Assessment of Damages in Mountain Tunnels due to the. Taiwan Chi-Chi Earthquake

Transcription

1 Assessment of Damages in Mountain Tunnels due to the Taiwan Chi-Chi Earthquake W.L. Wang 1, T.T. Wang 2, J.J. Su 1, C.H. Lin 1, C.R. Seng 1, and T.H. Huang 2 1 United Geotech Inc., Taipei, Taiwan 2 Department of Civil Engineering, National Taiwan University, Taipei, Taiwan Corresponding author T.T. Wang Department of Civil Engineering National Taiwan University 1 Section 4 Roosevelt Road Taipei, Taiwan Tel: Fax: wangseeu@ms11.hinet.net 1/29

2 Abstract Tunnels, being underground structures, have long been assumed to have the ability to sustain earthquake with little damage. However, investigations of mountain tunnels after the Chi-Chi Earthquake in central Taiwan revealed that many tunnels suffered significant damage to various extents. This work describes the findings of a systematic assessment of damages in the mountain tunnels in Taiwan after the earthquake. It was found that among the 57 tunnels investigated 49 of them were damaged. The damage patterns are summarized based on the characteristics and the distribution of the lining cracks. This systematic investigation, involving geological conditions, design documents, construction and maintenance records of these tunnels, has been conducted to assess the potential factor that may have influences on the various damage patterns and the earthquake loading on tunnel engineering. The results show that the degree of damage is associated with the geological condition and structural arrangement of the tunnel. A tunnel passing through displaced fault zone will definitely suffer damage. Extent of geological weak zone, distance from the epicenter, and existence of slope face are also significant influencing factors. The seismic capacity of the tunnel is influence by its structural arrangement, type if lining, invert setup, lining reinforcement, and others. Keywords: tunnel damage, earthquake, lining cracks. 2/29

3 1. Introduction Mountain tunnels, being situated deep within rock layers, have generally been assumed to be sustainable against damage from earthquakes. Previous studies have found earthquake damage in tunnels to be localosed at sections with two important characteristics: those running through displaced faults, which were damaged by shear forces that developed during the earthquake, and those near surface slopes (especially at portal sections), which were damaged owing to slope failures (Dowding and Rozen 1978, Yoshikawa 1981, Sharma and Judd 1991, Asakura and Sato 1996 and 1998). Most of the design codes relating to earthquake mitigation for mountain tunnels are currently designed for use at portals and sections near slope surfaces, and seldom for other sections, including deeper mined parts and areas near intersections. Nevertheless, the Chi-Chi Earthquake incurred much damage on many tunnels in central Taiwan, such as cracking, spalling of concrete lining and deformation of steel reinforcement. These damages provide sufficient evidences to suggest that the effects of earthquakes on tunnels should be further studied. To study the damage influencing factors, the results of investigations of 57 mountain tunnels affected by the Chi-Chi Earthquake were presented in this paper. For each tunnel the damage patterns are examined on the basis of crack mapping results, and the degree of damage was assessed based on its functionality after the earthquake. Tunnel damage, geological and geotechnical conditions and tunnel structural characteristics are systematically investigated to evaluate the factors influencing tunnel damage in an earthquake. This paper also discusses the seismic effects on tunnel engineering. 2. Brief description of the Chi-Chi Earthquake On September 21, 1999, at 1:47 AM local time, a strong earthquake with a magnitude of 7.3 on the Richter scale occurred near the town of Chi-Chi in central Taiwan (N23.78, E120.84), at a depth of 3/29

4 approximately 7.5km. The island suffered catastrophic damage during the earthquake, with 2375 lives being lost, over 10,000 people being injured and more than 30,000 buildings collapsed. The Chi-Chi earthquake resulted from the reactivation of the Chelungpu Fault. The fault was long ago identified as a thrust fault, running in a N-S direction for a total length of 60km, while dipping eastward at an angle of 30 or less. However, visually identified surface ruptures following earthquake indicated that the fault is extensive. It has a total length of 85km, including newly formed branches at northern tip of the fault, as shown in Fig. 1. This growth of the fault represents the largest known onshore thrust-faulting event of the 20th century. Geologically, the fault is easily identifiable. The formation on the east of the fault, i.e., the hanging wall, consists of late Pliocene Chinshui Shale and early Pleistocene shaley Cholan Formation, while the west of the fault, the footwall, consists of gravelly late Pleistocene Toukershan Formation. The Chi-Chi earthquake caused significant ground deformation. GPS survey results indicated that an area east of the fault with a width of about 15km was displaced north-westwards, with a maximum horizontal displacement of 9.06m (10m horizontal displacement was measured on the ground), and vertically uplifted by around 9.8m at the Shihkang Dam, near the north tip of the fault line. Fig. 1. shows the surface rupture and peak ground acceleration (PGA) contour caused by the earthquake. 3. Damages to mountain tunnels Following the earthquake, a systematic investigation was conducted on 57 tunnels located in central Taiwan. Firstly, quick visual inspections were performed within a couple of days of the earthquake to gather preliminary information on tunnel damage. Detailed surveys were then performed for tunnels that were significantly damaged, using lining crack mapping, photo recording, and measuring of the major crack characteristics (including width, depth and relative displacement direction). Nondestructive inspection methods, such as ground penetration radar (GPR), were also used in several 4/29

5 severely damaged tunnels. Fig. 2 indicates the locations of the investigated tunnels and the intensity of the earthquake on the ground surface. Various types of damage were observed, including lining cracks, portal failure, concrete lining spalling, groundwater inrush, exposed and buckled reinforcement, displaced lining, rockfalls in unlined sections, lining collapses caused by slope failure, pavement or bottom cracks, and sheared off lining. Table 1 lists the basic information and damage conditions of the investigated tunnels. Tunnel damage differs in magnitude and location among different sites in central Taiwan. To study the factors influencing damage, the tunnels are divided into 3 categories, according to their location relative to the displaced Chelungpu Fault: passing through the displaced fault, located in the hanging wall, and located in the footwall and other areas, as shown in Fig. 3. The numbers of tunnels suffering various types of damage are plotted in Fig. 4. Lining cracks and portal failure are clearly the most commonly observed types of damage suffered by mountain tunnels following the earthquake. Huang et al. (1999) suggested assessing the degree of damage to a tunnel based on its functionality after earthquake. Considering the potential hazard to vehicles, the degree of damage can be evaluated according to the width and length of cracks in mined sections of typical traffic tunnels, as presented in Table 2. Meanwhile, in this paper the degree of damage to the portal section can be accessed by the stability of the slope above the tunnel. For simplicity, the same damage evaluation standards are also adopted herein for water conveyance tunnels. Among the 57 tunnels investigated, only 8 are classified as totally undamaged, while the other 49 tunnels suffered various degrees of damage, as summarized in Table 1. Table 3 lists the degree of damage to tunnels in different categories. The tunnels passing through the displaced fault zone suffered catastrophic damage, and the lining was sheared off. Meanwhile, for the 50 tunnels in the hanging wall group, 5 tunnels (10%) are classified as undamaged, 21 tunnels (42%) were lightly damaged, 11(22%) moderately damaged and 13(26%) severely damaged. Finally, for the 6 tunnels in the footwall and other areas, 3 (50%) suffered no damage at all, 2 (34%) were lightly damaged and 1 (16%) was severely damaged. Evidently, the tunnels located in the hanging 5/29

6 wall area suffered more damage than those in the footwall area. 4. Classification of damage patterns Numerous damage conditions were observed in the tunnels, and some of the major patterns with significant characteristics are illustrated below Sheared off lining All buildings, bridges, roads, tunnels or other structures lying across the displaced fault zone were destroyed, regardless of their size and stiffness, when the Chelungpu Fault line underwent shearing. The Shihgang Dam and its water conveyance tunnel are the best examples of this type of failure. Investigations by Chang and Chang (2000) revealed that this gravitational type Shihgang Dam was displaced 7.8m vertically and 7.0m horizontally towards the north, destroying the dam s water retaining function. The dam s water conveyance tunnel was also sheared off at a point 180m downstream from the inlet because of the displaced fault. The tunnel was separated roughly 4m vertically and 3m horizontally, as displayed in Fig. 5(a), causing the tunnel to fail. Furthermore, severe spalling of the concrete lining and cracks developed along the tunnel, as illustrated in Fig. 5(b) Slope failure induced tunnel collapse When surface slopes fail during an earthquake, tunnels can be damaged by the failure surface at sections near the slope face. Figs. 6(a) to 6(c) illustrate two representative cases from tunnel located at Sta. 42k+573 of Highway No. 8 and the Chingshue Tunnel of Highway No. 149A. The damage patterns are presented Fig. 6(d) Longitudinal cracks Longitudinal cracks in the concrete lining were developed in some tunnels, and were generally extended. The crack length often exceeds the diameter of the tunnel, as illustrated in Fig. 7(a). This 6/29

7 damage pattern can be further classified into 3 types, singular crack at the vault of the crown, symmetrical cracks, and non-symmetrical cracks, as shown in Figs. 7(b) to 7(d). Most of the singular cracks and symmetrical cracks are of the open and non-sheared types. The No. 1 San-I railway tunnel, New Chi-Chi Tunnel on Highway No. 16 and the headrace tunnel of New Tienlun power station are the most representative examples of this type of damage Transverse Cracks Cracks in the concrete lining also developed perpendicular to the direction of tunnel axis, as illustrated in Fig. 8. These cracks were generally observed above the road, and were characterized by the spalling or relative displacement of the lining. The No. 1 San-I railway tunnel and the No. 1 Maaling Tunnel on Highway No. 8 are the most representative examples of this kind of damage Inclined Cracks Singular cracks inclined at 30~60 to the horizontal develop in concrete lining at one side of the tunnel and generally terminating at the segmental joints, as illustrates in Fig. 9. Damage of this type was found in the No. 1 San-I railway tunnel Extended Cross Cracks Inclined cracks in the concrete lining develop at variable inclinations and run continuously, possibly crossing segmental joints, along the concrete lining segments and around the tunnel, as illustrated in Fig. 10(a). The No.1 Shuangtung Tunnel, shown in Fig. 10(b), and the West Shuanglung Tunnel of Tou-6 highway are typical examples of this kind of damage Pavement or bottom Cracks Cracking of the pavement or the bottom of the tunnel usually runs continuously over a long distance, as shown in Fig. 11(a), such as in the Shuanglung Tunnel of the Tou-6 highway. More serious damage may also occur in the form of up heaving, such as the adit of the No.1 San-I railway tunnel, as shown in 7/29

8 Fig. 11(b) Wall Deformation Fig. 12(a) shows tunnel damage caused by significant inward deformation of the sidewalls. The deformation caused numerous cracks in the concrete lining on the inner face of the sidewalls and collapse of the side ditch, such as in the adit of the No.1 San-I railway tunnel shown in Figs. 11(b) and 12(b) Cracks that develop near opening It is very common to see cracks developed near an opening such as electronic niche, fireplug and fire extinguisher niche, refuge and so on. These cracks illustrated in Fig. 13(a) are usually localized and limited within a few meters. However, as those openings become large and are arranged symmetrically, the cracks can extend from both sides and join together. The collapse at the large refuges of the No. 1 San-I railway tunnel is an example, as shown in Fig. 13(b). 5. Study of the possible influences related to tunnel damage Based on the above damage patterns, possible causes of tunnel damage are investigated. For each damaged tunnel, relevant geological investigation reports, design documents and details on construction and maintenance were collected. Details of each section of the damaged tunnels were systematically investigated to examine possible factors controlling the occurrence of a certain type of damage. Table 4 illustrates an example of such an investigation, focusing on the 8 damaged sections in the No. 1 San-I railway tunnel. The major factors associated with increased damage include: tunnel being located adjacent to surface slopes or portals, tunnel running through faults, absence of concrete lining, unusual or unfavorable concrete lining conditions, steep sidewalls, absence of invert, and so on. These factors are 8/29

9 listed in Table 5 corresponding to the damage patterns, and can be broadly classified into 3 major categories, as discussed below Earthquake intensity at each tunnel The intensity of seismic force experienced by each tunnel differs owing to their different distances from the displaced fault zone and the epicenter of the earthquake. The distance to the ground surface or to nearby slopes also influences the seismic effect. Seismic waves propagate in the ground and lose energy because of dispersion and ground resistance, causing tunnels to be under greater seismic forces if they are closer to the displaced fault zone or the epicenter. Additionally, when seismic waves reach the ground surface, they release energy due to reflection or refraction, and thus tunnels near the surface, and especially those near slope faces, will absorb a greater seismic energy Condition of the surrounding ground Most mountain tunnels run through very hard ground, and a few tunnels pass through the displaced fault zone and fractured zones. Seismic waves propagate faster in hard and dense materials, and thus less energy will be released at places where the tunnels lie in ground that is harder than the tunnel structure, meaning that such tunnels will tend to deform with the ground and suffer less damage. On the other hand, if the tunnels lie in relatively weaker ground they will absorb larger amounts of energy and thus suffer greater damage. Concrete linings can particularly be damaged easily by ground displacement or ground squeeze where soft and hard grounds meet, as soft and hard grounds behave differently during earthquakes. Any unfavorable events such as cave-in or collapse during tunneling would extend the plastic zone around the tunnel, weaken the surrounding rock and cause excessive vibration when seismic waves pass through. In addition, if the ground has previously experienced vertical stress from loosening, plastic stress owing to squeezing, inclined stress or any other weakening processes, tunnels in these areas will suffer greater damage to their concrete linings during an earthquake. 9/29

10 5.3. Seismic capacity of the tunnel The seismic capacity of tunnels can be assessed by studying the amount of damage sustained, a higher seismic capacity implies less substantial the damages should be. Based on a general review of the 57 tunnels investigated, the seismic capacities of tunnels depend on structural arrangements such as cross sections and refuge openings, the presence of linings and inverts, the presence of lining reinforcements, lining thickness, and any unusual conditions such as porous structures, presence of cavities and serious concrete deterioration in the linings. 6. Concluding remarks Among the 57 tunnels investigated in central Taiwan, 49 tunnels suffered various degrees of damage after the Chi-Chi earthquake. The most and often serious damages were found on the east of the Chelungpu fault line (hanging wall) while damages on the footwall and other areas suffered less. The most severely damaged tunnel sections in the hanging wall are those close to surface slopes or portal openings, while sections with a thick overburden generally suffered less. Nevertheless, however badly the tunnels were damaged, they remained relatively unscathed when compared to surface structures. The extent of damage to tunnel linings was influenced by the position of the tunnels in relation to fault zones, ground conditions, and closeness to the epicenter and surface slopes. Additionally, the presence and type of lining and lining reinforcement, and any unusual condition in the linings are also important influence factors. It is difficult in a short time to gather the basic data of damaged tunnel, especially information on ground conditions, support structure designs and unusual construction events immediately after an earthquake. It is deemed necessary to establish a database of basic information on existing tunnel structures and damage assessment. To prevent slope failures and fault displacements from damaging tunnels, efforts should be made to avoid passing through active faults and avoid placing tunnels too close to slope faces when planning 10/29

11 future tunnels. The effects of earthquakes on mountain tunnels have seldom been investigated. Up to now, no established methods can be employed for assessing and evaluating tunnel stability during earthquakes, and design codes for earthquake protection in tunneling are lacking. To ensure the functionality of existing tunnels and enable future tunnels to withstand earthquakes, further investigation of the above topics is necessary. Acknowledgment The Highway Bureau, MOTC, the Taiwan Railway Administration and RSEA Engineering Corporation are appreciated for providing valuable information on the tunnels, helping in on site investigation, and assisting in other various tasks. 11/29

12 Reference Asakura, T. and Sato, Y Damage to maintain tunnels in hazard area. Soils and Foundations, Special Issue, Asakura, T. and Sato, Y Mountain tunnels damage in the 1995 HYOGOKEN-NANBU Earthquake, Quarterly Reports of RTRI 39(1):9-16. Chang C. T. and Chang, S. Y Preliminary inspection of dam works and tunnels after Chi-Chi Earthquake. Sino-Geotechnics 77: Dowding, C. H. and Rozen, A Damage to rock tunnel from earthquake shaking. Journal of the Geotechnical Engineering Division, ASCE 104(GT2): Huang, T. H. et al Quick investigation and assessment on tunnel structures after earthquake, and the relevant reinforced methods (in Chinese). Report for the Public Construction Commission, Taipei, Taiwan. National Central University, Department of Earth Science and Institute of Geophysics, Chi-Chi Earthquake Report. Sharma, S. and Judd, W. R Underground opening damage from earthquakes. Engineering Geology 30: United Geotech, Inc Assessment and recommendation report for Emergency repair work of Tsao-Ling Tunnels at Highway No. 149A and Yun-Chia Tunnel at Tsao-Rei Highway after Chi- Chi. Design document for Taiwan Highway Administration Bureau, Taipei, Taiwan. United Geotech, Inc Investigation and assessment report on the tunnels damaged under Chi-Chi 12/29

13 Earthquake in central Taiwan (5 volumes for Highway No. 8, No. 14, No. 16, No. 149 and Tou-6 highway, respectively), Taipei, Taiwan. Wang, W. L. and Wang, T. T A Study on the Damage of San-I No.1 Railway Tunnel Impacted by Chi-Chi Earthquake. Annual Celebration Conference for the reorganization of the China Institute of Technology, , Taipei. Wang. W. L., Wang, T. T., Su, J. J., Lin, C. H., Seng, C. R. and Huang, T. H The seismic hazards and the rehabilitation of the tunnels in central Taiwan after Chi-Chi Earthquake. Sino-Geotechnics 81: Yoshikawa, K Investigation about past earthquake disasters of railway tunnels. Quarterly Reports of RTRI 22(3): /29

14 Table 1. Tunnel investigated and their damage types Basic Information Damage Description Damage classification No. Tunnel and Location Tunnelling Method a Length (m) Width (m) Lining Distance to the Epicenter (km) Distance to Che-Lung-Pu Fault (km) Portal failure Sheared off lining Slope failure induced tunnel collapse Lining cracks Concrete lining spalling Water inrush Exposed reinforcement Lining displaced Pavement cracks Rockfalls in unlining section Overall Mined section Portals 1 Shih-Gang Dam, NA Water Conveyance Tunnel TM - - Y ˇ C 2 Highway 8, 13k+381 T N ˇ A A A 3 Highway 8, 27k+710 Li-Lang Tunnel T N A A A 4 Highway 8, 34k+668 T Y ˇ ˇ B A B 5 Highway 8, 34k+775 T Y ˇ ˇ B A B 6 Highway 8, 35k+908 old Ku-Kuan Tunnel T Y ˇ ˇ ˇ B A B 7 Highway 8, 36k+908 Ku-Kuan Tunnel T Y ˇ ˇ B A B 8 Highway 8, 38k+500 No.1 old Maa-Ling Tunnel T Y ˇ ˇ ˇ ˇ C B C 9 Highway 8, 38k+500 No.1 Maa-Ling Tunnel T Y ˇ ˇ ˇ ˇ ˇ ˇ C C C 10 Highway 8, 39k+075 No.2 Maa-Ling Tunnel T Y ˇ C A C 11 Highway 8, 40k+830 No.3 Maa-Ling Tunnel T Y ˇ ˇ C A C 12 Highway 8, 41k+311 No.4 Maa-Ling Tunnel T Y A A A 13 Highway 8, 41k+311 No.4 old Maa-Ling T Y ˇ ˇ ˇ C C A 14 Highway 8, 42k+573 T N ˇ ˇ C C C 15 Highway 8, 43k+040 T N ˇ ˇ B A B 16 Highway 8, 45k+266 T Y ˇ ˇ C B C 17 Highway 14, 37k+405 Shuang-Fu Tunnel T Y ˇ A A A 18 Highway 14, 37k+981(L) Gang-Lin Tunnel T Y ˇ ˇ ˇ ˇ C A C 19 Highway 14, 38k+000(R) Gang-Lin Tunnel T Y ˇ ˇ ˇ A A A 20 Highway 14, 39k+921(L) Yu-Ler Tunnel T Y ˇ ˇ A A A 21 Highway 14, 39k+921(R) Yu-Ler Tunnel T Y ˇ ˇ A A A 22 Highway 14, 45k+182 Pei-Shan Tunnel T Y ˇ A A A 23 Highway 14, 48k+616(L) No.1 Kuan-Yin Tunnel T Y ˇ A A A 24 Highway 14, 48k+616(R) No.1 Kuan-Yin Tunnel T Y ˇ A A A 25 Highway 14, 48k+787(L) No. 2 Kuan-Yin Tunnel T Y ˇ ˇ A A A 26 Highway 14, 48k+787(R) No.2 Kuan-Yin Tunnel T Y ˇ ˇ A A A 27 Highway 14, 49k+253(L) No. 3 Kuan-Yin Tunnel T Y ˇ ˇ ˇ B A B 28 Highway 14, 49k+253(R) No. 3 Kuan-Yin Tunnel T Y ˇ ˇ A A A 29 Highway 16, Chi-Chi Tunnel T Y ˇ ˇ A A A 30 Highway 16, NA New Chi-Chi Tunnel (L) TM Y ˇ A A A 31 Highway 16, NA New Chi-Chi Tunnel (R) TM Y ˇ ˇ ˇ C C A 14/29

15 Basic Information Damage Description Damage classification No Tunnel and Location Highway 21, 54k+326(L) Da-Yuan Tunnel Highway 21, 54k+326(R) Da-Yuan Tunnel Highway 21, 66k+940(L) Shue-Sir Tunnel Highway 21, 66k+940(R) Shue-Sir Tunnel Highway 21A, 17k+303 No. 1 Huan-Hu Tunnel Highway 21A, 17k+253 No. 2 Huan-Hu Tunnel Highway 149, Tsao-Ling Tunnel Highway 149, Ching-Shue Tunnel Tou-6 highway, No. 1 Tu-Cheng Tunnel Tou-6 highway, No. 2 Tu-Cheng Tunnel Tou-6 highway, Shuang-Lung Tunnel (E) Tou-6 highway, Shuang-Lung Tunnel (W) Tou-6 highway, No. 1 Shuang-Tung Tunnel Tou-6 highway, No. 2 Shuang-Tung Tunnel Chi-Chi line railway, No. 1 tunnel Chi-Chi line railway, No. 2 tunnel Chi-Chi line railway, No. 3 tunnel Chi-Chi line railway, No. 5 tunnel Da-Kuan power station, headrace tunnel New Tien-Lun power station, headrace tunnel Mountain line railway, No. 1 San-I Tunnel Mountain line railway, No. 2 San-I Tunnel Mountain line railway, No. 3 San-I Tunnel Mountain line railway, No. 4 San-I Tunnel Old mountain line railway, No. 1 San-I Tunnel Old mountain line railway, No. 2 San-I Tunnel Tunnelling Method a NA TM NA TM NA TM NA TM Length (m) Width (m) Lining Distance to the Epicenter (km) Distance to Che-Lung-Pu Fault (km) Portal failure Sheared off lining Slope failure induced tunnel collapse Lining cracks Concrete lining spalling Water inrush Exposed reinforcement Lining displaced Pavement cracks Rockfalls in unlining section Overall Mined section Portals Y ˇ A A A Y ˇ A A A Y ˇ A A A Y A A A T Y A A A T Y A A A NA TM Y ˇ ˇ ˇ ˇ C B C T N ˇ ˇ ˇ C C C T Y ˇ ˇ A A A T Y ˇ ˇ B B A T Y ˇ ˇ B B A T Y ˇ A A A T Y ˇ ˇ ˇ B B B T Y ˇ ˇ A A A T Y ˇ A A A T Y <1 11 ˇ A A A T Y ˇ B B B T Y ˇ B B B T Y ˇ ˇ C C A NA TM Y ˇ ˇ ˇ C C A NA TM Y ˇ ˇ ˇ ˇ C C A NA TM Y A A A NA TM Y A A A NA TM Y A A A T Y ˇ ˇ A A A T Y ˇ ˇ A A A Note: a. Tunnelling method: T means traditional tunnelling method, NATM means New Austria Tunnelling Method. b. Damage classification refers to Table 2 and texts for details. 15/29

16 Table 2. Tunnel damage classification chart for emergency investigation phase (Huang et al. 1999) Tunnel classification a,b Damage level Damage description Traffic strategy No immediate danger A No damage No damages detectable by visual inspection. Light damages detected on visual inspection, no effects on traffic Slight (w c <3mm, l c <5m). Normal operation B Moderate Spalling, cracking of linings (w>3mm, l>5m), exposed reinforcement, displacement of segmental joints, leaking of water. Operable with Some disruption to traffic. regulations Dangerous Slope failure at openings, collapse of main tunnel structure, up C Severe heave or differential movement of road and road shoulder, flooding, damaged ventilation and lighting system in long tunnels. Not operable Total disruption of traffic. Note: a. Classification of a tunnel is based on its functionality (Traffic condition for road tunnels, ability to withstand water pressure for water conveyance tunnels) and extent of damage in the tunnel. b. Classification of a tunnel should be based on the least safe section being assessed to be conservative. c. W means width of crack, L means length of crack 16/29

17 Table 3. Damages of mountain tunnels caused by Chi-Chi Earthquake (after Wang et al. 2000) Location No. Of tunnels assessed Tunnel Tunnel classification Damage level damaged Damaged in portal Damaged in mined section Displaced fault zone 1 A Slight B Moderate C Severe 1-1 Hanging wall area 50 A Slight B Moderate C Severe Footwall and other areas 6 A Slight B Moderate C Severe /29

18 Table 4. Assessment example on damages influencing factors, No. 1 San-I railway tunnel (after Wang and Wang 2000) Secti on Location Dama ge Overburde n (m) Adverse geological Rock mass classificatio Constructio n hazard Auxiliary method Convergenc e (%) Support stress Opening c Concrete condition type a condition n (RMR) A Approx. 161k+300 4,9 45 San-I Fault zone (19) Cement grouting (5 times) 4.1 Large refuge Local cavern existing B Approx. 4,9 35 San-I (14) Cave-in Cement grouting 0.3 b Small Good 161k+360 Fault zone and collapse refuge C 161k+375 ~410 3,5,9 25~35 San-I Fault zone (27~30) 0.3 b 8 tons in bolts Small refuge Cavern existing D Approx. 164k+740 3,9 120 (43) 1.5 Small refuge Local cavern existing E 164k+758 3,5 125 (46~67) 1.5~ MPa Good ~810 in shotcrete F 164k+842 ~ ~150 (19) Cement grouting 2.3 Cavern existing G 165k+600 ~660 3,4,5 105~110 (35~37) Squeezing and support Forepoling, cement grouting, ring excavation 3.6~5.5 Cavern existing damaged and re-supporting H Approx. 165k+800 3,9 125 Fractured zone (28) 2.5 Small refuge Cavern existing Notes:a. Damage types: 1) sheared off lining, 2) slope failure induced tunnel collapse, 3) longitudinal cracks, 4) transverse cracks, 5) inclined cracks, 6) extended cross cracks, 7) pavement or bottom cracks, 8) wall deformation, 9) cracks nearby the opening. b. Section B and C were located close to breakthrough point Sta. 161k+385, the monitoring results might not reflect the actual tunnel behavior. c. The large refuge is 4.5m in width and 3.5m in height, the small one is 2.0m in width and in height, and the excavation size of tunnel is 11m in width and in height. 18/29

19 Table 5. Summary of factors influencing tunnel damage Possible Factors Passing through displaced fault zones Damage Type a Passing through faults, fractured zone and unfavorable ground condition Interface of hard and soft ground Nearby slope surface or portal Unfavorable events such as cave-in, collapse occurred during construction Anomaly such as lining cracks occurred before quake Badly structural arrangements b Absence of lining Plane concrete lining Steep sidewalls, absence of invert, etc. Deteriorated lining material Cavity existed behind lining Notes:a. Damage types refer to Table 4. b. Arrangements such as opening, refuge, and so on, especially symmetrically located in both sidewalls. c. presents significant influence, presents moderate influence. 19/29

20 Figure 1. Intensity of Chi-Chi earthquake felt at different locations in Taiwan (after National Central University, 2001) Figure 2. The location of tunnels investigated and the earthquake intensity on ground surface 20/29

21 Figure 3. Tunnels location relative to Chelungpu thrust fault Figure 4. The numbers of tunnels suffering various types of damage 21/29

22 Figure 5. Damage pattern - sheared off lining. (a) sheared off damage at the water conveyance tunnel of Shih-Gang Dam; (b) sketch of sheared off lining damage. Figure 6. Damage pattern slope failure induced tunnel collapse. (a) photo of Chi-Shu Tunnel before Chi-Chi Earthquake; (b) photo of Chi-Shu Tunnel after Chi-Chi Earthquake; (c) slope failure induced tunnel collapse at Sta. 45k+573 of Highway No.8; (d) sketch of damage pattern. 22/29

23 Figure 7. Damage pattern longitudinal cracks. (a) sketch of longitudinal cracks damage; (b) typical mapping result of singular crack at the vault of the crown; (c) typical mapping result of symmetrical cracks; (d) typicla mapping result of non-symmetrical cracks. 23/29

24 Figure 8. Damage pattern transverse cracks Figure 9. Damage pattern inclined cracks 24/29

25 Figure 10. Damage pattern extended cross cracks. (a) sketch of extended cross cracks; (b) extended cross cracks observed in No.1 Shuang-Tung Tunnel. 25/29

26 Figure 11. Damage pattern pavement or bottom cracks. (a) sketch of pavement of bottom cracks; (b) bottom cracks observed in the adit of No1. San-I railway tunnel. 26/29

27 Figure 12. Damage pattern wall deformation. (a) sketch of inward deformation of sidewall; (b) inward deformation of sidewall in the adit of No.1 San-I railway tunnel. 27/29

28 Figure 13. Damage pattern cracks nearby the opening. (a) sketch of the cracks nearby the opening; (b) lining collapse occurred above the refuge of No.1 San-I railway tunnel. 28/29