Research contents / 研究内容

Shear behaviors and water retention characteristics of partially saturated soils<br>_ー 不飽和土のせん断挙動と保水特性

The state in which both water and air coexist in the voids between soil particles is referred to as unsaturated soil. A major difference from dry or saturated soils is that capillary forces, arising from the surface tension of water and the hydrophilicity of soil particles, act on liquid bridges and bulk water within the soil, thereby increasing the interparticle bonding forces. Therefore, to correctly interpret and model the macroscopic mechanical response of unsaturated soils, it is essential to understand microscopic characteristics such as changes in water retention capacity, variations in the configuration of pore water, and changes in the contribution of capillary forces. In this study, laboratory experiments combining X-ray micro-computed tomography (micro-CT) and image analysis, together with numerical simulations using the discrete element method (DEM), are employed to elucidate the mechanical behavior of unsaturated soils from a microscopic perspective.

土粒子の間隙に水と空気が混在して存在する状態を不飽和土と呼びます．乾燥土や飽和土と大きく異なる点は，水の表面張力と土粒子の親水性に起因する毛管力が，土中の液架橋やバルク水に作用し，土粒子間の結合力を高めることです．したがって，不飽和土のマクロな力学応答を正しく解釈し，モデル化するためには，保水性の変化，間隙水の存在形態の変化，毛管力の寄与の変化といったミクロな特性を把握することが重要です．本研究では，X線マイクロCTと画像解析を組み合わせた実験と，個別要素法（DEM）による数値解析を併用し，ミクロな視点から不飽和土の力学挙動の全貌解明を目指します．

Major Publications / 主要な論文

1. Kido, R. and Higo, Y.: Microscopic characteristics of partially saturated dense sand and their link to macroscopic responses under triaxial compression conditions, Acta Geotechnica, Vol.15, No.11, pp.3055-3073, 2020. https://doi.org/10.1007/s11440-020-01049-w
2. Kido, R., Higo, Y., Takamura, F., Morishita, R., Khaddour, G. and Salager, S.: Morphological transitions for pore water and pore air during drying and wetting processes in partially saturated sand, Acta Geotechnica, Vol.15, No.7, pp.1745-1761, 2020. https://doi.org/10.1007/s11440-020-00939-3
3. 木戸隆之祐，肥後陽介，高村福志：三軸圧縮下の不飽和砂の進行的なせん断帯発達過程における間隙水の微視的特性，土木学会論文集C（地圏工学），Vol.73, No.2, pp.233-247, 2017. https://doi.org/10.2208/jscejge.73.233

Bearing mechanism of closed-ended and open-ended piles
<br>_ー 閉端杭および開端杭の支持力メカニズム

For a structure to remain stable on the ground, it is essential that the pile foundations supporting it exhibit sufficient bearing capacity. The bearing capacity of a pile foundation is developed through the interaction between the ground and both the pile tip and shaft; therefore, accurately understanding this mechanism can contribute to improving the accuracy of bearing capacity estimation and rationalizing design methods. This study aims to elucidate the microscopic interactions between piles and the surrounding ground—interactions that are normally difficult to observe—such as the deformation behavior of soil at the pile tip and along the shaft, and to interpret their relationship with bearing capacity characteristics. In addition, efforts are being made to develop technologies that enhance the bearing capacity of existing piles by incorporating ground improvement techniques.

構造物が地盤上で安定するためには，それを支える杭基礎が十分な支持力を発揮することが重要です．杭基礎の支持力は，杭先端および周面との地盤の相互作用によって発揮されるため，そのメカニズムを正確に把握できれば，支持力推定の高精度化や設計法の合理化に寄与すると考えられます．本研究では，通常は観察が困難な杭と地盤のミクロな相互作用（例えば杭先端や周面における土の変形特性）を解明し，支持力特性との関係を解釈することを目指します．また，地盤改良技術を取り入れて，既存杭の支持力を向上させる技術開発にも取り組んでいます．

Major Publications / 主要な論文

1. Kido, R.*, Suezawa, R., Sawamura, Y. and Kimura, M.: Experimental investigation of bearing mechanism of closed- and open-ended piles supported by thin bearing layer using X-ray micro CT, Soils and Foundations, Vol.62, No.4, 101179, 2022.https://doi.org/10.1016/j.sandf.2022.101179
2. 末澤理希，木戸隆之祐*，澤村康生，木村　亮：杭の鉛直支持力特性と地盤の変形挙動に対する支持層厚の影響，土木学会論文集C（地圏工学），Vol.76, No.4, pp.429-440, 2020. https://doi.org/10.2208/jscejge.76.4_429

Performance evaluation of reinforced earth walls
<br>_ー 補強土壁の性能評価

Reinforced earth walls are structures that exhibit high seismic resistance through the pullout resistance of reinforcements embedded in the ground, and they have been widely constructed for applications such as expressways and underpasses. However, some existing reinforced earth walls have experienced numerous rainfall and earthquake events, and there is a possibility that they may exhibit damage in the future, such as reinforcement pullout or overturning. Therefore, it is important to evaluate the pullout resistance performance of individual reinforcements and to extend the service life of existing reinforced soil walls against combined hazards of rainfall and earthquakes. In this study, fundamental investigations are conducted to visualize and quantify the interaction between reinforcements and soil at the particle scale, together with verification of the dynamic behavior of reinforced earth walls through 1-g shaking table tests. By integrating these micro- to macro-scale approaches, this research aims to address the aforementioned challenges.

補強土壁は，地中に敷設された補強材の引抜き抵抗力により高い耐震性を発揮する構造物であり，高速道路やアンダーパスにおいて多くの施工実績があります．しかし，既設の補強土壁の中には，多数の降雨や地震の履歴を受けてきたものも存在し，将来的に補強材の引き抜けや転倒などの変状が生じる可能性があります．このため，補強材単体の引抜き抵抗性能評価をはじめ，降雨・地震の複合災害に対する既設補強土壁の長寿命化が重要です．本研究では，補強材と土の相互作用を土粒子スケールで可視化・定量化する基礎的検討と，重力場振動台実験による補強土壁の動的挙動の照査を組み合わせ，ミクロからマクロにわたる多角的アプローチによりこれらの課題解決を目指します．

Major Publications / 主要な論文

1. Kido, R.*, Sawamura, Y., Kimura, K. and Kimura, M.: Investigation of soil deformation characteristics during pullout of a ribbed reinforcement using X-ray micro CT, Soils and Foundations, Vol.61, No.3, pp.642-657, 2021. https://doi.org/10.1016/j.sandf.2021.01.013
2. 久保田篤之，木戸隆之祐，北村明洋，奥西一裕，木村　亮：風力発電事業におけるチェーン補強土壁の施工及び動態観測，地盤工学ジャーナル，Vol.17, No.4, pp.573-586, 2022. https://doi.org/10.3208/jgs.17.573
3. 玉置　尊，木戸隆之祐*：等方圧縮条件下での補強材引抜き抵抗特性に関する三次元DEM解析，地盤と建設，Vol.43, No.1, pp.53-63, 2025. https://jgschugoku.jp/asset/00032/GE/Vol43/vol43_ronbunhokoku04.pdf

Infiltration characteristics of bentonite slurry into soils
<br>_ー 土中へのベントナイト安定液浸潤特性

In the earth drill method and slurry shield tunneling method, excavation is carried out using a bentonite-based stabilizing slurry. During this process, the slurry infiltrates into the ground and forms a thin, impermeable mud cake, allowing the slurry pressure to counteract the earth pressure and thereby maintain ground stability during excavation. However, if the mud cake is not properly formed and slurry loss occurs, there is a risk of ground collapse; thus, the slurry mix must be carefully designed. Ideally, the formation of the mud cake would be predicted in accordance with the ground conditions and construction parameters at each site, enabling the optimal slurry composition to be determined. In practice, however, slurry mix design still largely relies on empirical rules derived from past construction experience. A major reason for this is the lack of a clear understanding of the conditions under which a good mud cake is formed and of the actual formation process itself. This study aims to elucidate and predict the mud cake formation mechanism through visualization of the slurry infiltration characteristics and the development of a particle–fluid coupled analysis method.

アースドリル工法や泥水式トンネル工法では，ベントナイト安定液を用いて地盤を掘削する工程があります．このとき，安定液は地山へ浸潤して薄い不透水性の泥膜を形成し，安定液圧が土圧に抵抗することで，地山を安定させながら掘削を進めることができます．しかし，泥膜が適切に形成されず逸水が生じると，地山が崩壊するおそれがあるため，安定液の配合は慎重に設定する必要があります．本来であれば，現場の地盤特性や施工条件に応じて泥膜形成を予測し，最適な配合を設計することが望ましいものの，現状では過去の施工実績に基づく経験則に頼っているのが実情です．その主な要因は，良好な泥膜が形成される条件や，その形成プロセスの実態が十分に解明されていないことにあります．本研究では，泥膜形成メカニズムの解明と予測を目的として，安定液の浸潤特性の可視化および粒子－流体連成解析法の開発に取り組んでいます

Major Publications / 主要な論文

1. Kido, R.* and Ueda, H. Infiltration characteristics of bentonite-based stabilizing fluid into pore spaces of sand investigated by X-ray micro-CT analysis. Acta Geotechnica. (2025). https://doi.org/10.1007/s11440-025-02781-x
2. 堀　温人，上田　聖，木戸隆之祐*：X線マイクロCTを用いた性状の異なる安定液浸潤試験後の土粒子間隙の可視化，材料，2025. (in press)
3. Kido, R.* and Ueda, H.: Microscale investigation of bentonite distribution in pore spaces of sand after infiltration of stabilising solution, Géotechnique Letters, Vol.15, No.2, pp.110-115, 2025. https://doi.org/10.1680/jgele.24.00053

Mitigation of re-liquefaction
<br>_ー 再液状化の抑制

During the 2011 Great East Japan Earthquake and the 2024 Noto Peninsula Earthquake, repeated seismic shaking caused multiple occurrences of liquefaction at the same locations (reliquefaction), resulting in severe damage, particularly in residential areas as well as port and coastal zones. Developing technologies to prevent such reliquefaction or mitigate damage when it occurs is an urgent challenge. This study focuses on elucidating the mechanism of reliquefaction occurrence and developing ground improvement technologies aimed at its suppression.

2011年の東日本大震災や2024年の能登半島地震では，繰り返しの地震動により同一地点で複数回の液状化（再液状化）が発生し，住宅地や港湾・沿岸部を中心に甚大な被害が生じました．このような再液状化の発生を防止し，あるいは発生時の被害を軽減するための技術開発は喫緊の課題です．本研究では，再液状化発生のメカニズム解明と，その抑制法について研究しています．

Major Publications/ 主要な論文

1. Kido, R.*, Shigemoto, S. and Hata, T.: Improvement of sand re-liquefaction resistance by microbially induced calcite precipitation under cyclic undrained triaxial loading conditions, Biogeotechnics, No.100243, 2026. https://doi.org/10.1016/j.bgtech.2026.100243