由山岳隧道圍岩依時變形案例探討襯砌受力特性

Abstract

隧道為關鍵交通基礎設施，其服務對現代社會的安全穩定至關重要。當今隧道工程設計理論多基於施工開挖引致擾動後，圍岩應力－應變重分布與支撐系統再平衡的假設下，隧道力學中襯砌常被視為安全儲備，僅承受自重理論上內應力甚低。惟諸多營運中隧道案例出現襯砌裂縫現象，意即其所承受應力已超出混凝土開裂強度，源於超乎預期的外部載重，如地震、地下水壓、圍岩依時變形效應，或是襯砌老劣化等因素所引致。其中，圍岩依時變形引起的襯砌裂縫具有累積性與延續性，將隨時間持續惡化削弱隧道的服務性能，繼而衝擊隧道使運輸網絡的韌性下降。因此，圍岩依時變形對隧道襯砌受力與全生命週期韌性評估的影響，亟需深入探討與重新檢視。 曾文水庫越域引水工程的西隧道，提供了此類問題的典型案例。該隧道多數區段於初期開挖與支撐系統設置後，圍岩變形迅速收斂並趨穩，然而，原訂計畫混凝土襯砌的澆置工程，卻因莫拉克颱風帶來的強降雨重創工址與鄰近環境而遭延宕。此後，西隧道部分區段陸續觀察到圍岩變形現象，甚至造成支撐構件局部破壞，該隧道開挖後1500餘日的圍岩變形監測與相關施工記錄、支撐破壞現象，提供了罕見的長期圍岩依時變形議題研究資料。 本文研析西隧道相關監測資料，歸納圍岩變形包括彈性、塑性與黏性等部分，且顯露滯動現象，為合理描述完整變形歷程，本文提出一套創新修正型柏格模型(Modified Burgers Model)，並將應力門檻引入馬克斯威爾阻尼(Maxwell Dashpot)機制，用以模擬岩體在不同階段之依時變形特性。為了滿足數值模擬與實例隧道相關性，本文依據現地地質紀錄與圍岩變形監測資料，反算所提模型所需參數，並透過數值模擬評估圍岩依時變形對襯砌應力的影響。另亦配合室內潛變試驗，觀察不同應力條件下岩體的變形曲線，據以推估合理的應力門檻範圍。模擬結果顯示相同時間下，考量依時變形特性會造成襯砌內部顯著的應力增量，以及開挖中若無施作仰拱結構，長期下襯砌底部將隨時間累積大量集中應力，使隧道路面產生無法忽視的隆起變形，對襯砌安全與耐久性具實質威脅，顯見圍岩依時特性的韌性評估中之關鍵性。 隧道規劃設計階段的地質調查與地工評估應釐清圍岩是否具依時變形特性，將其效應納入隧道支撐系統與襯砌設計考量，配合適當的施工監測確認其影響。本研究提出修正柏格模式與施工監測數據反算圍岩依時變形相關參數，進而評估襯砌長期受力的方法，則可提供現代化隧道工法遭遇具依時變形特性地盤調查、設計、施工監測以至於營運階段維護管理的參考。

Tunnels are critical transportation infrastructure, and their service integrity is essential to the safety and stability of modern society. Contemporary tunnel engineering design theories are largely based on the assumption of stress-strain redistribution in surrounding rock following excavation-induced disturbances, with the support system subsequently re-establishing equilibrium. In tunnel mechanics, the lining is often regarded as a safety reserve, theoretically bearing minimal internal stress from self-weight alone. However, numerous operational tunnels have exhibited lining cracks, indicating that the stress exerted has exceeded the concrete's tensile strength. This is often attributable to unexpected external loads such as seismic activity, groundwater pressure, time-dependent deformation of surrounding rock, or aging and deterioration of the lining. Among these, cracks induced by time-dependent deformation of surrounding rock are cumulative and progressive, continuing to worsen over time and thereby undermining the tunnel’s service performance and the resilience of the transportation network it supports. Therefore, the influence of surrounding rock time-dependent deformation on tunnel lining stress and lifecycle resilience assessment necessitates further investigation and reevaluation. The West Tunnel of the Zhengwen Reservoir Inter-basin Water Diversion Project serves as a representative case study of such issues. In most sections of the tunnel, rock deformation stabilized promptly following initial excavation and installation of the support system. However, the planned concrete lining works were delayed due to severe rainfall and site damage caused by Typhoon Morakot. Subsequently, ongoing deformation of the surrounding rock was observed in certain sections of the West Tunnel, with partial damage to support components. Over 1,500 days of post-excavation monitoring data, construction records, and support failure observations provide a rare dataset for studying long-term time-dependent deformation in surrounding rock. This study analyzes monitoring data from the West Tunnel, identifying that surrounding rock deformation comprises elastic, plastic, and viscous components, with evident retardation phenomena. To accurately describe the complete deformation process, an innovative Modified Burgers Model is proposed, incorporating a stress threshold into the Maxwell dashpot mechanism to simulate time-dependent deformation characteristics at various stages. To ensure correlation with actual tunnel conditions, model parameters were back-calculated using geological records and monitoring data. Numerical simulations were then used to evaluate the impact of time-dependent deformation on lining stress. Complementary creep tests were conducted to observe rock deformation under various stress conditions, enabling estimation of reasonable stress threshold ranges. Simulation results reveal that over time, consideration of time-dependent deformation leads to significant increases in internal lining stress. Moreover, the absence of an invert structure during excavation results in substantial stress accumulation at the lining base over the long term, leading to pronounced uplift deformation of the tunnel floor. This poses a tangible threat to the safety and durability of the lining, underscoring the critical role of surrounding rock time-dependency in resilience assessments. Geological investigations and geotechnical evaluations during the planning and design stages should identify whether surrounding rock exhibits time-dependent deformation characteristics and incorporate such effects into support and lining design. This should be supported by appropriate construction monitoring to confirm their influence. The modified Burgers model proposed in this study, along with back-analysis of monitoring data to derive relevant deformation parameters, offers a method for assessing long-term lining stress. This can serve as a reference for modern tunnel construction methods encountering time-dependent ground behavior in site investigation, design, construction monitoring, and maintenance management during operation.