岩石隧道受震行為及襯砌破壞機制之研究

Abstract

臺灣自1935年新竹－臺中地震即造成鐵路隧道損害案例，1999年921集集大地震亦造成約五十座岩石隧道受震損壞，除洞口段受損外，隧道襯砌亦發生裂縫或損壞，其中以鐵路山線三義壹號隧道與石岡壩引水隧道破壞最為嚴重，分別影響西部鐵路交通運輸及中部地區民生用水。國外如日本、美國、土耳其及中國大陸岩石隧道受震破壞案例亦時有所聞。臺灣地震發生頻繁，岩石隧道總長度已逾八百公里，相較於土壤隧道已有明確可行之耐震分析與設計規範可供依循，岩石隧道不僅受震反應欠缺耐震分析規範，亦無適當考慮岩體工程特性的量化分析方法可以參考，究其原因，主要係目前有關岩石隧道受震損壞類型缺乏系統性探討，受震反應認識未臻完善所致。隧道及地下結構物相關受震課題漸受重視之際，岩石受震破壞機制與影響因素亟待建立分析模式深入探討。 本研究旨在建立適用於岩石隧道受震反應的分析模式。透過國內外震後岩石隧道破壞案例之蒐集，整理隧道受震時損壞類型、部位與型態，藉以探討岩石隧道受震損害力學機制與重要的影響因素；繼而據以建立數值分析模式，經解析解驗證模式的正確性後，應用於隧道實際破壞案例，透過破壞類型與受損部位的解釋，佐證本研究所提數值分析模式的應用性。最後應用所提數值分析模式，探討岩石隧道受震反應的地盤影響因子與工程影響因子兩大部份，以明瞭岩石隧道受震反應、可能的破壞型態及破壞機制，提供岩石隧道耐震分析與設計的參考。 國內外隧道受震破壞案例蒐集與整理得知，襯砌受震破壞型態包括：(1)襯砌遭斷層剪斷破壞、(2)隧道因邊坡坍滑而破壞、(3)襯砌縱向裂縫、(4)襯砌環向裂縫或環向施工縫錯動、(5)襯砌環狀剝落、以及(6)襯砌斜向裂縫與剝落等6種，其中第(1)項係隧道與斷層錯動帶相交引致，第(2)項係隧道疪鄰邊坡破壞引致，而第(3)至(6)項則係隧道所處位置地盤震動過大所致。 透過數值模擬途徑建立分析模式，利用有限元素法進行動態歷時分析，考慮平面應變條件下隧道受震引致的襯砌應力增量，探討岩石隧道受震反應以及襯砌破壞機制。經比較解析解進行驗證後，再應用於探討隧道受震損壞案例的損壞機制與影響因素，確認本研究所提分析模式的應用性。利用數值模擬所得受震引致襯砌應力增量最顯著位置，交叉比對隧道受震損害之裂縫空間分佈獲知：襯砌縱向裂縫主要受地震波中S波垂直或45°或高頻P波垂直入射，引致襯砌軸力與彎矩增加而影響。環向裂縫或環向施工縫錯動則受到水平向傳遞P波造成的張力破壞以及Love波造成襯砌應力增加的影響。環狀剝落與隧道承受地盤較顯著的水平向應力、S波45°入射及交叉或擴挖段引致應力集中有關，而斜向裂縫則係震波造成軟硬互層圍岩的應變差異以及襯砌結構勁度特性所致，併可由受震損害區段因震波受地表自由面反射與隧道散射效應，導致襯砌受震引致應力大幅增加現象而發生損害。 應用本研究所提分析模式探討岩石隧道受震地盤因子之影響獲知，震波引致襯砌最大正規化軸應力、剪應力及撓曲應力增量隨覆蓋深度之分佈，與輸入波之頻率或波長(λ)相關，且於正規化覆蓋H/λ=0.25時有尖峰值，研判隧道襯砌因入射波、自由面反射波及隧道散射作用互制影響而放大；同時可發現阻尼比越大，隨覆蓋越深，正規化襯砌相關最大應力值越小，或為淺覆蓋發生損壞機率較高原因之一。而依據本研究分析模式考慮不同深度、襯砌勁度、阻尼、斷面形狀等之第一尖峰值皆發生於H/λ=0.25附近，與三義壹號隧道於1999年集集地震中損壞區段覆蓋厚度與岩盤卓越頻率的組合皆在H/λ=0.25範圍相符，可解釋三義壹號隧道地質較差之岩體，受損位置發生於較淺區段，地質較佳之岩體，受損位置發生於較深區段，亦可解釋地質較佳岩體於深覆蓋發生損壞原因。此外，於相同輸入波與頻率情況下，地質越差岩體或趨近於第五類或第六類岩體類別，其波長越短，波長與隧道直徑之比值越小，襯砌相關最大應力越大，此可解釋國內外隧道案例於較差岩體較易損壞之原因。 應用本研究所提分析模式探討岩石隧道受震工程因子的影響獲知，襯砌勁度越大，震波引致最大應力增量正規化值愈大，即震波引致應力增量值愈大，因此隧道襯砌耐震設計不能全以提高勁度為主要方法。隧道襯砌與岩盤互制作用研究結果顯示，採用勻滑開挖、防水膜及襯砌與岩盤填補增加滑動性材料等，可減少襯砌受震引致軸向應力增量，達到減震效果；岩石隧道混凝土襯砌外側之再加厚襯砌與輔助工法等加勁措施，經過採用等值勁度模擬分析結果顯示，提高加勁勁度將增加襯砌軸向應力，但減少剪應力及撓曲應力；加大加勁範圍可減少襯砌軸向應力，但將增加剪應力及撓曲應力；開挖引致的鬆動區具有減少加勁勁度的效果，將減少襯砌軸向應力，但增加剪應力及撓曲應力。

Since 1935, the railway tunnels Taiwan were damage due to Hsinchu-Taichung earthquake.After the 921 earthquake in 1999, about fifty rock tunnel had been damaged. In addition to the damages at portal slopes, the tunnel lining were cracked or damaged, The most serious damage were Sanyi railway no.1 tunnel and Shek Kong dam diversion tunnel that affected railway transportation and water supply in the western part of Taiwan. According to the literature shows that there were rock tunnels damaged triggered by earthquake in Japan, the United States, Turkey and China. The earthquakes occur frequently and the total length of the tunnels stretch over eight hundred kilometers in Taiwan, while the soil tunnel has been clear and feasible for the seismic analysis and specification to follow. Nerveless, there are fewer seismic design specifications for reference. In this manner, this study review and analyze the rock tunnels damaged due to earthquake in Taiwan and overseas first. It is found the failure mechanism of rock tunnel could be separated into ground shaking, fault dislocation, ground failure (including portal slope failure). The ground shaking is studied due to complex failure mechanism compared to the other two. Besides, this study explore the failure mode, the failure mechanism and influence factors of the seismic damage of rock tunnel through the analysis of the dynamic numerical model verified by analytical solution.Considering the features of wave propagation in the surrounded rock and construction methods of the tunnel, this study explores the seismic behavior, failure mode and failure mechanism of the tunnel. Results of this study include: the comparison the characteristics of the soil tunnel and rock tunnel, the seismic damage pattern and location, failure mechanism, establishment of the dynamic numerical model, seismic behavior , failure mechanism through numerical results. The sub-subjects are divided into geological factors and engineering factors for the seismic response of tunnel. The geological factors include the depth of the tunnel, damping ratio of rock mass and rock type. Engineering factors include the tunnel cross-section shape, lining stiffness, interaction between the lining and rock, reinforcement of the surrounding rock by rock bolt and auxiliary methods, disturbance zone due to poor excavation. the seismically induced stress ofr the tunnel lining from the numerical results are, maximum axial stress, maximum shear stress and maximum flexural stress according to practical application of lining design. Finally, this study analyzes seismic damage to the tunnel by representative case ,the literature and numerical results, the failure mode of rock tunnel and its mechanism for academic and engineering applications. The rock mass and tunnel lining are assumed to be homogeneous, isotropic and elastic. Seismic wave is simulated as harmonic S wave, P wave and Rayleigh wave with the frequency within the range of predominant frequency in rock mass, 1~ 5Hz. The maximum axial stress, maximum shear stress and maximum flexural stress of the lining are normalized by maximum stress of incident wave. Tunnel depth H is normalized by wavelength λ. Analysis result of the study about geological factors shows the distribution of maximum axial stress, shear stress and flexural stress varied with depth are related to frequency or wavelength (λ) of the incident wave. The peak values of the related stresses occur when the value of H / λ is 0.25. It could be the effect of the incident wave, reflected wave from free surface and the scattering of the tunnel. The greater the damping ratio can be, the smaller maximum related stresses of the lining with depth are. This could explain the reason for the higher probability of seismic damaged in shallow depyh. Also, under the assumptions of this study, different depths, lining stiffness, damping, and cross section shape of the first peak occur at the H / λ value of 0.25. Finally, by establishing the H / λ = 0.25 relationship with Sanyi no.1 tunnel damaged due to Chi-Chi earthquake in Taiwan. This comparative case study shows the all the damage positions of rock tunnel are located within the predominant frequency 1 ~ 5Hz for the relationship of H / λ = 0.25. These indicate the reasonableness of the relationship. It could be the reason the seismic damage occurred in the shallow depth when the geology of rock mass and in the deeper depth when the geology of rock mass is good. In addition, the worse the geology of rock mass like rock type V or VI is and the shorter the wavelength is, when the same incident wave with the same frequency. When the ratio of the wavelength to the diameter of the tunnel is small, the maximum stresses of lining are larger. This could explain rock tunnels in poor geological condition were easily damaged. Analysis of engineering factors shows that the greater the lining stiffness is, the greater maximum stress of lining is. That is it is not suitable to enhance the stiffness as the main seismic design of tunnel lining. For the study about the interaction between tunnel lining and rock mass, smooth excavation face, waterproof membrane and filled materials to increase the slip behavior between the rock and lining can reduce the stresses and achieve the reduction of seismic action. This study simulates reinforcement measures of rock tunnel such as shotcrete lining and the auxiliary methods by the increase of equivalent stiffness of related rock mass. The increase of stiffness of lining will increase the maximum axial stress of the lining, but it will reduce the maximum shear stress and flexural stress. The increase of the area of the reinforced section will decrease the maximum axial stress of the lining, but it will increase the maximum shear stress and flexural stress. On the other hand, loosen area in surrounding rock due to poor excavation will reduce the reinforcement stiffness. It will decrease the maximum axial stress of the lining, but it will increase the maximum shear stress and flexural stress. Finally,the four significant damage patterns of lining are also elucidated，including the longitudinal cracks，circumferential cracks or construction joint dislocation，circularity-spalling of lining concrete，as well as oblique cracks and associated spalling of lining concrete. The seismically induced exceeding axial- and flexural stress caused by the S wave with vertical and 45˚ incident elevation angle yield to the longitudinal cracks. The tensile failure caused by horizontally propagating P wave and the stress increment caused by Love wave dominate the circumferential cracks or construction joint dislocation. The stress increment caused by the S waves with 45˚ incident elevation angle，occurred nearby opening refuges and amplified while a tunnel subjected to significantly horizontal stress results in the circularity-like spalling. The propagating S wave induced distinct strains in-between of the soft and hard rock layer lead to the oblique cracks and associated spalling. This study clarifies the seismically induced damage patterns of rock tunnels and associated affecting factors，which is conductive to seismic design for tunnels.