Underground facilities as an integral part of the infrastructure of modern society are used nowadays for a wide range of applications, ranging from small pipelines such as those used in natural gas transmission or water transport to large underground structures including subway and highway tunnels. The underground facilities in earthquake active areas are subjected to both static and seismic loadings during earthquake. In other words, they need to satisfy the serviceability criteria which generally deal with the performance of the structure for a longtime after its construction under the surrounding sustained load as well as the seismic criteria which would occur in a short time. Therefore, the design of underground structures has to be carried out considering these two aspects in a parallel manner. Besides, the importance and cost of this kind of structures make it necessary to analyze the behavior of underground structural systems including surrounding soil media accurately.

The past inspections of underground structures have revealed a kind of contradiction in their design concept. According to the observations made on medium-size underground structures which are constructed just few years ago, large amount of cracks and leakage can occur in the elements of this type of structures which brings about the necessity of their repair and maintenance. On the other hand, fewer cracks and damage are observed in large-size underground structures, mostly subway and highway tunnels, built several years or even decades ago. Therefore, two opposing stances exist among the engineers on the design of underground structures; one suggests that their design procedure needs to be upgraded in such a manner that stronger structures would be constructed to carry longtime sustained load from surrounding media while the other argues that the current underground structures are overdesigned and the design needs to be revised.

The attitude towards the performance of underground structures from seismic point of view has also changed by time. Historically, fewer damages to the large underground structures were reported in earthquakes compared to the surface structures. This made many structural engineers believe that underground reinforced concrete structures might not be seriously damaged during earthquakes; however, followed the several severe earthquakes in recent years, the earthquake induced damages to large underground structures and the corresponding earthquake resistance design have received considerable attention.

Therefore, the need for a general and comprehensive view towards the design of underground structures by investigating their response from different aspects can be obviously felt. This is the major objective of this study which is achieved by employing a finite element program and conducting some experiments. First of all, in order to verify the accuracy of the numerical model used in the program, different analyses were performed to simulate the behavior of soil-structure interaction in a couple of experiments which were conducted by some other researchers or the author. The results show the FEM program can effectively simulate the numerical models which involve the nonlinear behavior of soil and RC structures under both static and dynamic loads. Then, the program is used to model the strong coupling effect of highly nonlinear soil and inelastic underground RC structure during a seismic excitation. Through this study, the seismic features of this type of structures under different conditions are evaluated. Furthermore, a parametric study is performed to determine the effects of sheet piling as a widely used method to suppress the uplift of underground ducts in liquefiable soils. Finally, the long-term behavior of these structures under permanent loads was studied to investigate the serviceability and durability features of this kind of structures.

The results prove that although conventional methods of design of underground structures such as "simplified frame analysis model" or "free-field deformation method" provides good approximation for the design of underground structures in some cases, they should not be generalized to all the situations. In other words, nowadays thanks to the development of powerful tools and techniques, a numerical FEM model covering the nonlinearity of soil, RC structure and their interactions can be employed for their design as the best representation of soil-structure system. Through this study, it turns out the results of the FEM analysis sometimes could be much different from those of conventional methods. For example, when the duct is stiffer than the surrounding soil layers, the conventional methods are very conservative in the design of underground structures or when soil layers may liquefy during an earthquake, the ductility demand of underground structure based on the conventional methods is much higher than what really needed.

Liquefaction may bring about various troubles to RC underground ducts. One is the rigid-body motion like uplift and even floatation of structures. The other is the structural destruction associated with cracking and crushing of concrete and yield of reinforcing steel, which is highly related to deformation during seismic events. The simulations in this study prove that these two forms of trouble - shear deformation and the uplift - of underground structures are in relation of trade-off. Besides, it is found that if the soil is loosely deposited with high risk of liquefaction, the structural damage may be less than the case of unsaturated dry soil with the same stiffness. Finally, it should be pointed out that although installation of sheet piles as a widely0udes measure can drastically alleviate the uplift of underground RC ducts, it may cause the structure to suffer more damage due to the remained shear stiffness of the soil surrounded by the sheet piles. Hence, a rational design of sheet piling should consider both of its positive and negative effects on the underground RC structures.

The serviceability limit states needs to be paid attention for the long-term behavior of underground structures. By analyzing some numerical models, it can be concluded the design requirements of underground structures under permanent loads can be lightened by conducting more accurate analysis, considering soil-structure interaction. Furthermore, it is found the construction procedure and environmental condition are two important factors influencing the long-term behavior of underground structures and should be considered in design procedure.

Finally, it should be noted that the conventional methods widely-used for the design of underground structures in the past, have some defects in satisfying both ultimate and serviceability limit states. Also, the durability performance of a duct designed based on its elastic behavior under surrounding earth pressure lacks the sufficient accuracy and the consideration of influential factors. In the recent years, in order to reduce the constriction cost and practical problems of this kind of structures, their ultimate designs have been improved thanks to the development of powerful tools and techniques which make their analyses more reliable and available. However, it can lead to their poor durability performance which has been observed especially in small to medium-size ducts. Therefore, it is recommended that the serviceability design of underground structures needs to be elevated as well as the ultimate design by conducting more accurate and comprehensive analyses.

地下空間の有効利用は,人口と社会・経済活動が高密度に集中する都市空間の維持に不可欠である。一般に都市構造が過密化するほどに,地下空間の構築と更新,維持管理は技術的に困難を伴う。近接する社会基盤施設が多々ある中で新たな空間を求める場合には,困難な施工環境ゆえに多大なコストを要する。高密度に人間活動が展開していることから,安全性と環境負荷に対する配慮は一層重要な課題である。地下空間の利用は,放射性高廃棄物の隔離からも今日の喫緊の課題となっている。

地下空間を形成する社会基盤施設は専ら長期耐久性に優れるコンクリート構造が採用され,地震時における耐震性と長期荷重に対する耐久性が,設計における主たる限界状態に設定されてきた。これまで,地震被害の教訓などを取り入れて基準類の改定が進められてきたが,比較的深部に位置する大型地下構造では過剰に性能が担保される一方で,浅地中の小中規模の構造では,過大なひび割れや漏水などの耐久性上の問題がこれまでも指摘されてきた。これは設計システムに改良すべき点が少なくないことを示唆している。本研究は,地下鉄やダクト管路等の線状地下空間の非常時荷重に対する終局限界状態と,常時永久荷重に対する使用限界状態に関する設計システムの改良を目指し,非線形地盤-構造連成解析を中核に据えた性能照査型設計の提案を行うものである。

第1章は本論文の目的について述べ、地下構造の損傷事例や崩壊事例の分析,設計上の問題点について整理を行い,本研究の方法論をまとめたものである。

第2章は,地下構造に求める設計施工上の要求性能を明確にすべく,短期非常時荷重に対する鉄筋コンクリート地下構造の終局限界状態と,常時荷重下における使用限界状態を示している。終局限界状態は地震時の地盤変形に連動するコンクリート躯体の圧縮破壊と軸力を受けるコンクリート部材のせん断破壊の二者で照査し,使用限界状態は主としてひび割れ幅とひび割れからの漏水量で制御する論旨を,既往の知見から整理している。

第3章では,本研究で開発し,提案する地下構造性能照査の中核をなす地盤-構造連成非線形応答解析システムについて論じている。地盤を形成する土粒子骨格構成モデルには,東畑のマルチスプリングモデルの概念を採用した。これを鉄筋コンクリート構造の非線形応答解析と整合するように,有限時間差分内のひずみに対して区間経路積分法で記述し,地盤-構造システムに対する応答数値解の収束性を高めることに成功している。早期に液状化する地盤内での鉄筋コンクリート部材の降伏と損傷,鉛直支持力と地盤の崩壊過程と寸法効果,液状化が逐次広がる地盤内に打ち込まれた鉄筋コンクリート杭の損傷過程を数値解析することで,構造システムが終局限界状態に至るまで定量評価できることを検証している。一方,長期耐久性を予測するために,既往の多方向非直交ひび割れ構成モデルをひび割れ以後の時間依存挙動解析に採用し,地盤-構造連成系でのクリープおよび応力緩和解析を可能にした。

第4章では地震時の地中ダクト本体のひび割れと鋼材降伏に関する損傷制御について,広範な地盤条件と構造条件を想定した検討を展開している。軟地盤中に位置する地中鉄筋コンクリート箱型ダクトでは過大な地震時地盤せん断変形によって構造損傷が加速する一方で,地盤の液状化が進行すると,急速な地盤剛性の低下から構造損傷は軽減されることを示している。液状化のリスクを定量的に設計に取り入れることで,軟弱地盤中の地中構造の過剰に配置される鉄筋を改善できることを提示している。

第5章は,液状化による地中構造物の浮き上がりとシートパイルによる制御について論じたものである。シートパイルが地中構造物の下部における液状化した地盤の回り込みを抑制することが可能である。一方で構造物周辺の剛性を維持する結果となり,構造物本体のひび割れや鋼材降伏の増加が懸念された。数値評価の結果,シートパイル施工による躯体自体の損傷は限定的であって,浮き上がりを抑制する効果に比較して無視できる程度であることが新たに示された。

第6章では,持続荷重下における地中カルバートの長期たわみとひび割れ幅の増加を数値解析で追跡し,使用限界状態の照査方法に対して新たな提案を与えている。重交通を受ける道路床下の比較的浅い位置に設置された都市型洞道や共同溝を想定し,鉛直支持力実験を実施した。これにより,非線形応答解析の有する精度を併せて検証された。近接構造に拘束された施工環境では,地中カルバート上床版の50年程度の長期たわみは,供用初期のたわみの数倍に及ぶことが判明し,都市部の浅地中空間の現況と適合する結果が得られた。ただし,終局限界状態に対しては十分な耐荷機構が大変形領域に至っても確保されいることが,解析と実験両者から示された。

第7章で本研究の結論をまとめ、今後の課題について概括している。

本論文の眼目は,地盤-地中構造システムの終局限界状態と使用限界状態を分離することなく,ライフサイクルのなかで要求性能を位置づけ,現行設計の改善点を明らかにした点にある。そのために不可欠な技術,高度非線形領域と長期間におよぶ応答予測技術を開発した。本論文で示された限界状態と照査方法は,地下構造の設計の合理化と都市地下環境の保全技術の向上を与えるものであり,工学上の貢献が大である。よって,本論文は博士(工学)の学位請求論文として合格と認められる。