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局地的集中豪雨の出水において, 河川堤防の決壊が発生し, 堤内地の甚大な被害をが報告されている. こうした被害を軽減するためには, 決壊に至るメカニズムを理解し, 水の流れと地形変化を精度よく予測できる数値モデルを構築して既存の堤防の安全性評価を行うとともに, その対策を検討することが重要である. 本研究は, 堤体内への浸透による裏法尻の破壊について再現性の高い解析手法を確立することを目的とし, 表面流と浸透流を同時に予測できる開水路流れモデルと, 砂の弾塑性構成式を基本とし, GIMP法によって離散化した土の変形モデルを連成させ, 数値モデルの構築を行った. 浸透破壊に関する水理模型実験に適用し, 裏法尻から破壊が進行していく様子を再現できることを示した., Severe damages caused by embankment failures during floods have been recently reported. For disaster mitigation, it is of great importance to understand its failure process, evaluate the emkankment stability by simulating flows and bed deformation accurately and consider countermeasures. The objectives in this study are to develop a numerical model and predict the embankment failure process at the back side toe due to seepage flows. By coupling the 3D flow model to simulate simultaneously overtopping and seepage flows, and the soil deformation model based on GIMP method, a numerical model is developed. Then, applying the model to the hydraulic experiment, it is shown that continuous embankment failure at the back side can be reasonably simulated.

本研究では，水を吸収すると膨張する吸水性粒子の間隙に浸入する自由液面流れを有限体積法により計算する手法を提案する．吸水性粒子の膨張により，水に接する粒子の固体領域が広がり，その影響を粒子表面の流速と圧力の境界条件として考慮する．この計算手法を用いて非吸水性粒子間隙の lock-exchange 問題や単一の吸水性粒子の膨張問題を計算し，高密度流体側のフロント位置や水の体積保存性について基本的な検証を行った．さらに，複数の吸水性粒子間隙に浸入する dam-break 流れに対する数値実験を行った．その結果，粒子間隙におけるフローパターンは粒子の膨張に大きく影響を受けること，また粒子の吸水速度が大きくなりすぎると粒子間隙へのフロントの進行が阻害され，領域全体の粒子吸水量が低減する可能性があることを示した．A computational method was proposed for the free-surface flows intruding between multiple absorbent particles on the basis of the finite volume method. In the proposed method, the solid area of the particle, which is in contact with water, expands due to the swelling of the absorbent particle. The influences of particle swelling and water absorption on flows are considered with velocity and pressure boundary conditions on the particle surfaces. The proposed method was first applied to the lock-exchange problem in porous media, which consist of multiple non-absorbent particles. Second, the swelling of a single absorbent particle was calculated with the present method in saturated and unsaturated conditions. The basic applicability of the proposed method was confirmed through the discussions about the front position of the heavy current in the first calculations as well as the volume conservation of water in the second investigations. In addition, the proposed method was applied to the dam-break flows in the porous media, which consist of multiple absorbent particles. As a result of the numerical experiments, it was demonstrated that the flow patterns intruding the porous media are largely affected by the swelling particles. In particular, it was found there is a possibility that the total amount of water absorption decreases due to the blocking of the flows by the swelling particles even in case the water absorption speed of the particles is sufficiently high.

東日本大震災では津波により多くの防潮堤が崩壊した．防潮堤の崩壊の原因には波力の作用とともに浸透力の作用が指摘されている．同様の崩壊は河川における堰でも河川水位の差によって発生することが報告されている．しかし，津波などの波力に加えて浸透力が作用する土構造物の安定性は，極限平衡法や支持力公式等の既存の設計手法では評価することが難しい．本研究では，地盤材料に関する剛塑性構成式を用いた剛塑性有限要素法ならびに浸透流解析によるハイブリッド解析手法を開発し，浸透力が作用する地盤の極限支持力・破壊形態を合理的に評価できることを明らかにする．, In 2011 off the Pacific coast of Tohoku Earthquake, lots of seawalls were destroyed by Tsunami. By the survey of destroyed seawalls, not only the wave pressure to dyke but also seepage force in ground have been reported to contribute to the instability of seawalls. However, it has been difficult to consider the effect of seepage force in the stability analysis of seawalls. This paper presents the hybrid stability analysis of rigid plastic finite element method and seepage finite element method in order to evaluate the total stability of seawall against tsunami. To simplify the problem, ultimate bearing capacity of caisson is investigated under the specific seepage condition by setting the sea water levels of upstream and downstream of caisson. Through case studies, the effect of seepage force on the instability of seawall is examined by handling the magnitude of seepage force. The effect of embedment of caisson on ultimate bearing capacity is also investigated. The depth of embedment works to the stabilization of seawall from the two aspects. One is the mechanical stabilization of caisson by overburden pressure and the assistance in shear strength of overburden ground. The other is the decrease in seepage force by embedment. The obtained ultimate bearing capacity shows the proposed analysis method well simulated the stability of caisson under the seepage force condition.

本研究では，均一型砂質堤防の越水模型実験を実施し，砂質堤防の越流侵食現象が再現可能な数値解析モデルの構築を図った。実験では，堤防の越流破壊現象について粒径および浸透流の進行が越流侵食に与える影響について考察を行った。数値解析では，粒径および浸透の進行によって変化するせん断抵抗の影響を考慮した堤防の越流侵食モデルの構築を図った。流れの水深積分モデル，堤体内の浸透流解析，非平衡流砂モデルの枠組みによる土砂輸送，斜面安定解析の4つを結合し同時に解析することにより越流侵食破壊現象の再現を試み，一部の検証ケースにおいて計算モデルの結果は概ね良好な結果が得られ，堤防越水時の不飽和堤土の侵食評価において，浸透水によるせん断抵抗の変化を考慮することの重要性が示された。, In this study, experiments were conducted to clarify the mechanism of embankment erosion due to overtopping flow under saturated and unsaturated sediment, and a numerical model was developed to compute the erosion of river embankment by flow overtopping. An influence of saturation and sediment size in the erosion process was discussed using the results of experiments. In the computation of erosion process of embankment, the infiltration process and shear strength due to suction on the unsaturated sediment bed were considered as a new equation. In order to simulate a phenomenon of embankment erosion, the numerical model consists of four modules, two-dimensional shallow water flow, seepage flow, sediment transport using framework of non-equilibrium model and two-dimensional slope stability. The developed model was tested for erosion of embankment. The numerical results of embankment surface erosion and moisture movement in the embankment were agreeable with the results of experiments.

埋め込み境界法や相平均モデルでは，離散化した運動方程式右辺に現われる流体と固体の相互作用項に，流体計算の時間増分 ∆t が含まれる．本報では，この相互作用項に含まれる ∆t が計算結果に与える影響を確認するため，相平均モデルを用いてダルシー則が成り立つ低レイノルズ数領域を対象とした粒子間隙流れの数値実験を行った．均一な球を規則的に配置した場合，および複数の異なる粒径を有する球をランダムに配置した場合のそれぞれについて，空間分解能と時間分解能に関する検討を行った結果，空隙に対するセル解像度 Rs を 5000～10000 程度に設定することで十分な空間分解能を設定できることを確認し，このとき，拡散数 D を 6.0 × 10⁻³ 以下に設定すれば，∆t が計算結果に与える影響を十分小さくできることがわかった．

ハイブリッド型モデルの考え方を浸透流問題に適用した新しい離散化手法として，ハイブリッド型ペナルティ法（HPM）を提案する．まず，水頭（ポテンシャル）の連続性を付帯条件として，これをペナルティにより一般的な重み付き残差法定式に導入した定式化を示す．続いて，水頭をテーラー展開し，1次項までで水頭場を表現して離散化方程式を誘導する．本手法は，水頭場を部分領域毎に独立に設定するため，任意の部分領域形状を用いることができ，複雑な地盤領域も容易に細分割することができる．本論文において定式化とともに簡単な数値計算例により，本手法による解の特性を検討する．, In this paper, new discrete method which applied the approach of the hybrid model to the seepage flow problems are developed. First, the formulation which introduced the potential continuity by penalty in general weighting residual procedure is shown. Compatibility of the potential on the sub-domain boundary edge is approximately introduced using the penalty. Next, the Taylor's series expansion is used for potential function. It is possible that this model uses the sub-domain of the arbitrary shape, because independent potential field is assumed for each sub-domain. The equivalent solution with the analytical solution when it examined the accuracy of numerical solution using this model is obtained.

線形ポテンシャル場を用いた有限体積法による地下水浸透問題の解析法を提案する．有限体積法(FVM)は，非直交格子を用いた有限差分近似として流体力学や移動現象論などの分野において広く用いられている．特に，地盤環境の問題である地下水による汚染拡散のような水以外との連成問題を解く手法として利用されている．これらの方法は，主に，水頭場を一定として取り扱っている．しかし，FVMは非Galarkin重みを有する有限要素法(FEM)の特殊なケースと考えることができるため，水頭場の次数を高めることで，より精度の高い解析が可能となる．本研究では，FEMの四角形要素と同一の水頭場を仮定し，FEMの概念を導入した定式化を試みた．さらに，本手法によって得られる解の特性を検討するため，簡単な数値計算例によりFEM解との比較検討を行った結果を示す．, A numerical method of seepage flow problems by finite volume method using the linear potential field is proposed. Finite volume method (FVM) is widely used as a finite difference approximation using the non-orthogonality grids in fields such as fluid dynamics and transport phenomenon. Especially, it is used as the technique which solves the coupled problem like the pollution diffusion by the groundwater which is a problem of the ground environment. In these methods, there are many cases in which the water head field is handled as being constant. However, FVM can be considered to be a particular case of finite elements with a non-Galarkin weighting. Therefore, the analysis accuracy is improved by raising the degree of the water head field. In this paper, water head field which is identical with the quadrilateral element of FEM is assumed, and the formulation which introduced the concept of FEM is developed. In addition, the result of the comparison examination with the FEM solution according to the simple model for examining characteristics of the solution by this method is shown.

土中を流れる浸透流が地震時の斜面崩壊にどのような影響を与えるのかを明らかにすることを目的として、浸透流を有する斜面模型に種々の地震外力を与える実験を行った。土中の加速度、間隙水圧、および斜面の変形に関するデータを計測することにより、土中の間隙水圧の変化と崩壊発生との関係を検討している。, In order to clarify the effects of the seepage flow on the slope failure due to earthquake, . Physical experiments were done by using shaking table under the several conditions of seismic force, depth of seepage water, shaking direction and so on.The relationships between pore pressure and the occurrence of slope failure are discussed form experimental data.