Faculty Profiles - NAKAI, Kentaro

Papers - NAKAI, Kentaro

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Seismic assessment of sheet pile reinforcement effect on river embankments constructed on a soft foundation ground including soft estuarine clay

Nakai Kentaro, Noda Toshihiro, Kato Kenta

CANADIAN GEOTECHNICAL JOURNAL Vol. 54 ( 10 ) page： 1375 - 1396 2017.10

A 3D soil-water coupled FE analysis of hollow cylinder test concerning non-uniform deformation Reviewed

Xu, B., Nakai, K., Noda, T. and Takaine, T.

Soils and Foundations Vol. 53 ( 6 ) page： 923-926 2013.12

A means for assessing the seismic stability of a temporary structure sited on soft ground and preventing its collapse Reviewed

Nakai, K. and Noda, T.

Japanese Geotechnical Journal Vol. 5 ( 3 ) page： 499-510 2010.9

Authorship：Lead author Language：Japanese Publishing type：Research paper (scientific journal)

While a temporary structure is relatively simple to erect and remove, the fact that no particular measures such as pile installations or soil improvements are taken for its protection can mean that there is a risk of it collapsing in the event of an earthquake. This paper reports a numerical analysis of the seismic stability of a temporary structure erected on a foundation of alternating clay and sand layers in a case where the surface layer is sandy and clayey. It finds that the main cause for the structure's collapse lies in 1) the increase in the amplitude of the tremors resulting from a coincidence of the predominant ground wave motion with the specific vibration of the structure, and 2) the decline in the bearing force and the consequent unevenness in the settlement of the structure as a result of liquefaction in the surface sand layer. One important prevention measure is to ensure that the specific vibration does not coincide with the predominant ground motion by making appropriate changes in the height and width of the structure; but another cheap but effective safeguard is to increase the area of contact between the structure and its foundation by laying a base immediately below the temporary structure.

Seismic response analysis of a coastal artificial reclaimed ground containing a soft layer Reviewed

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

IOP Conference Series: Materials Science and Engineering Vol. 10 page： CD, No.012107. 2010.7

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

The seismic resistance of a coastal artificial ground situated within the Port of Nagoya against the Tokai-Tonankai-Nankai triple-segment earthquake was assessed. The analysis was carried out using the elasto-plastic constitutive equation for soils (SYS-Cam clay model) mounted on the soil-water coupled finite element deformation analysis program The model focuses attention on the action of the soil skeleton structure, and the analysis allows description of the mechanical behavior of a wide range of soils without distinguishing between static and dynamic behavior. When considering earthquake damage, attention usually tends to focus only on the phenomenon of liquefaction in sandy grounds. In this paper, finite element analysis was employed to simulate the phenomenon of liquefaction of soft sand and, in addition, to show that because of disturbance of the soil during the earthquake, there is a danger of large settlement occurring over a long period after the earthquake in the soft clay layer directly beneath the area that is under the action of vertical loads.

部分的に固化改良したゆるい砂地盤の地震時／地震後応答解析 Reviewed

竹内秀克, 中井健太郎, 野田利弘

応用力学論文集 Vol. 13 2010.6

Different failure modes of a clay foundation-embankment system corresponding to different incident seismic waves Reviewed

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

ASCE Geotechnical Special Publication No.201, Soil Dynamics and Earthquake Engineering page： 125-130 2010.6

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal)

In recent years, damage to embankments from earthquakes has been frequently reported. In this paper, the deformation and failure behavior of an embankment constructed on a weak clay foundation during and after an earthquake is investigated using numerical analysis, GEOASIA, assuming a 40 g centrifugal field. The main conclusions obtained were as follows. 1) Although the embankment remained stable without large deformation during the earthquake, a slip plane subsequently penetrated through the embankment, resulting in delayed failure (circular arc slip). 2) The deformation behavior (failure mode) of the foundation and embankment varies greatly depending on the characteristics of the incident seismic motions.

Modeling and seismic response analysis of a reclaimed artificial ground Reviewed

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

ASCE Geotechnical Special Publication No.201, Soil Dynamics and Earthquake Engineering page： 294-299 2010.6

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

Compared with natural grounds formed by sedimentation over many years, reclaimed artificial grounds are often weak. In this work, modeling of a reclaimed offshore artificial ground and its seismic response analysis were carried out, and the vulnerability of the weak reclaimed layers to seismic activity is pointed out. This study was carried out using a soil-water coupled finite deformation analysis program GEOASIA incorporating an elasto-plastic constitutive model (the SYS Cam-clay model). This model is capable of describing, within a single framework, the behaviors of sands, clays, and intermediate soils consisting of sand/clay mixtures through their soil skeleton structures (structure, overconsolidation, anisotropy) and its works. In reclaimed sand, lateral flow and upward lifting of lightweight underground structures occur because of liquefaction during an earthquake. In reclaimed clay, long-term settlement, which continues for several years after the earthquake, occurs.

Seismic stability assessment of a lightweight structure on sandy/clayey foundations Reviewed

Nakai, K. and Noda, T.

Proceedings of the Fourth Japan-China Geotechnical Symposium page： 564-571 2010.4

Authorship：Lead author Language：English

For a temporary structure, which is comparatively easy to placement and withdrawal, special countermeasures such as pile penetration or ground improvement are not taken into considerations. As a consequence, when an earthquake occurs during the placement/construction, personal injury might be caused by overturning of the temporary structure. In this paper, co- and post-seismic responses of interactive systems consisting of a temporary structure - alternately layered sandy/clayey foundations were numerically investigated by using a soil-water coupled elasto-plastic finite deformation analysis. The findings are mainly as follows. 1) An unstable structure such as temporary structure become at high risk of overturning by means of liquefaction of sand layer. 2) Although it is possible to consider countermeasure against overturning by widen the breadth and lower the height of the temporary structure, it turned out that installation of a construction board is very effective.

*Structure upgradation concept applied to cyclic mobility of sand and high ductility of natural clay Reviewed

Nakai, K., Noda, T., Nakano, M. Asaoka, A. and Kawai, T.

Proceedings of 17th International Conference on Soil Mechanics and Geotechnical Engineering page： 175-178 2009.10

Authorship：Lead author Language：English

Naturally deposited sands/clays are mostly found in structured states. In addition, these are usually in overconsolidated states. Furthermore, they exhibit more or less a condition of anisotropy. In order to describe the mechanical behavior of these natural soils, the authors proposed an elasto-plastic constitutive model, namely Super/subloading yield surface Cam-clay model (E.g. Asaoka et al., 2002), in which three mechanical concepts and their evolution rules were introduced into the Cam-clay model; the first one describes `degradation of soil structure' due to ongoing plastic deformation, the second, `loss of overconsolidation', and the third, `development/loss of anisotropy'. It can be emphasized in the model that, for sand, the degradation of structure proceeds more rapidly than the loss of overconsolidation, while for clay, the loss of overconsolidation proceeds more rapidly than the degradation of soil structure. Thus, the model can describe sand and clay consistently within the same theoretical framework.
For evolution rule of soil structure, most simple one is to consider only the degradation process. But through the exhaustive observation of repeated undrained shear behavior of sand, especially pre and post cyclic mobility behavior, upgradation process of the soil structure can be truly found. So, in the present paper, the process of re-upgradeation for the soil structure due to the plastic volume expansion, i.e. the (positive) dilation, is newly introduced to the model, dividing the contribution of plastic deformation to the change of soil structure into the volumetric and deviatoric part as a sophisticated evolution rule for the soil skeleton structure. And, the importance of upgradation concept of soil skeleton structure can be showned in shear behavior of both sand and clay demonstrating their model responses
1) Sand: Cyclic mobility behavior can be described. Fig.1 shows the calculated result of repeated undrained shear behavior of medium dens

*Co-seismic and post-seismic behavior of an alternately layered sand-clay ground and embankment system accompanied by soil disturbance Reviewed

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

Soils and Foundations Vol. 49 ( 5 ) page： 739-756 2009.10

*Numerical simulation of a delayed circular slip failure after the occurrence of the earthquake Reviewed

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

Proceedings of 4th International Workshop on New Frontiers in Computational Geomechanics page： 29-32 2008.10

Authorship：Lead author Language：English

By using a soil-water coupled finite element method, deformation/failure behavior during and after the occurrence of an earthquake in a clay foundation-embankment system was numerically examined. The elasto-plastic constitutive model for the soil skeleton is the Super/subloading Yield Surface Cam-clay model. The embankment was calculated as a saturated soil. The results showed that the embankment maintained stability without any significant large deformation during the earthquake, while slip failure penetrating to the crest was suddenly generated a few hours after the earthquake.

Postseismic consolidation settlement of clayey soil due to the coseismic disturbance Reviewed

Noda, T., Takeuchi, H., Nakai, K., Nakano, M. and Asaoka, A.

Proceedings of 4th International Workshop on New Frontiers in Computational Geomechanics page： 57-60 2008.10

*Delayed failure of a clay foundation-embankment system after the occurrence of an earthquake Reviewed

(6) Noda, T., Nakai, K. and Asaoka, A.

Theoretical and applied mechanics JAPAN Vol. 57 page： 41-47 2008

Liquefaction-induced consolidation of embankment soil after earthquake Reviewed

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

Proceedings of 13th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering (13th ARC) page： 602-605 2007.12

Effects of embankment rigidity on behavior of naturally deposited soils during/after earthquakes Reviewed

Noda, T., Asaoka, A, Nakano, M. Nakai, K. and Takeuchi, H.

Proc. of International Workshop on Earthquake Hazards and Mitigations page： 23-26 2007.12

Structural re-upgradation in clay and sand accompanying plastic swelling Reviewed

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

Proceedings of 13th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering(13th ARC) page： 23-26 2007.12

Explanation of cyclic mobility of soils: approach by stress-induced anisotropy Reviewed

Zhang, F., Ye, B., Noda, T., Nakano, M. and Nakai, K.

Soils and Foundations Vol. 47 ( 4 ) page： 635-648 2007.8

High ductility of structured clay and cyclic mobility of sand Reviewed

Nakai, K., Noda, T., Nakano, M., Tashiro, M. and Asaoka, A

Proc. of 3rd International Workshop on New Frontiers in Computational Geomechanics (IWS-Xian2006) page： 183-186 2006

High ductility of structured clay and cyclic mobility of sand Reviewed

Nakai, K., Noda, T., Nakano, M., Tashiro, M. and Asaoka, A.

Proceedings of 3rd International Workshop on New Frontiers in Computational Geomechanics (IWS-XIAN 2006) page： 183-186 2006

Large compression of soil due to decay of structure during drained repeated shearing Reviewed

Masaki NakanoAkira AsaokaKentaro NakaiMutsumi Tashiro

The 16th International Conference on Soil Mechanics and Geotechnical Engineering,Osaka, Japan. page： 929-932 2005.9

Language：English

Huge volume compression of loose sand can be easily observed not under monotonic loading but under repeated shear stress application with small amplitudes, which is called compaction of sand. Although it takes long time the similar phenomenon is commonly observed even in highly structured natural clay, which has been called, for long years, secondary consolidation/delayed compression of natural clay. This study describes that these large-scaled volume compression observed both in clay and sand is due to the decay/collapse of soil skeleton structure that occurs with on going plastic shear deformation. Distinguishing the mechanical difference between sand and clay, the super/subloading elasto-plastic constitutive model still explains the mechanism of the volume compression of soils consistently.

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Papers - NAKAI, Kentaro