受逆斷層作用下加勁基礎與土堤系統之有限元素分析

Abstract

近幾十年來，全球多起重大地震所觀察到的嚴重破壞，使逆斷層作用之影響逐漸受到廣泛關注。多個逆斷層場址的現地調查顯示，相關災害具有高度破壞性，往往造成大規模基礎設施損毀，並導致重大人員傷亡(Alaska (1974), San Fernando (1971), Algeria (1980), Taiwan (1999), Turkey (1999), Wenchuan(2008))。儘管透過詳細的地質調查已能辨識斷層跡線，並劃設建議之斷層避讓帶，實務上卻極少能完全避開這些區域。橋梁、公路、鐵路及管線等重大基礎設施系統，往往難以重新配置路線；繞避活動斷層不僅需龐大的規劃工作，亦伴隨高昂的經濟成本，且未必能提供有效的替代方案。因此，當無法避免跨越斷層跡線時，採取適當的減災對策即顯得至關重要。 近年來雖已提出多種減災策略，惟地表斷層破裂行為之高度複雜性，仍迫切需要更為穩健且可靠的解決方案。其中，加勁材料因具備環境永續性，且已經證實可有效降低斷層破裂時的地表變形，進而有助於上部基礎設施之快速修復，而逐漸受到重視。本研究採用有限元素分析方法，探討在實體試驗中難以直接評估之行為特性。為此，本研究選用兩種進階組成律模式：土壤硬化模式（HS model）與NorSand 模式（NS model）。由於HS model廣泛應用於各類大地工程模擬，且具有良好之模擬表現故採用之；相較之下，NS model納入應變軟化行為，特別適用於描述地表斷層破裂所伴隨之高度非線性土壤反應。 研究初期，兩種組成律模式皆經校正，用以重現物理模型試驗結果。雖然兩者在預測剪切帶之形成與發展皆展現良好準確度，但NS model因具備軟化機制，在捕捉剪切傳播所主導之主要變形機制方面，表現更為優越。 為更全面瞭解加勁基礎與路堤系統之行為特性，本研究進一步採用 NS model進行全尺度數值模擬，以分析系統於逆斷層作用下之反應。結果顯示，蜂巢格網（geocell mattress）在斷層破裂減災方面具有顯著潛力。透過對回填土提供更佳的側向圍束，蜂巢格網可提升土壤之抗剪強度，進而攔截傳播中的剪切帶；同時，其彎曲剛度的提升有助於應力於較寬之斷層影響區域內重新分布。此一加勁機制可有效降低地表變形，並顯著減少路堤牆後之側向土壓力，最終大幅改善路堤之變形表現，確保其於大位移條件下仍具備良好穩定性。 研究結果顯示，於錯動量為S/Hf = 25% 時，配置單層蜂巢格網之基礎可使路堤牆面前傾位移量降低約12%；當改採雙層蜂巢格網時，其降幅可進一步提升至約22%。此外，結合單層蜂巢格網與提高路堤加勁材之勁度之配置展現出最佳的整體效益，在S/Hf = 25% 條件下可使最大前傾位移量降低30–40%，因此可視為本研究所獲得之最適設計策略。 研究結果亦進一步指出，蜂巢格網之加勁效果受斷層傾角影響甚鉅；隨著傾角降低，其抑制變形之效果呈現逐步衰減的趨勢，且當斷層傾角為α = 45°且錯動量達S/Hf = 25% 時，蜂巢格網之加勁效果已趨於不顯著。

The effects of reverse faulting have received increasing attention due to the severe impacts observed during major earthquakes worldwide over recent decades. Field investigations at several reverse-faulting sites (Alaska (1974), San Fernando (1971), Algeria (1980), Taiwan (1999), Turkey (1999), Wenchuan(2008)) have shown that the associated hazards can be highly destructive, leading to extensive infrastructure damage and significant loss of life. Although detailed geological surveys have enabled the identification of fault traces and the establishment of recommended fault-setback zones, completely avoiding these areas is rarely practical. Major infrastructure systems such as bridges, highways, railway lines, and pipelines often cannot be feasibly rerouted, as contouring around active faults requires substantial planning, incurs considerable economic cost, and may not yield effective alternatives. Consequently, where crossing fault traces are unavoidable, the implementation of appropriate mitigation measures becomes essential. Various mitigation strategies have been proposed in recent years such as applying isolation sheets, heavy foundations, embedded walls and geosynthetics; however, the complex nature of surface faulting continues to demand more robust and reliable solutions. Among proposed mitigations measures, the use of geosynthetic materials has attracted considerable attention due to their environmental sustainability and their demonstrated capacity to reduce surface deformation during fault-rupture events, thereby facilitating quicker repair of overlying infrastructure. The research presented herein proposes the use of geocell as mitigation measure against reverse faulting and relies on the use of finite elements analyses to investigate aspects that are difficult to assess during physical experiments. To achieve this, two advanced constitutive models were selected: the Hardening Soil model (HS) and the NorSand model (NS). The HS model was adopted because of its widespread application and proven performance in a broad range of geotechnical simulations. In contrast, the NS model, which incorporates strain-softening behavior, is particularly well-suited for representing soil response under the highly nonlinear conditions associated with surface faulting. Initially, both models were calibrated and used to reproduce physical model tests. The findings show that, while both models exhibited good accuracy in predicting the formation and evolution of the shear band observed experimentally, the NS model, due to its softening formulation, demonstrated a superior ability to capture the magnitude of deformation associated with shear propagation. To obtain a more comprehensive understanding of the embankment-foundation system behavior, full-scale simulations were conducted using the NS model to analyze the system’s response when subjected to reverse faulting. The results highlight significant potential of geocell mattress in mitigating the effects of fault rupture. By providing additional pressure confinement to the infill soil, the geocell increases its shear strength, enabling interception of the propagating shear band, additionally improving bending stiffness, promoting redistribution of stress over a wider influential zone within the overlying embankment. This reinforcing mechanism leads to reduced surface deformation and a marked decrease in lateral earth pressure behind the embankment wall, ultimately yielding substantially improved deformation performance of the embankment, ensuring the embankment’s stability under large offsets. The results indicate that wall inclination is reduced by approximately 12% for foundations reinforced with a single geocell layer at a fault offset of S/Hf = 25%. The inclusion of a double layer geocell mattress enhances the reduction to 22%. The combined use of a single geocell mattress and increased embankment reinforcement stiffness provides the greatest benefit, achieving a maximum reduction of approximately 30–40% at S/Hf = 25%, and is therefore identified as the optimum design strategy. The results further demonstrate that the effectiveness of the geocell mattress is highly dependent on the fault dip angle; its performance progressively diminishes with decreasing dip angle and becomes ineffective at S/Hf = 25% for a dip angle of α = 45°.