refereed]
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Undrained shear strength is used for estimating the bearing resistance of foundations and the safety factor of a slope. For this test, the pore-pressure coefficients, namely A and B as proposed by Skempton, are available. These coefficients can be obtained from the undrained triaxial test. According to Terzaghi's effective stress theory, the B-value must be 1.0 in a saturated soil. Therefore, a B-value of 1.0 is used as the target for saturating a specimen. However, it is difficult to achieve a B-value of 1.0 in the triaxial test. Because of this, a specimen with a B-value greater than 0.95 is regarded as saturated. The relationship between saturation and the B-value is still not clear. In this study, the factors influencing the B-value were numerically investigated with a soil/water/air coupled model that considers the solvability of pore-air to pore-water. Consequently, the pore-water pressure behavior, which depends on the initial degree of saturation and initial stress conditions, could be expressed.
CRC PRESS-TAYLOR & FRANCIS GROUP, Sep. 2014, Computer Methods and Recent Advances in Geomechanics, 403 - 408, English[Refereed]
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In this study, we use conservation laws to construct a method for describing the movement of dissolved material. This is then applied to a mathematical model. Recently, soil pollution, induced by toxic materials in the ground water, is spreading. The mobility modes for this dissolved material can be divided into advection, induced by distribution of fluid head, and dispersion, induced by distribution of concentration content. First, Darcy's law is derived from the momentum conservation law using Bear's theory. Secondly, the concept of total fluid head is extended for application to various fluids. Additionally, an infiltration equation and an advection-dispersion equation are derived from mass conservation laws. Coupling the two equations, we construct a mass transfer model for unsaturated/saturated ground. Using the model, the effect of density flow is considered assuming basic one-dimensional areas. The results show that mobility of the dissolved material depends on its density. We concluded that density flow dominates along the vertical rather than the horizontal direction and mobility of the fluid is dulled in the unsaturated area.
CRC PRESS-TAYLOR & FRANCIS GROUP, Jul. 2014, Proc. International Conference on Unsaturated Soils: Research and Applications (UNSAT2014), Vol.2, 1179 - 1187, English[Refereed]
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In order to study the triggering factors for shallow landslides during rainfall, sandy soil slopes were prepared in laboratory angles of 30 and 45 degrees and those slopes were poured with rainfall intensities of 30 and 60 mm/hours through a specially developed rain simulator. Moreover, the rainfall on slope was simulated with using unsaturated soil/water/air coupled F.E. code, and the numerically calculated depth of water front and values of suction at different location in the slopes at various angles of inclinations matched well with the experimentally observed values. The combined results obtained with the numerical and experimental studies were utilized to predict the variation in the depth of water and values of suction at different intensities of rainfall as well as deformation of slopes during rainfall.
CRC PRESS-TAYLOR & FRANCIS GROUP, Jul. 2014, Proc. International Conference on Unsaturated Soils: Research and Applications (UNSAT2014), Vol.1, 621 - 627, English[Refereed]
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The pore pressure coefficient B, which is measured on triaxial test, is useful for evaluating degree of saturation of specimen. However, the influential factors on this B value other than degree of saturation have not been investigated in detail up to now. In this study, first, Henry's law that indicates dissolved air in pore water was applied to the existing constitutive model for unsaturated soil in order to express the behavior of pore air on unsaturated soil extremely close to fully saturated state. Next, the influential factors, such as degree of saturation, confining stress and back pressure, were investigated through numerical simulations with using newly obtained model. Consequently, it was found out that B value depends on not only degree of saturation but also stress state.
Japan Society of Civil Engineers, 2014, Journal of Japan Society of Civil Engineers, Ser. A2 (Applied Mechanics (AM)), 70 (2), I_145 - I_153, Japanese[Refereed]
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The East Japan Earthquake and Tsunami caused serious damages in the coastal area of eastern Japan. Many houses and cars on the road were swept away by the tsunami. Also, industrial facilities such as chemical plants in the coastal area were completely destroyed. Damage from saline seawater was not limited only to the rice field areas, and there were also serious anxieties regarding the possibility of ground contamination as a result of harmful heavy metals that leaked from destroyed chemical plants. In light of this scenario, we carried out a quantitative survey to examine these areas. This paper reports on the results of our survey. It was found that high salt concentrations were detected on the ground surface but predictions state that these concentrations can be diluted with rainfall. It was also revealed that serious ground contamination due to the harmful heavy metals was not detected.
2014, Theoretical and Applied Mechanics Japan, 62, 141 - 148, English[Refereed]
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Recently, the hydraulic/mechanical/chemical (HMC) analytical method has been studied with the aim of evaluating the long-term performance of Trans-Uranium (TRU) geological repositories. In this particular research, the hydraulic/mechanical modelling of bentonite materials for HMC analyses has been studied, with bentonite materials considered as the engineered barrier. It is said that the bentonite material in the TRU disposal facilities is altered chemically by cement leachate. Moreover, there is a concern that chemical alteration changes the expansion characteristics and water permeability of the bentonite material. In this paper, using past test results, the influence of montmorillonite content and the exchange of sodium and calcium ions on mechanical and hydraulics behaviour is examined. Hydraulic/mechanical mathematical modelling is also presented, describing the mechanical behaviour of bentonite from an initially unsaturated state while also taking into account the hydraulic and mechanical characteristics of bentonite mentioned above. We also simulate the saturation process from an initially unsaturated state in a TRU disposal facility, which is modelled one-dimensionally.
GEOLOGICAL SOC PUBLISHING HOUSE, 2014, CLAYS IN NATURAL AND ENGINEERED BARRIERS FOR RADIOACTIVE WASTE CONFINEMENT, 400, 221 - 235, English[Refereed]
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In order to evaluate the long-term behaviour of the engineered barriers in geological disposal sites for transuranic element-bearing (TRU) waste, an evaluation by numerical analysis is required. Although chemical and hydraulic/mechanical analyses have been conducted independently until now, essentially both type of phenomena occur simultaneously and produce synergistic effects. Therefore, we focused attention on the buffer (bentonite) engineered barrier and conducted a study of which involved incorporating hydraulic/mechanical phenomena into the chemical analysis of bentonite alteration. The simulations employed weakly-coupled chemical and hydraulic/mechanical effects to study the behaviour in one dimension. The results showed that the dissolution of the montmorillonite is suppressed in the buffer section nearest the cement material. Moreover, in order to achieve a fully coupled analysis in future, the present study also identifies issues that need to be resolved.
MINERALOGICAL SOC, May 2013, Clay Minerals - Journal of Fine Particle Science, 48 (2), 331 - 341, English[Refereed]
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© Springer Science+Business Media Dordrecht 2013. In this chapter, the stress–strain relationship on the singular point of the yield surface of the original Cam clay model and the Sekiguchi-Ohta’s model is derived in order to compute the volume change correctly. Metastable space, the range of increment of strain in which effective stress does not get away from the singular point of yield surface, was theoretically derived. In metastable space, the increment of volumetric strain is larger than the increment of deviatoric strain.
2013, Geotechnical, Geological and Earthquake Engineering, 25, 229 - 239[Refereed]
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© Springer Science+Business Media Dordrecht 2013. An elasto-plastic constitutive model for unsaturated soils is improved to realize numerical stability of computations at the singular point on the yield surface and an accurate prediction of the mechanical behavior of overconsolidated (or elastic state) soils in this chapter. The authors introduce the exponential contractancy model (EC model) by Ohno et al. (J Appl Mech JSCE 9:407–414, 2006) and the subloading surface model by Hashiguchi and Chen (Int J Numer Anal Method Geomech 22:197–227, 1998) to the elasto-plastic constitutive model for unsaturated soils by Ohno et al. (J JSCE 63(4):1132–1141, 2007). The applicability of the constitutive model is verified by simulating triaxial shear tests under constant net stress undrained conditions.
2013, Geotechnical, Geological and Earthquake Engineering, 25, 215 - 228[Refereed]
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Results obtained in a lab compaction test are difficult to apply to the design and compaction control at an actual geotechnical engineering site. This is attributed to the fact that the mechanism for compaction has not been explained using the principles of soil mechanics. The main theme of this study is interpretation of the compaction mechanism with unsaturated soil mechanics. Here, static compaction tests were simulated with soil/water/air coupled finite element analysis code DACSAR-MP. Consequently, the shape of the compaction curve was successfully expressed. Moreover, the effects of compaction on compressibility and permeability of compacted soil could be reasonably explained. Additionally, shear deformation was applied to the specimen obtained from static compaction simulations in soil/water/air coupled analysis. The relationships between shear strength and compaction curve showed good agreement with the actual behavior.
2013, Proc. Int. Conf. on Soil Mechanics and Geotechnical Engineering, 1, 1129 - 1132, English[Refereed]
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Compacted soil is widely used for earth structures. However, the mechanism of compaction has not been explained, so it is difficult to grasp distributions of stress, void ration, and soil moisture within compacted earth structure. Therefore, estimation of safety and stability of compacted earth structures exposed to natural disaster is complicated. Compaction is decreasing void ratio by applying stress under certain water content and pushing air out of soil mass. Thus, the mechanics of unsaturated soil, which includes air within void, is needed for understanding of compaction. In this study, we regarded compaction as compression and expansion phenomenon of unsaturated soil under drained air and undrained water conditions and formulated it in initial-boundary-value problem. Here, the influences of permeability and soil water retention characteristics on compaction were considered and static compaction tests were simulated. Moreover, multi-layered compaction was examined.
Japan Society of Civil Engineers, 2012, Journal of Japan Society of Civil Engineers, Ser. A2 (Applied Mechanics (AM)), 68 (2), I_299 - I_306, Japanese[Refereed]
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This study presents a new constitutive model for expansive soils. Under any arbitrary state of degree-of-saturation, the mechanical void ratio predicted by the elasto-plastic constitutive models, i.e. EC model and Se-hardening model are combined with the additional void ratio in order to characterize the typical swelling behavior of expansive soil. Herein, the fully saturation curve which derived from compaction and consolidation experimental tests are taken into consideration. The proposed model is examined by simulating the typical experimental test on expansive soil, i.e. normally consolidation and swelling pressure tests. © 2012 IEEE.
2012, 5th Asia-Pacific Conference on Unsaturated Soils 2012, 1, 303 - 308, English[Refereed]
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Static friction in granular wedges can stabilize own weight on a horizontal plane without lateral supports. This sort of problem provides fundamental knowledge about pressure distribution along the foundation of earth works which have a long triangular shape in particular. The present study reviews the authors' recent development on an analytic model of a sand heap which adopted Nadai's hypothesis on the major principal stress orientation and a framework of below-yielding limit. Because deformation analyses of many geotechnical engineering problems often require the magnitude of coefficient of lateral earth pressure at-rest K-0 to describe in situ lateral earth pressure, emphasis is placed on the evaluation of stress ratio under the apex of sand heap which represents Jaky's basic assumption on the derivation of K-0 in the prismatic sand heap sloped at an angle of repose. Comparison of the analytical stress ratio along the central plane of granular wedge with Jaky's expressions was reported.
IOS PRESS, Sep. 2011, Proc. International Symposium on Deformation Characteristics of Geomaterials (IS-Seoul 2011), Vol.2, 924 - 932, English[Refereed]
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An evaluation method for the mechanical behavior of unsaturated soils is studied in this paper. Although the mechanical behavior of unsaturated soils is complicated, a simple modeling is preferable in practice. This is because the soil properties are not homogeneous and ground data is limited when structures are being designed. In addition, in order to evaluate the reliability of the design, the physical meanings of the parameters applied in the prediction model should be clear. Firstly, the authors study the relationship between compaction curves and compression indexes in the unsaturated state that is used in the proposed constitutive model. Based on the constitutive model, the stress paths for constant volume shear tests are formulated under a constant void ratio condition and the stress paths for undrained shear tests are calculated under a constant water content condition. In the case of unsaturated specimens, the volume of these specimens changes with the shear deformation and the stress paths depend on the initial degree of saturation. The results of the calculation qualitatively describe the test results by considering the changes in effective confining pressure in the undrained condition and the water retention curves. © 2010 Springer Science+Business Media B.V.
Mar. 2011, Geotechnical and Geological Engineering, 29 (2), 171 - 180, English[Refereed]
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Due to its low permeability and excellent expansion characteristics, bentonite is an excellent candidate with potential use as a buffer in the disposal of nuclear waste. Its expansion characteristics, activated by wetting, can be interpreted based on the full saturation line, depicted as a unique line on the density and the confining pressure relationship as proposed by Kobayashi et al. (2007). In addition, its elasto-plastic constitutive relation can also be formulated by introducing additional irreversible strain component describing the expansion of the montmorillonite present in the bentonite material. A constitutive model can consistently express the mechanical behavior of the compacted bentonite material from the unsaturated to the fully saturated state. This paper describes the density homogenization process that was conducted through a series of soil-water coupled elasto-plastic finite element simulations. Specifically, bentonite specimens, with different initial densities, were permeated with a constant water head. Stresses and strains developing in bentonite, particularly the density change, were carefully examined. A series of numerical simulations, performed on the two specimens, showed that specimens did not homogenize to a unique value of density upon reaching the fully saturated state. To confirm the simulation results, we carried out a series of experiments. The experimental results also support our simulation results. Copyright © 2011 by ASME.
2011, Proceedings of the International Conference on Radioactive Waste Management and Environmental Remediation, ICEM, (PARTS A AND B), 949 - 954, English[Refereed]
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In structural mechanics, Nadai's sand hill analogy is the interpretation of an ultimate torque applied to a given structural member with a magnitude that is analogously twice the volume of stable sand heap which can be accommodated on a transverse cross-section basis. Nadai's analogy is accompanied by his observation of a loose triangular embankment, based on the fact that gravitating loose earth is stable if inclined just under the angle of repose. However, Nadai's analysis of stress distribution in a planar sand heap was found to be inaccurate because the total pressure obtained from Nadai's solution is greater than the self-weight calculated from the heap geometry. This raises a question about the validity of his observation in relation to the analogy. To confirm his criterion, this article presents and corrects the error found in Nadai's solution by analyzing a radially symmetric stress field for a wedge-shaped sand heap with the purpose of satisfying both force balance and Nadai's closure. The fundamental equation was obtained by letting the friction state vary as a function of angular position and deduce it under the constraint that the principal stress orientation obeys Nadai's closure. The theoretical solution sufficiently agreed with the past experimental measurements. © 2010 Elsevier Ltd. All rights reserved.
Oct. 2010, Journal of the Mechanics and Physics of Solids, 58, 1506 - 1523, English[Refereed]
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Desertification due to salt damage has recently become a serious problem throughout the world. Salt damage is a phenomenon where salt in the groundwater gets concentrated at the ground surface and kills vegetation. In this study salt damage due to disequilibrium of water balance was simulated with unsaturated soil/water coupled simulation code, as applied the material transport equation. Evaporation and precipitation cycles were applied to a model of ground that contained salty groundwater in its deep region. This allowed the successful reproduction of the phenomenon of salt accumulation on the ground surface. Additionally, identical climatic conditions were applied to model grounds consisting of two types of soil, different in terms of their soil-water retention characteristics. It was found that two methods of overlay and insertion of a layer of coarse-grade material, known as 'mulching', are effective in the prevention of salt accumulation. © 2011 Taylor & Francis Group, London.
Sep. 2010, Proc. 5th Int. Conf. on Unsaturated Soils, 2, 1133 - 1139, English[Refereed]
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Pressure distribution in sand heaps is affected by a particular arching action during the formation process. The closure of fixed principal axes successfully links arching criterion to capture the phenomenon of central stress minimum in conical sand heaps. However, it is insufficient to explain a negligible stress dip observed in planar sand heaps, which are of practical interest to civil engineers because many earthworks are triangular in shape. This study explains the closure of polarized principal axes and the stress distribution formulation, which was found to be agreeable with experimental data. The direction of major compression along a circle traced about the apex might provide a suitable arching criterion in the 2D space of sand heaps.
2010, Theoretical and Applied Mechanics Japan, 58, 41 - 48, English[Refereed]
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To predict the long-term elasto-plastic behavior of unsaturated earth structures, a constitutive model, which can express the following typical mechanical behavior of unsaturated soil, is needed. This includes shrinkage and increase of stiffness upon drying (i.e. suction increase), collapse with reduction of stiffness upon wetting (i.e. suction decrease), and the influences of suction history related to the hysteresis of soil-water retention characteristics. In this study, a new constitutive model is proposed for unsaturated soil where the effective degree of saturation is used as a parameter relating to stiffness. The model shows good agreement with experimental results. In addition, a soil/water coupled finite element simulation code (simulator), as applied to this model, is introduced. The appropriateness of the simulator is demonstrated for the case of a virtual fill.
CRC PRESS-TAYLOR & FRANCIS GROUP, Nov. 2009, Proc. 4th Asia-Pacific Conference on Unsaturated Soils, 711 - 718, English[Refereed]
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When troublesome sludge discharged from industries and purification plants are burnt by 1300 degree, they change to innocuous micro-porous ceramic materials. The authors are seeking for practical use of such micro-porous ceramic materials in the geotechnical engineering field, particularly for geohazard mitigation. Then, in order to grasp fundamental mechanical properties of the micro-porous ceramic particles, a series of laboratory tests, that is, triaxial compressive shear tests, permeability tests and water retention tests, were performed. It is found that the stress-strain characteristics and permeability of the micro-porous ceramic particles are very similar to those of standard sandy materials but they have very high water retention capability. Next, applicability of the unsaturated elasto-plastic constitutive model proposed by Ohno et al. (2007) to the mechanical behavior of micro-porous ceramic particles was examined in this paper.
CRC PRESS-TAYLOR & FRANCIS GROUP, May 2009, Proc. of the International Symposium on Prediction and Simulation Methods for Geohazard Mitigation (IS-Kyoto 2009), 339 - 345, English[Refereed]
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Discontinuity at the vertex of the Sekiguchi-Ohta model causes numerical troubles when associated flow rule is applied because the singularity encountered in the derivatives of yield function. Based on Koiter's associated (low rule, the numerical implementation to accommodate the vertex singularity had been generalized. The set of applied strains had been formulated and defined as the domain of metastability. This study is focused on validity of the domain of metastability by a regularized method. Infinitesimal vicinity adjacent to the vertex is considered to regularize all of plastic flow in outwards normal direction to the yield surface. It was confirmed that the metastable domain of the Sekiguchi-Ohta model is bounded by an elliptical cone in principal strain space.
2009, Theoretical and Applied Mechanics Japan, 57, 89 - 94, English[Refereed]
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It is empirical knowledge that vegetation can influence the behavior of earth structure. It prevents surface failure and erosion due to rain and wind, and contributes to the stability of earth structure. Moreover, it encourages water circulation with transpiration, fixes atmospheric nitrogen and carbon via photosynthesis, and improves the environment. Therefore, surface seeding is purposely performed when an earth structure, such as an embankment or an earth dam, is constructed. Typically, vegetation grows on unsaturated soil. Consequently, a constitutive model for unsaturated soil is needed for understanding the effects of vegetation on the ground. In this study, we focus on the effect of vegetation-induced water uptake. The effect of water uptake due to evapo-transpiration can be regarded as reduction of water content within soil and appropriately represented in simulation studies. The constitutive model proposed by Ohno et al. (2007) is used for the simulation. In this model, the effective degree of saturation is treated as a parameter expressing hardening/softening. Moreover, the 'root element,' where the reduction of water content occurs, is applied to the soil/water coupled analysis with unsaturated soil mechanics. To verify the applicability of this model, the accident in Poland where non-uniform settlement and building damage were caused by water uptake of vegetation was simulated. The results show that water uptake increased suction and induced non-uniform settlement. This method is effective for understanding the effects of vegetation on the ground. © 2009 IOS Press.
2009, Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering: The Academia and Practice of Geotechnical Engineering, 1, 526 - 529, English[Refereed]
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The soil/water-coupled analysis for unsaturated soil mechanics with the finite element method is used to express the behavior of compacted earth structures exposed to rainfall. The constitutive model, which is proposed by Karube et al. (1996, 1997) and modified by Karube and Kawai (2001), is formulated for initial and boundary problems and coded as DACSAR-U (lizuka et al., 2000). A deep fill, which consists of compacted, unsaturated soil, was analyzed with DACSAR-U. Different patterns of rainfall were applied to deep fills in finite element simulations. As a result, non-uniform settlement of deep fill can be calculated and the magnitude of settlement of deep fills is dependent on the amount of collapse, namely, the compression due to the decrease in suction. Moreover, delayed settlement due to rainfall and an accumulation of an irreversible settlement due to cyclic rainfall was observed.
JAPANESE GEOTECHNICAL SOC, Apr. 2007, Soils and Foundations, 47 (2), 195 - 206, English[Refereed]
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A new constitutive model of swelling soil material is proposed to evaluate its mechanical characteristics. In the case of the compacted bentonite material, nonlinear behavior appears more dominantly than ordinary clay materials in the consolidation test. The original Cam-clay model cannot describe these characteristics observed in the compacted bentonite materials. Therefore, a new modeling that strain increment consists of linear summation of the swelling component and the elast-plastic component is introduced to describe the nonlinear behavior in the unloading stage. From the result of the simulation of the triaxial compression test, it is demonstrated that the stress pass and the stress and strain relationship are well simulated by the new model.
2007, Structural Engineering/Earthquake Engineering, 24 (2), 102 - 111, English[Refereed]
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In this paper, the authors focus on the confining effect brought by geosynthetic-reinforcement. The reinforcement effect is not brought by the geosynthtics itself but mobilized complexly as a composite material consisting of geosynthetics and compacted soils. If the geosynthetics and the compacted soils are unified more tightly, higher reinforcement effect may be expected. Then, in this paper, presented is an experiment, that is, making soil cantilevers with compacted soils wrapped by geosynthetics which are tightly unified by prestressed steel bars. Two types of soil cantilevers were constructed. The soil structures were, first, constructed and wrapped by geosynthetics on the bases of styrene forms and, next, prestressed through the steel bars penetrated from the top surface to the bottom of the geosynthetic-reinforced soil structures. The soil cantilever was formed by melting and removing the styrene forms in order from the edge with monitoring the vertical displacements. It was found that amazing reinforcement effect appears by tightly confining the dilative deformation of compacted soils with the geosynthetics and the prestressed steel bars. The reinforcement effect is numerically investigated. A series of elasto-plastic FE numerical simulation was carried out. The compacted soil was modelled to be elasto-plastic body and the geosynthetics was expressed by the elastic material.
MILLPRESS SCIENCE PUBLISHERS, Sep. 2006, Proc. of 8th International Conference on Geosynthetics (CD-ROM), 1213 - 1218, English[Refereed]
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This paper discusses the volume change characteristics of compacted soil based on oedometer test results. After making specimens with different water contents and under different compaction loads, compression tests are conducted under both saturated and unsaturated conditions. The effects of the compaction conditions on the compression curves are discussed. It is confirmed that the yield stress and the water content are important factors when evaluating a compression curve, while dry density and suction are important factors when evaluating the yield stress. Copyright ASCE 2006.
2006, Geotechnical Special Publication, (147), 837 - 848, English[Refereed]
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Changes in the instantaneous shear modulus associated with the non-coaxiality of NC clay materials are presented. A series of hollow cylindrical shear tests was performed to measure the changes in the instantaneous shear modulus with shear history. After each normally consolidated clay specimen was compressively sheared until a prescribed shear level, it was continuously torsionally sheared under a constant volume to measure the instantaneous shear modulus. Coaxial constitutive relations, such as these of the conventional Cam-clay type of models, predict that the instantaneous shear modulus obtained, does not change regardless of the compressive shear history. On the contrary, it is known that non-coaxial constitutive relations theoretically bring about changes in the instantaneous shear modulus in relation to the compressive shear history. The test results supported the predictions made by the non-coaxial constitutive models. Finally, the proper formulation for the non-coaxiality between the stress and the incremental plastic strain is discussed.
地盤工学会, 15 Apr. 2005, Journal of the Japanese Geotechnical Society, 45 (2), 135 - 143, English[Refereed]
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Changes in the instantaneous shear modulus associated with the non-coaxiality of NC clay materials are presented. A series of hollow cylindrical shear tests was performed to measure the changes in the instantaneous shear modulus with shear history. After each normally consolidated clay specimen was compressively sheared until a prescribed shear level, it was continuously torsionally sheared under a constant volume to measure the instantaneous shear modulus. Coaxial constitutive relations, such as these of the conventional Cam-clay type of models, predict that the instantaneous shear modulus obtained, does not change regardless of the compressive shear history. On the contrary, it is known that non-coaxial constitutive relations theoretically bring about changes in the instantaneous shear modulus in relation to the compressive shear history. The test results supported the predictions made by the non-coaxial constitutive models. Finally, the proper formulation for the non-coaxiality between the stress and the incremental plastic strain is discussed.
JAPANESE GEOTECHNICAL SOC, Apr. 2005, Soils and Foundations, 45 (2), 135 - 143, English[Refereed]
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The prediction of back-fill settlement due to the excavation work is getting important, particularly in the urban area. The finite element simulation would be a suitable choice to answer it. However, it is less feasible in practice to carry it out because input parameters needed in the F.E. simulation are not sufficiently provided in advance at the stage of design work. In the past, some simple evaluation methods for predicting the back-fill settlement due to excavation works were presented by Peck, Long and Moormann et al. and so on. In this paper, a new method capable for predicting the back-fill settlement at the stage of design work is proposed, taking account of design parameters, i.e. soil parameters, retaining wall parameters and construction sequences. The proposed method is built up based on a lot of monitored records obtained from sites and the numerical results obtained from a series of F.E. simulations for imaginary excavation works. It is designed to be simple and useful at the stage of design work. This method consists of two parts, predictions of the maximum amount of back-fill settlement and the distance from the retaining wall. Finally, some case studies in which the proposed new method was applied to actual excavation sites in Japan are presented.
TAYLOR & FRANCIS LTD, 2005, 5th International Conference on Getechnical Aspects of Underground Construction in Soft Ground, 719 - 723, English[Refereed]
International conference proceedings
This paper presents the modeling of the composite soil-geosynthetic structure and discusses the effect of the reinforcement arising from the confining of the deformation of the soil by geosynthetics. A series of compressive shear tests for compacted sandy soil specimens, wrapped in geosynthetics, is carried out for the purpose of quantitatively examining the effect of the geosynthetic reinforcement arising from the confining of the deformation of compacted soils during shearing. Furthermore, an elasto-plastic modeling for compacted soil and a rational determination procedure for input parameters, needed in the elasto-plastic modeling, are presented. In this paper, the subloading yielding Surface is introduced to the elasto-plastic modeling in order to describe the irreversible deformation characteristics of compacted soil during shearing. Since compacted soil is essentially unsaturated and its mechanical behavior is influenced by the suction working in the compacted soil media, the suction effect is taken into account in estimating of the degree of compaction. Finally, an elasto-plastic finite element simulation is conducted and the geosynthetic-reinforcement effect is presented. (C) 2004 Elsevier Ltd. All rights reserved.
ELSEVIER SCI LTD, Oct. 2004, Journal of Geotextiles and Geomembranes, 22 (5), 329 - 358, English[Refereed]
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There are some constitutive models for unsaturated soil. However, their mathematical structures, particularly for expression of collapse phenomenon, are essentially the same. Therefore, it is possible to unify the constitutive models which were proposed in the past. In this paper, a generalized constitutive model for unsaturated soils is presented. First, we outline existing constitutive models for unsaturated soil and, next, we propose a comprehensive constitutive model for unsaturated soils in terms of the effective stress for unsaturated soils. To this new elasto-plastic model, it is easier to introduce sophisticated methods such as a rotational hardening, a subloading surface and so forth. This model is formulated in the form of incremental general stress and strain relationship and is incorporated into a finite element code, DACSAR-U, for unsaturated soils. The performance of the new constitutive model is demonstrated by comparing with the models proposed by Karube et at and by Honda et al
Japan Society of Civil Engineers, Sep. 2004, Journal of applied mechanics, Vol.7 (No.1), 505 - 513, Japanese[Refereed]
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柴田(1963)、柴田・軽部(1965)、太田・畠(1971)による一連の研究によって提案された弾塑性構成式(太田・畠モデル)において、土のダイレイタンシー挙動は、間隙比(e)と応力比(q/p')との関係において直線としてモデル化されている。しかし、これまでの研究により、土によっては、その関係が必ずしも直線とならないことがわかっている。本研究では、柴田(1963)、柴田・軽部(1965)、太田・畠(1971)による一連の研究を踏襲しながら、より適用性の高いダイレイタンシーの表現式を提案することで、新しい土の弾塑性構成式を導出している。
The Japanese Geotechnical Society, 2004, Proceedings of the Japan National Conference on Geotechnical Engineering, 39, 261 - 262これまで,土構造物の挙動予測を行う際に関口・太田による弾(粘)塑性構成モデルが組み込まれた二次元土/水連成弾(粘)塑性有限要素解析プログラム(DACSAR)が多く用いられてきた.しかし,現行のDACSARで適用されている負荷判定法は粘塑性体積ひずみと定める閾値の大小関係で行われているため,閾値の定め方によって負荷判定結果が左右される.そこで,本報告は飯塚らが提案した閾値を必要としない負荷判定法をDACSARに組み込み,K0圧密および静的非排水繰返しせん断の解析結果を比較することにより,新しく提案した負荷判定法の適用性について検討したものである.
The Japanese Geotechnical Society, 2004, Proceedings of the Japan National Conference on Geotechnical Engineering, 39, 913 - 914[Refereed]
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A highway embankment built in Akita prefecture using ground stabilization methods was constructed on very soft peat and clay deposits on a sloping base. This paper presents a case study of a soil/water coupled elasto-viscoplastic finite element simulation of an actual embankment construction, that is, the deformation analysis of a soft clayey foundation deposited on an inclined base with embankment loading. For execution of the numerical simulation, determining the input parameters needed in the analysis and evaluating the shape of sloping bedrock under the ground were problems. The former is discussed for a practical procedure of determining input parameters using unconfined compression strengths and the latter is accomplished by drawing a contour map of the top face of the bedrock under the ground, with reference to the geographical features of the area around the site and to a topographic map. The performance of the numerical simulation is discussed in comparison with in-situ monitored records. The effectiveness of ground stabilization methods employed at the site is also a topic of discussion.
JAPANESE GEOTECHNICAL SOC, Oct. 2003, Soils and Foundations, 43 (5), 81 - 92, English[Refereed]
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Experimental evidence suggests that a mutual relationship between anisotropy and structure of clays exists. It was hypothesized that development and decay of the structure of clays (called structuralizing and destructuralizing) can be regarded as development of anisotropy and decay of anisotropy (isotropization), respectively. Using this hypothesis, an anisotropic rotational hardening rule was incorporated in the Sekiguchi-Ohta elasto-plastic constitutive model. The subloading surface concept was also introduced to model the plastic deformation inside the yield surface more accurately. The proposed model provided some rational as well as physical explanations for the experimental facts. Using the proposed model, it was possible to simulate the apparent softening behavior observed in anisotropically consolidated specimens. For isotropically consolidated specimens, no softening was observed, which is consistent with the experimental data. At the critical state, the undrained shear strength of the anisotropically consolidated specimens was the same as that of the isotropically consolidated specimens. This is due to both specimens having the same soil structure after large shearing. The shape of yield surface obtained from stress probe tests was investigated by simulating the tests with the proposed model. Typical data of the stress probe tests of natural clays shows that the yield surface generally has a smooth elliptical shape. By modeling the rotation of yield surface, the proposed model was capable of simulating this 'apparent' yield surface.
JAPANESE GEOTECHNICAL SOC, Jun. 2003, Soils and Foundations, 43 (3), 95 - 104, English[Refereed]
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本研究の目的は,室内試験を境界値問題と捉え,正規圧密粘土の非排水三軸圧縮・伸張試験,平面ひずみ圧縮試験をシミュレーションし,供試体の破壊挙動を解析的に検討することである.計算を行う道具として,有限変形弾塑性理論に基づく土/水連成3次元有限要素プログラムを開発した.シミュレーションにはMises型のCam-clayモデルを用いているため,解析解として得られる破壊強度に圧縮側,伸張側あるいはその中間で違いがあれば,一般に実験で得られるMohr-Coulombの規準は粘土の材料としての性質ではなく,他の要因に起因するものと考えられる.
The Japanese Geotechnical Society, 2003, Proceedings of the Japan National Conference on Geotechnical Engineering, 38, 293 - 294既存のモデルでは,構造・過圧密の変化を表す概念として上・下負荷面モデル,異方性の変化を表す概念として回転硬化則を用いていたが,両者の関係に関しては十分な議論がなされていなかった.本研究では,構造の劣化と異方性の変化には不可分な関係があると考え,関口・太田モデルに上・下負荷面の概念と回転硬化則を導入し,構造・過圧密・異方性の変化を統一的に取扱える構成モデルを開発した.具体的には,回転硬化の発展則を上負荷面の発展則の関数で表すことによって,構造と異方性を結びつけた.開発した構成モデルを用いて,構造を持った土の各種室内力学試験を解析し,これまで表すことができなかった実験事実を説明し,妥当性を示した.
The Japanese Geotechnical Society, 2003, Proceedings of the Japan National Conference on Geotechnical Engineering, 38, 283 - 284[Refereed]
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The mechanical behavior of unsaturated soils strongly depends on the pore water distribution as well as the suction. The water characteristic curve (W. C. C) specifying the pore water content and the suction value plays a very important role in describing the mechanical modeling for unsaturated soils. However, the W. R. 0 shows the hysteresis loop in the wetting/drying processes. This paper emphasizes the importance of considering the hysteresis of W.C.C in the modeling. The accumulation of irreversible strains in the soil structures caused by rainfalls cannot be explained well without considering the hysteresis of W. C. C. An existing model of W. C. C is modified so as to express the accumulation of irreversible strains as the cyclic suction loading. Finally, it is shown that the modified model well explains the experimental results.
Japan Society of Civil Engineers, Sep. 2002, 応用力学論文集, 5, 777 - 784, Japanese[Refereed]
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The reinforcement technique using flexible materials such as geosynthetics has been widely employed in the current engineering practice. However, the reinforcement mechanism has not revealed sufficiently, i.e. the mechanical interaction between the reinforcements and soils, the confining effect induced by reinforcements and so forth, and then it has not yet led into a proposal of rational design procedure. This paper reports in-situ loading tests carried out on the compacted soil structure reinforced by flexible beltlike materials. Elasto-plastic finite element simulations were also performed considering dilatancy characteristics of compacted soils during shearing. Throughout comparison with the field records monitored in the site and the results obtained from the conventional elastic computation, discussed are the applicability of the elasto-plastic analysis and the role of dilatancy characteristics of compacted soils in the reinforcement mechanism.
A A BALKEMA PUBLISHERS, Nov. 2001, Proc. of the International Symposium on Landmarks in Earth Reinforcement (IS-Kyushu 2001), Vol.1, 507 - 510, English[Refereed]
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Apart from following general outlines for input procedures and parameter findings when performing excavation analysis using FEM, some additional considerations should be taken. The influences of appropriate soil stiffness, pre-load efficiency, actual strut stiffness, soil/wall interface, crack in concrete retaining wall, and foundation piles installed before excavation have been investigated to weight the impact on analysis quality, enhancing acceptability in practice. Computed deformations and strut axial loads of a case study agreed quite well with field responses after applying these effects.
A A BALKEMA PUBLISHERS, Sep. 2000, Proc. of Coastal Geotechnical Engineering in Practice (IS-Yokohama), Vol.1, 351 - 356, English[Refereed]
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The prediction of back-fill settlement due to the excavation work is getting important, particularly in the urban area. The finite element simulation would be a suitable choice to answer it. However, it is not feasible in practice to carry it out because input parameters needed in the F. E. simulation are not provided in advance in the stage of design work. This paper proposes a simple prediction method of the back-fill settlement due to the excavation work. The proposed method is built based on a lot of monitored data obtained from sites and the numerical results obtained from a series of F. E. simulations for imaginary excavation sites. It is designed to be simple and useful in the stage of design work. This method consists of two parts, prediction of the maximum amount of back-fill settlement and prospect of the distance from the retaining wall.
Japan Society of Civil Engineers, 21 Jun. 2000, Doboku Gakkai Ronbunshu, 652, 217 - 227, Japanese[Refereed]
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The specification of strength parameters is an important task in the design work of slope stability. Usually laboratory shear tests are employed to determine the strength parameters. The specimen in the laboratory tests is intended and generally assumed to represent a single point in the in-situ soil medium. And, the strength parameters obtained are interpreted as material properties of the soil element. However, since the uniformity of stress/strain distribution within the specimen is hardly achieved during shearing, the strength parameters thus obtained cannot be essentially regarded as material properties. In this paper, the shear box test under the condition of constant volume is considered and the distribution of stress/strain within the specimen is rigorously examined through numerical simulations based on finite incremental deformation theory. The mobilization of strength is explained associated with the development of shear bands in the specimen during shear.
A A BALKEMA PUBLISHERS, Nov. 1999, Proc. of International Symposium on Slope Stability Engineering, Geotechnical and Geoenvironmental Aspects (IS-Shikoku), 763 - 768, English[Refereed]
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The strain localization developing inside clay specimens during shear is examined through the experimental observation and numerical F.E. simulation. In the experiment, clay specimens of column and hollow-cylinder shapes are uniaxially sheared and the development of shear bands is observed. The numerical simulation of compression shear is carried out under the plane strain condition for normally consolidated clay specimens with impermeable boundaries by using the elasto-plastic soil/water coupled finite element computation technique based on the finite deformation theory. Although the geometric boundary conditions are different in the experiment and simulation, discussed are the development of shear bands on/in the specimen and the transition of the localization mode which are found commonly in the experiment and simulation.
A A BALKEMA PUBLISHERS, Nov. 1998, Proc. of 4th International Workshop on Localization and Bifurcation Theory for Soils and Rocks, 345 - 354, English[Refereed]
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Mechanical properties of soft clays, such as deformability and strength, are estimated using data obtained from in-situ and laboratory tests. These test methods were developed through experience gained in the development of geotechnical engineering. The undrained strength, for instance, of a soft clay estimated by using any one of these in-situ or laboratory tests is not usually the same as that estimated by another one of these tests. There are many possible reasons for the apparently different response of the clay to each of these testing methods. The differences caused by factors such as disturbance, stress history, detailed testing procedures, are sometimes surprisingly influential. This paper shows examples of the above mentioned complications encountered in engineering practice. The paper also describes the interpretation of such complications by means of an elasto-plastic constitutive model which was proposed by Sekiguchi and Ohta in 1977.
A A BALKEMA PUBLISHERS, Apr. 1998, Proc. of 1st International Conference on Site Characterization, Vol.1, 403 - 408, English[Refereed]
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This paper presents evaluation of undrained strengths obtained from unconfined compression tests, constant volume shear box tests and field vane shear tests which were carried out in a site. Theoretical expressions of these undrained strengths derived from the elasto-plastic constitutive model are summarized. And discussed are the correction factors that help converting the undrained strengths obtained from tests to ideal undrained strengths comparable to the theoretical strengths. Throughout comparison of the measured strengths and the theoretical predictions, reliable specification of the constitutive parameters for the model from the measured strengths is examined.
A A BALKEMA PUBLISHERS, Apr. 1998, Proc. of 1st International Conference on Site Characterization, Vol.2, 1265 - 1271, English[Refereed]
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The seismic deamplifying effect of soft clay layers is described. This deamplifying effect of soft clay layers on the propagated waves due to an earthquake is significantly influenced by the degree of consolidation of the soft clay underlying the site. In this paper, two buildings on Rokko Island are analysed. First, the static soil/water coupled analyses are carried out to assess the initial equilibrium conditions of the site just before the earthquake and, secondly, the dynamic response analyses are performed based on the data obtained from the static analyses. The emphasis is placed on evaluating the factor of safety against liquefaction at the various stages of consolidation and at the completion of consolidation. The results seem to be consistent with the overall phenomena observed in Port island and Rokko Island.
A A BALKEMA PUBLISHERS, Jul. 1997, Proc. of 14th International Conference on Soil Mechanics and Foundation Engineering (ICSMFE), Vol.2, 859 - 862, English[Refereed]
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A full-scale model test was conducted to investigate the mechanism of soil reinforcement by geosynthetic materials, A beam shaped soil structure reinforced by geosynthetics was designed and the global momentum stiffness was examined. This paper presents a brief description of the theoretical estimate of the global momentum stiffness of the reinforced structure for the laboratory model test and the numerical simulation of the full scale model test. The numerical simulation and the field test indicate that the effect of soil confinement by the geosynthetic material was particularly significant. A comparison of the numerical and field results is also presented.
A A BALKEMA, Nov. 1996, Proc. of International Symposium on Earth Reinforcement (IS-Kyushu), Vol.1, 255 - 260, English[Refereed]
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This paper describes the effect of non-coaxiality arising from material anisotropy on bifurcation phenomena such as shear band formation. The elasto-plastic model originally proposed by Sekiguchi and Ohta [7] is one of the typical models which include anisotropy and it is used to examine the effect of anisotropy on shear band formation. First, we shall extend this elasto-plastic model for infinitesimal strain to a model for finite strain and discuss the mathematical structure of this model. The stress induced anisotropy is found to bring about a vertex-like effect, such as the non-coaxiality between the Cauchy stress tensor and a plastic stretching tensor, into the constitutive relation. Then, we shall examine the effect of this non-coaxiality on bifurcation conditions in relation to the material rigidity which changes with the angle of simple shear. Finally, it will be concluded that this non-coaxiality arising from the anisotropy does not contribute much to triggering instability by localization of the deformations which result in shear band formation, while on the other hand, the non-coaxiality due to the yield vertex effect is rather inclined towards instability by localization of the deformations.
SPRINGER VERLAG, Mar. 1992, Archive of Applied Mechanics, 62, 104 - 114, English[Refereed]
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After the Great East Japan Earthquake occurred on 11 March 2011, many earthquake-induced damages of levees and reclaimed lands were reported. According to the investigation to the failure characteristic of levees summarized by Tohoku Regional Development Bureau of MLIT, damages caused by liquefaction of foundation ground would involve basal settlement and saturated condition of loosening construction materials inside levees submerged under water. This study aims to highlight that initial stress states influenced by passive arch action across a basal deflection plays a dominant role in mechanisms of weakening resistance against liquefaction and will contribute to embankments stability evaluation against earthquake.
National Committee for IUTAM, 2013, NCTAM papers, National Congress of Theoretical and Applied Mechanics, Japan, 62 (0), 114 - 114, JapaneseIntroduction other
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Statics of a sand heap under gravity associates to stress indeterminacy. The additional relationship is usually given by constitutive relations. Thus, radial stress field function satisfying plane equilibrium, constitutive relation and stress initial-boundary condition can provide a basic solution. Since stress distribution in sand heap is also depended on stress history during formation, therefore it is difficult to specify the constitutive relation which can characterize this complicated problem. The recent researches has linked arching criterion to describe a local constitutive relation and successfully captured a phenomenon of vertical pressure dip. However, a clear degree of arching effect to a stress distribution beneath the base of the heap has not been clarified. This study investigated the influence of arch shapes formed in sand heap and found particular patterns which are acceptably agreed with the experimental data. Therefore, arch formed during formation could be considered as stress memories of sand heap.
National Committee for IUTAM, 2009, NCTAM papers, National Congress of Theoretical and Applied Mechanics, Japan, 58 (0), 190 - 190, JapaneseIntroduction scientific journal
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By a consequence of surveys on salt damage areas in the northeastern part of Thailand during October, 13-20, 2005 and November 26-29, 2006, a co-organized workshop on numerical method for unsaturated soil behaviors prediction was held in Bangkok on November 27, 2006. The sites and related organizations were visited to investigate the backgrounds of salt disaster. It was found that though each organization has clear different missions, there are some missing links among organizations to solve the salt disaster which is caused by multiple factors. It is expected that the workshop will be benefit to Thai experts while promoting mutual cooperation together.
地盤工学会, 01 Mar. 2007, 土と基礎, Vol.55 (No.3,Ser.No.590), 22 - 24, JapaneseIntroduction scientific journal
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本研究では,デコルマ帯の滑り込み部表層を対象に,スメクタイトからイライトへの変質を考慮した弾塑性構成モデルを開発し,土/水連成の力学体系に組み込んだ.プレート沈み込みを想定して,せん断力を受けながらデコルマ帯が変質する場合の数値シミュレーションを行なった.変質に伴う過剰間隙水圧の発生が現れ,この過剰間隙水圧の発生は,デコルマ帯付近で観測された事実と良く整合していた.さらに,変質に伴い過圧密化することにより,せん断力を受け続けるとやがて軟化して強度が低下することが分かった.このことは,デコルマ帯で固着が生じ,ゆっくり地震が発生するメカニズムに一致する.
本研究は,塩害による沙漠化抑止と修復の技術提案を通して,地盤工学の地球環境問題への積極的な寄与を目的としている.過伐採,過放牧,塩害が沙漠化の三大要因であり,全体の約8 割を占めている.森林などの植生伐採に伴う気圏・地圏間の水循環の狂いが原因である.特に塩害による沙漠化は,地下水中の希薄な塩分が高濃度化して地表に析出することにより生じ,ユーラシア大陸などの大陸の広範囲な地域で発生している.このような塩害による沙漠化メカニズムを表現できるFE シミュレータを開発し,さらに,環境負荷の低減を可能とするPS 灰や浄水場汚泥のリサイクルによる粒状材料などを活用できる塩害抑止工法を開発した.
ユーラシア大陸地下には,広く岩塩層が堆積していることが知られる.地層褶曲などにより岩塩層が比較的地表近くまで迫出している場合には,表層地下水にまで塩分が含まれていることが多い.典型的な事例をタイ東北部(内陸)に見ることができる.乾期,雨期の乾湿繰返しに伴って地表面への塩の析出と塩害に伴う農業生産の低下を引き起こしている.一方,東南アジアの国々では,製紙業から排出されるPS(ペーパースラッジ)や浄水場発生汚泥の処理は深刻な環境問題となっている.このような汚泥を1300度程度で焼くと,微細な多孔構造を持つ安定した多孔質材とすることができる.本研究課題は,タイ東北部をフィールドに,この多孔質材料を塩害防止・対策に用い,塩害防止と農耕地保全に役立てることを目的とし, 1.水収支の狂いによる塩分析出シミュレータの高度化, 2.タイ東北部で計測された塩害,地盤変状の解析, 3. PS焼成材,浄水場排出焼成材の力学特性の把握, 4.塩害防止,農耕地保全の構造体の提案を行った.
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本研究は, 塩害に伴う地盤変状をシミュレートする飽和/不飽和土・水・移流拡散連成有限要素解析手法の開発とそれを用いた塩害軽減・修復技術の開発を目的としている. 多くの「塩害→植生枯死→砂漠化」の地盤環境変状・地盤環境崩壊プロセスは, 「森林の伐採」などによって, 降雨などの供給水量と植生からの蒸散水量と地表からの蒸発水量のバランスが崩れ, 塩害をもたらすと説明されてきた. 一旦, 地表面付近に塩分が析出すると, もはや少種の草木を除いて, 植生は育成することができず, さらなる地盤の乾燥化を招き, 砂漠化に至る. しかしながら, 世界の多くの地で問題視される「塩害」は, このような単純なプロセス(一次的塩害)では収まりきらない. 森林伐採は人類の生活圏の拡大に伴って生じる. 例えば, タイの場合, 新たな水田の確保のために森林が伐採された. 水田への農業用水の供給は, ため池などの地上水からの供給ばかりでなく, 地下水の汲み上げによっても補われている. 降雨の少ない乾燥地ほど地下水への依存度は高い. 地下水には, もともと希薄であったにせよ, 塩分が含まれている. 地下水への塩分の溶解は, その上流部もしくは地中深部に存在する塩岩層に起因していることが多い. 塩分を含んだ地下水を地表に汲み上げることにより, 乾季における蒸発によって塩分濃度が上昇し, さらなる塩害をもたらす. また灌漑水路などの建設は, 塩分含有地中水の濃度上昇をもたらし, これまた塩害を引き起こす(二次的塩害). 一方, 地下水の汲み上げは, 地盤変状をもたらし, 農地の機能損失, 道路の沈下・陥没などとして現われる. すなわち, 塩害の発生と,水路や道路の建設などの人為的力学作用との相互作用が重要となる. この目的を遂行するため,(1) 飽和・不飽和状態にある土の透水・力学特性,(2) 植生を介しての蒸散などの水循環, (3) 降雨による水供給と地表面蒸発などの水循環,(4) 地中水に含有している塩分などの移流拡散,(5) 地盤の変形・応力・浸透を考慮した数理モデルを組み立てた. この数理モデルのキャリブレーションを行うために, タイのカセサート大学の協力を得て, 平成18年11月27日に, バンコクでワークショップを開催した. さらに, タイ東北部のサコナコン, ウドンタニにて, 地表塩分渡度の現地測定を行った.さらに,(6) 本研究で開発した数値シミュレータを用いて, 塩害地盤の代表的な修復法であるフラッシングの効果を定量的に検討できる手法を開発し,(7) 塩害防止策として, 粗粒材料を用いたマルティング法およびパイリング法を考案した.
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Competitive research funding
土構造物に対する補強とは,剛な固定端で反力をとって土構造物を支えようとする機構ではない.土と補強材が一体となって剛性や耐荷力が向上しているメカニズムである.では,その力学的相互作用がどのように土構造物の剛性や耐荷力の向上に結びついているのであろうか.本研究では,土のダイレタンシー変形を補強材が拘束するメカニズムがポイントであると考えた.しかも不飽和状態におかれている土を考えねばならない.土構造物に荷重が作用する.土はせん断変形しようとするが,ダイレタンシー特性により体積膨張変形を生じようとする.この体積膨張変形を補強材が拘束することにより,土全体の剛性と強度が増し,補強効果が発現すると考えている.もしそうだとすると,土の締固め度合い(または地盤の過圧密度合い)と補強材の剛性との間に,最適な組み合わせがありうるはずである.さらに,最適な補強材の配置が見出されるはずである.本研究は,締固め土(不飽和土)を対象に,まず,1)ダイレタンシー拘束による補強効果発現メカニズムを検証し,2)ダイレタンシー特性を決定する最大締固め荷重(過圧密度合い)の推定法を提案した.これらの成果を組み込んだ数理モデル(有限要素解析手法)を開発した.これにより補強効果の定量化が可能となる.そして,3)補強材の最適な剛性,配置,量の算定を可能とし,4)最終的には,杜会ニーズに応えるべく,補強された土構造物の経年的機能変化,品質変化を予測できる手法としてまとめあげられている.
基礎地盤もしくは土構造物内に補強材を敷設して,土構造物の剛性や強度を増加せしめようとする補強土工法のアイデアは,古くから土木技術の一つの要であった.多くの土構造物は多種多様な補強土工法の恩恵の上に,機能を持続し続けている.しかし昨今の社会ニーズは,新規構造物の建設から既設構造物の維持・補修に移りつつある.過去に土構造物が受けた履歴と無関係に,その構造物のグローバルな安定に寄与する形にまとめあげられている現行設計法では対処しづらいケースが増えつつある.構造物の部分的なリニューアル,機能更新が求められるからである.本研究課題はこのような社会ニーズに合理的に答えうる技術体系の確立を目的とした.また,層状に敷設されたジオシンセティックス,基礎の底面端部に付けられたスカート,テールアルメの定着帯,盛土底部横断方法に設置された松杭連結の梯子,松杭,砂杭や摩擦杭,…これらが立派に地盤を補強して(地盤の力学特性の改良=地盤改良に寄与して)機能していることは,経験で知ってはいても理屈がついてこない.構造物のグローバルな安定を考える現行設計法の限界である.これらの「経験」に「理屈」を提供しようとしているとも言える.土構造物に対する補強とは,剛な固定端で反力をとって土構造物を支えようとする機構ではない.土と補強材が一体となって剛性や耐荷力が向上しているメカニズムである.では,その力学的相互作用がどのように土構造物の剛性や耐荷力の向上に結びついているのであろうか.本研究では,土のダイレタンシー変形を補強材が拘束するメカニズムがポイントであると考えた.土構造物に荷重が作用する.その荷重によって土はせん断変形を生じようとするが,土はダイレタンシー特性を有するため,同時に体積膨張を生じようとする.この体積膨張を補強材が拘束することにより,土の剛性と強度が増し,補強効果が発現する.これを室内試験,模型実験,数値解析により検証し,その発現メカニズムを明らかにした.このような補強効果は,土自身の材料特性ばかりでなく,その土の締固め度合い,含水状態などによって変化すると考えられる.そこで本研究では,不飽和状態にある土の力学特性,特に締固め度合いや含水状態の影響を,一連の室内三軸試験を実施して議論している.この成果は,新しい水分特性曲線モデルの提案に結びついた.さらに,土構造物の品質を評価する解析手法を開発し,実際の土構造物の品質評価への適用性を検討している.以上の成果を,順次,学会論文集や国際会議などに発表していった.
地盤材料は少なくとも土と間隙水との混合体であって,両者の力学的な相互作用の考慮が重要である.せん断帯の生成のみを追跡していたのでは地盤材料の破壊を説明することはできない.土供試体の破壊形態はせん断速度か異なる.供試体内ひずみの局所化は,土と間隙水との相互作用の発現の仕方に左右されて,せん断が進むにつれて,「はじめほとんど均質場→ひずみが局所化→局所的に体積膨張→吸水軟化→強度低下」というプロセスを経ることが予見される.供試体が正規粘土であれば当然,供試体全体での平均過剰間隙水圧は正であるが,局所化変形領域では,負の過剰間隙水圧が発生する.すなわち「ダイレタンシーの局所化」の発現である.本研究は,まず,土のダイレタンシー特産を考慮できる弾塑性構成モデルを検討し,このようなダイレタンシーの局所化メカニズムに注目しながら,土供試体のせん断破壊現象を土/水連成の有限要素法を用いてシミュレーションしている.さらに,土のダイレタンシー挙動が,実際の土構造物において,どのような力学的相互作用を引き起こしているか,ジオシンセティックスのような異種材で補強された土構造物において,どのような補強効果を担っているのかを,現場実測事例と有限要素シミュレーションによって検討している.多くの人工土構造物は,間隙が水で満たされた飽和土から成らない.間隙に水ばかりでなく空気も存在する「不飽和」状態にある.このような不飽和状態におけるダイレタンシー挙動の発現メカニズムについても検討を行っている.不飽和土内部の間隙水分布の違いによって,その挙動は大きく異なることが分かった.さらには,そのような挙動を考慮できる数理モデルの開発した.
本研究では,まず,正規圧密された粘性土供試体を軸圧縮せん断し,供試体内に発達するせん断帯の様子を観察した.次いで,有限変形理論に基づく土/水連成の弾塑性有限要素解析プログラムを開発し,平面ひずみ軸圧縮せん断のシミュレーションを行った.その結果,粘性土材料に発達するせん断帯は常に膨張を伴い,そのような局所変形が間隙水のマイグレーションを引き起こし,このマイグレーションがさらなるせん断帯の生成に支配的に影響を及ぼしていることがわかった.すなわち,間隙水のマイグレーションによって,せん断帯の生成モードが大きく変化する.また,供試体に発達するせん断帯の間隔には周期性がみられ,その周期性は境界条件によって変化することがわかった.通常の土質試験では,供試体の縦横比は2:1に取られることが多い.この場合,供試体が示すピーク強度は,供試体成型時の初期不整や間隙水のマイグレーションに支配されるせん断帯の生成モードの違いによらず,ほぼ一定の値をとることが明らかになり,現行の土質試験における規定を裏付けるものとなった.以上の成果は,数年前に他の研究グループによって先駆的に示された結果と符合しているが,有限要素解析手法の一層の精緻化をはかった結果,本研究では,より定量的で詳細な議論を可能としている.また,弾塑性速度場におけるせん断帯生成理論によれば,材料の構成関係における非共軸性がせん断帯の生成に強い影響を及ぼすことが明らかとなっている.従って,この非共軸性とダイレタンシーの関係も重要となる.本研究では,せん断に伴う体積変化(ダイレタンシー特性)の影響を考慮できるように,Hillに代表されるせん断帯生成理論の拡張をはかった.すなわち,ダイレタンシーを考慮して,せん断に伴って,せん断帯の幅の変化を許す理論を考えてみた.その結果,粘土材料の持つ非共軸性が,せん断帯の生成・発達に極めて強い影響を及ぼすことが予測された.
本研究は,すべり線の発生のみならず,端面拘束などにより供試体が経験する非一様変形を一種の分岐現象として捉え,材料の構成関係を同定しながら,供試体が発現する強度を説明しようとするものである.得られた成果を要約すると以下のようになる. 1.有限要素法による解析を行う場合,極限状態近傍でのアワグラスモードやゼロエナジ-モードの発生は,本来の解を覆い隠してしまうため深刻な問題である.4つの4節点四角形要素を選び,それぞれの要素の固有ベクトルと固有値を調べることにより,極限状態近傍での特性を把握した. 2.供試体試験自身も境界問題であれば,材料の構成関係を陽な形で求めることはできない.そこでまず,従来の理論的フレームワークで,どこまで供試体の不均質場を追跡できるかを検討した.すなわち仮想粘性法を組み込んだ土・水連成FEプログラムを開発し,平面ひずみ状態での圧縮せん断を解析した.供試体中に発生する間隙水のマイグレーションが不均質場の形成に支配的な影響を及ぼすことがわかった.しかし,マイグレーションの影響が大きすぎて,材料の硬化則を同定するまでには至らなかった. 3.供試体が経験する分岐を順次追跡するためには有限変形理論が必要であるが,2より,さらに土・水の連成であることも不可欠である.そこで,Cam-clayモデルの硬化則を仮定した土・水連成FEプログラムを開発した. 4.1軸試験のようにハダカのままで圧縮せん断でき,かつそのときの有効応力を計測できる試験装置を用いて,Ko圧密された粘土に発生するスベリ線網を計測した.スベリ線網の間隔は離散的であり,供試体の壊れ方によって異なることがわかった.すなわち供試体の強度は,壊れ方に依存した離散的な量であることが予想される.