Ž . Tunnelling and Underground Space Technology 16 2001 133150 Tunnelling in Taiwan Assessment of damage in mountain tunnels due to the Taiwan Chi-Chi Earthquake W.L. Wang a , T.T. Wang b, , J.J. Su a , C.H. Lin a , C.R. Seng a , T.H. Huang b a United Geotech Inc., Taipei, Taiwan, ROC b Department of Ci il Engineering, National Taiwan Uni ersity, 1 Section 4 Roose elt Road, Taipei, Taiwan, ROC Received 15 March 2001; received in revised form 10 August 2001; accepted 14 August 2001 Abstract Tunnels, being underground structures, have long been assumed to have the ability to sustain earthquakes 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 damage 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 factors that may have influence on the various damage patterns and the earthquake loading for tunnels. The results show that the degree of damage is associated with the geological condition and structural arrangement of the tunnel. A tunnel passing through a displaced fault zone will definitely suffer damage. The extent of geological weak zones, distance from the epicenter, and the existence of a slope face are also significant influencing factors. The seismic capacity of the tunnel is influenced by its structural arrangement, type of lining, invert setup, lining reinforcement, and other parameters. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Tunnel damage; Earthquake; Lining cracks 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 local- ized at sections with two important characteristics: those running through displaced faults, which were damaged by shear forces that developed during the Ž earthquake, and those near surface slopes especially at Corresponding author. Tel.: 886-2-2746-6777; fax: 886-2- 2364-5734. Ž . E-mail address: [email protected]T.T. Wang . . portal sections , which were damaged owing to slope Ž failures Dowding and Rozen, 1978; Yoshikawa, 1981; . Sharma and Judd, 1991; Asakura and Sato, 1996, 1998 . Most of the design codes relating to earthquake mitiga- tion 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 inflicted significant damage on many tun- nels in central Taiwan, such as cracking, spalling of concrete lining and deformation of steel reinforcement. This damage provides sufficient evidence to suggest that the effects of earthquakes on tunnels should be further studied. To study the damage influencing factors, the results 0886-779801$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. Ž . PII: S 0 8 8 6 - 7 7 9 8 01 00047-5
Transcript
Ž .Tunnelling and Underground Space Technology 16 2001 133�150
Tunnelling in Taiwan
Assessment of damage in mountain tunnels due to theTaiwan Chi-Chi Earthquake
aUnited Geotech Inc., Taipei, Taiwan, ROCbDepartment of Ci�il Engineering, National Taiwan Uni�ersity, 1 Section 4 Roose�elt Road, Taipei, Taiwan, ROC
Received 15 March 2001; received in revised form 10 August 2001; accepted 14 August 2001
Abstract
Tunnels, being underground structures, have long been assumed to have the ability to sustain earthquakes with little damage.However, investigations of mountain tunnels after the Chi-Chi Earthquake in central Taiwan revealed that many tunnels sufferedsignificant damage to various extents. This work describes the findings of a systematic assessment of damage in the mountaintunnels in Taiwan after the earthquake. It was found that among the 57 tunnels investigated 49 of them were damaged. Thedamage patterns are summarized based on the characteristics and the distribution of the lining cracks. This systematicinvestigation, involving geological conditions, design documents, construction and maintenance records of these tunnels, has beenconducted to assess the potential factors that may have influence on the various damage patterns and the earthquake loading fortunnels. The results show that the degree of damage is associated with the geological condition and structural arrangement of thetunnel. A tunnel passing through a displaced fault zone will definitely suffer damage. The extent of geological weak zones,distance from the epicenter, and the existence of a slope face are also significant influencing factors. The seismic capacity of thetunnel is influenced by its structural arrangement, type of lining, invert setup, lining reinforcement, and other parameters. � 2001Elsevier Science Ltd. All rights reserved.
Mountain tunnels, being situated deep within rocklayers, have generally been assumed to be sustainableagainst damage from earthquakes. Previous studieshave found earthquake damage in tunnels to be local-ized at sections with two important characteristics:those running through displaced faults, which weredamaged by shear forces that developed during the
Žearthquake, and those near surface slopes especially at
.portal sections , which were damaged owing to slopeŽfailures Dowding and Rozen, 1978; Yoshikawa, 1981;
.Sharma and Judd, 1991; Asakura and Sato, 1996, 1998 .Most of the design codes relating to earthquake mitiga-tion for mountain tunnels are currently designed foruse at portals and sections near slope surfaces, andseldom for other sections, including deeper mined partsand areas near intersections. Nevertheless, the Chi-ChiEarthquake inflicted significant damage on many tun-nels in central Taiwan, such as cracking, spalling ofconcrete lining and deformation of steel reinforcement.This damage provides sufficient evidence to suggestthat the effects of earthquakes on tunnels should befurther studied.
To study the damage influencing factors, the results
0886-7798�01�$ - see front matter � 2001 Elsevier Science Ltd. All rights reserved.Ž .PII: S 0 8 8 6 - 7 7 9 8 0 1 0 0 0 4 7 - 5
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150134
of investigations of 57 mountain tunnels affected by theChi-Chi Earthquake were presented in this paper. Foreach tunnel, the damage patterns are examined on thebasis of crack mapping results, and the degree ofdamage was assessed based on its functionality afterthe earthquake. Tunnel damage, geological andgeotechnical conditions and tunnel structural charac-teristics are systematically investigated to evaluate thefactors influencing tunnel damage in an earthquake.This paper also discusses the influence of seismic ef-fects on tunnel engineering.
2. Brief description of the Chi-Chi Earthquake
On 21 September 1999, at 01.47 h local time, astrong earthquake with a magnitude of 7.3 on theRichter scale occurred near the town of Chi-Chi in
Ž .central Taiwan N23.78�, E120.84� , at a depth of ap-proximately 7.5 km. The island suffered catastrophicdamage during the earthquake, with 2375 lives beinglost, over 10 000 people being injured and more than30 000 buildings collapsed.
The Chi-Chi earthquake resulted from the reactiva-tion of the Chelungpu Fault. The fault was long agoidentified as a thrust fault, running in a N�S directionfor a total length of 60 km, while dipping eastward atan angle of 30� or less. However, visually identifiedsurface ruptures following the earthquake indicatedthat the fault is more extensive. It has a total length of85 km, including newly formed branches at the north-ern tip of the fault, as shown in Fig. 1. This growth ofthe fault represents the largest known onshore thrust-faulting event of the 20th century.
Geologically, the fault is easily identifiable. The for-
Fig. 1. Intensity of Chi-Chi Earthquake felt at different locations inŽ .Taiwan after National Central University, 2001 .
mation on the east of the fault, i.e. the hanging wall,consists of late Pliocene Chinshui Shale and earlyPleistocene shaley Cholan Formation, while the west ofthe fault, the footwall, consists of gravelly late Pleis-tocene Toukershan Formation.
The Chi-Chi earthquake caused significant ground
Fig. 2. The location of tunnels investigated and the earthquake intensity on ground surface.
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Fig. 3. Tunnel locations relative to Chelungpu thrust fault.
deformation. GPS survey results indicated that an areaeast of the fault with a width of approximately 15 kmwas displaced north-westwards, with a maximum hori-
Žzontal displacement of 9.06 m 10 m horizontal dis-.placement was measured on the ground , and vertically
uplifted by approximately 9.8 m at the Shihkang Dam,near the north tip of the fault line. Fig. 1 shows the
Ž .surface rupture and peak ground acceleration PGAcontour caused by the earthquake.
3. Damage to mountain tunnels
Following the earthquake, a systematic investigationwas conducted on 57 tunnels located in central Taiwan.Firstly, quick visual inspections were performed within
a couple of days of the earthquake to gather prelimi-nary information on tunnel damage. Detailed surveyswere then performed for tunnels that were significantlydamaged, using lining crack mapping, photo recording,
Žand measuring of the major crack characteristics in-cluding width, depth and relative displacement direc-
.tion . Non-destructive inspection methods, such asŽ .ground penetration radar GPR , were also used in
several severely damaged tunnels. Fig. 2 indicates thelocations of the investigated tunnels and the intensityof the earthquake on the ground surface.
Various types of damage were observed, includinglining cracks, portal failure, concrete lining spalling,groundwater inrush, exposed and buckled reinforce-ment, displaced lining, rockfalls in unlined sections,lining collapses caused by slope failure, pavement or
Fig. 4. The numbers of tunnels suffering various types of damage.
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150 143
Table 2Ž .Tunnel damage classification chart for emergency investigation phase Huang et al., 1999
No A No damage No damage detectable by visual inspection. Normalimmediate A Slight Light damage detected on visual inspection, no operation
c cŽ .danger effects on traffic w �3 mm, l �5 m .Ž .Dangerous B Moderate Spalling, cracking of linings w�3 mm, l�5 m , Operable with
exposed reinforcement, displacement of segmental regulationsjoints, leaking of water.Some disruption to traffic.
Dangerous C Severe Slope failure at openings, collapse of main tunnel Notstructure, up heave or differential movement of operableroad and road shoulder, flooding, damagedventilation and lighting system in long tunnels.Total disruption of traffic.
a ŽClassification 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.
bClassification of a tunnel should be based on the least safe section being assessed to be conservative.c w�width of crack, l� length of crack.
bottom cracks, and sheared off lining. Table 1 lists thebasic information and damage conditions of the investi-gated tunnels.damage suffered by mountain tunnelsfollowing the earthquake.
Ž .Huang et al. 1999 suggested assessing the degree ofdamage to a tunnel based on its functionality after anearthquake. Considering the potential hazard to vehi-cles, the degree of damage can be evaluated accordingto the width and length of cracks in mined sections oftypical traffic tunnels, as presented in Table 2. Mean-while, in this paper the degree of damage to the portalsection can be accessed by the stability of the slopeabove the tunnel. For simplicity, the same damageevaluation standards are also adopted herein for waterconveyance tunnels. Among the 57 tunnels investi-gated, only 8 are classified as totally undamaged, whilethe other 49 tunnels suffered various degrees ofdamage, as summarized in Table 1. Table 3 lists thedegree of damage to tunnels in different categories.The tunnels passing through the displaced fault zonesuffered catastrophic damage, and the lining was
sheared off. Meanwhile, for the 50 tunnels in theŽ .hanging wall group, 5 tunnels 10% are classified as
Ž .undamaged, 21 tunnels 42% were lightly damaged, 11Ž . Ž .22% moderately damaged and 13 26% severely da-maged. Finally, for the 6 tunnels in the footwall and
Ž . Ž .other areas, 3 50% suffered no damage at all, 2 34%Ž .were lightly damaged and 1 16% was severely da-
maged. Evidently, the tunnels located in the hangingwall area suffered more damage than those in thefootwall area.
4. Classification of damage patterns
Numerous damage conditions were observed in theŽ .tunnels United Geotech, Inc., 1999a,b , and some of
the major patterns with significant characteristics areillustrated below.
4.1. Sheared off lining
All buildings, bridges, roads, tunnels or other struc-
Table 3Ž .Damage of mountain tunnels caused by Chi-Chi Earthquake after Wang et al., 2000
Location No. of tunnels Tunnel Damage Tunnel Damaged Damaged inassessed classification level damaged in portal mined section
Displaced 1 A Slight � � �
fault zone 1 B Moderate � � �
1 C Severe 1 � 1Hanging 50 A Slight 26 32 35
wall area 50 B Moderate 11 9 850 C Severe 13 9 7
Footwall 6 A Slight 2 3 2and other 6 B Moderate � � �
areas 6 C Severe 1 � 1
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150144
tures lying across the displaced fault zone were de-stroyed, regardless of their size and stiffness, when theChelungpu Fault line underwent shearing. The Shih-gang Dam and its water conveyance tunnel are the bestexamples of this type of failure. Investigations by Chang
Ž .and Chang 2000 revealed that this gravitational typeShihgang Dam was displaced 7.8 m vertically and 7.0 mhorizontally towards the north, destroying the dam’swater retaining function. The dam’s water conveyancetunnel was also sheared off at a point 180 m down-stream from the inlet because of the displaced fault.The tunnel was separated roughly 4 m vertically and 3m horizontally, as displayed in Fig. 5a, causing thetunnel to fail. Furthermore, severe spalling of the con-crete lining and cracks developed along the tunnel, asillustrated in Fig. 5b.
4.2. Slope failure induced tunnel collapse
When surface slopes fail during an earthquake, tun-nels can be damaged by the failure surface at sectionsnear the slope face. Fig. 6a�c illustrate two representa-tive cases from tunnels located at Sta. 42k�573 ofHighway No. 8 and the Chingshue Tunnel of HighwayNo. 149A. The damage patterns are presented Fig. 6d.
Ž .Fig. 5. Damage pattern � sheared off lining. a sheared off damageŽ .at the water conveyance tunnel of Shih-Gang Dam; b sketch of
sheared off lining damage.
Ž . Ž .Fig. 6. Damage pattern � slope failure induced tunnel collapse. a photo of Chi-Shue Tunnel before Chi-Chi Earthquake; b photo f Chi-ShueŽ . Ž .Tunnel after Chi-Chi Earthquake; c slope failure induced tunnel collapse at Sta. 45k�573 of Highway No. 8; d sketch of damage pattern.
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150 145
Ž . Ž .Fig. 7. Damage pattern � longitudinal cracks. a sketch of longitudinal cracks damage; b typical mapping result of singular crack at the vaultŽ . Ž .of the crown; c typical mapping result of symmetrical cracks; d typical mapping result of non-symmetrical cracks.
4.3. Longitudinal cracks
Longitudinal cracks in the concrete lining were de-veloped in some tunnels, and were generally extended.The crack length often exceeds the diameter of thetunnel, as illustrated in Fig. 7a. This damage patterncan be further classified into three types, singular crackat the vault of the crown, symmetrical cracks, andnon-symmetrical cracks, as shown in Fig. 7b�d. Most ofthe singular cracks and symmetrical cracks are of theopen and non-sheared types. The No. 1 San-I railwaytunnel, New Chi-Chi Tunnel on Highway No. 16 andthe headrace tunnel of New Tienlun power station arethe most representative examples of this type of da-mage.
4.4. Trans�erse cracks
Cracks in the concrete lining also developed perpen-dicular to the direction of tunnel axis, as illustrated inFig. 8. These cracks were generally observed above theroad, and were characterized by the spalling or relativedisplacement of the lining. The No. 1 San-I railwaytunnel and the No. 1 Maaling Tunnel on Highway No.8 are the most representative examples of this kind ofdamage.
4.5. Inclined cracks
Singular cracks inclined at 30�60� to the horizontaldevelop in concrete lining at one side of the tunnel and
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150146
Fig. 8. Damage pattern � transverse cracks.
generally terminating at the segmental joints, as illus-trated in Fig. 9. Damage of this type was found in theNo. 1 San-I railway tunnel.
4.6. Extended cross cracks
Inclined cracks in the concrete lining develop atvariable inclinations and run continuously, possiblycrossing segmental joints, along the concrete liningsegments and around the tunnel, as illustrated in Fig.10a. The No.1 Shuangtung Tunnel, shown in Fig. 10b,and the West Shuanglung Tunnel of Tou-6 highway aretypical examples of this kind of damage.
4.7. Pa�ement or bottom cracks
Cracking of the pavement or the bottom of thetunnel usually runs continuously over a long distance,as shown in Fig. 11a, such as in the Shuanglung Tunnelof the Tou-6 highway. More serious damage may alsooccur in the form of up heaving, such as the adit of theNo.1 San-I railway tunnel, as shown in Fig. 11b.
4.8. Wall deformation
Fig. 12a shows tunnel damage caused by significantinward deformation of the sidewalls. The deformationcaused numerous cracks in the concrete lining on theinner face of the sidewalls and collapse of the sideditch, such as in the adit of the No.1 San-I railwaytunnel shown in Fig. 11b and Fig. 12b.
Fig. 9. Damage pattern � inclined cracks.
Ž .Fig. 10. Damage pattern � extended cross cracks. a sketch ofŽ .extended cross cracks; b extended cross cracks observed in No. 1
Shuang-Tung Tunnel.
4.9. Cracks that de�elop near opening
It is very common to see cracks developed near anopening such as electronic niche, fireplug and fireextinguisher niche, refuge and so on. These cracksillustrated in Fig. 13a are usually localized and limitedto a few meters. However, as those openings becomelarge and are arranged symmetrically, the cracks canextend from both sides and join together. The collapseat the large refuges of the No. 1 San-I railway tunnel isan example, as shown in Fig. 13b.
5. Study of the possible influences related to tunneldamage
Based on the above damage patterns, possible causesof tunnel damage are investigated. For each damagedtunnel, relevant geological investigation reports, designdocuments and details on construction and mainte-nance were collected. Details of each section of the
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150 147
Ž .Fig. 11. Damage pattern � pavement or bottom cracks. a sketch ofŽ .pavement or bottom cracks; b bottom cracks observed in the adit of
No. 1 San-I railway tunnel.
damaged tunnels were systematically investigated toexamine possible factors controlling the occurrence ofa certain type of damage. Table 4 illustrates an exam-ple of such an investigation, focusing on the eightdamaged sections in the No. 1 San-I railway tunnel.
The major factors associated with increased damageinclude: tunnel being located adjacent to surface slopesor portals, tunnel running through faults, absence ofconcrete lining, unusual or unfavorable concrete liningconditions, steep sidewalls, absence of invert, etc. Thesefactors are listed in Table 5 corresponding to thedamage patterns, and can be broadly classified intothree major categories, as discussed below.
5.1. Earthquake intensity at each tunnel
The intensity of seismic force experienced by eachtunnel differs owing to their different distances fromthe displaced fault zone and the epicenter of theearthquake. The distance to the ground surface or tonearby slopes also influences the seismic effect. Seismicwaves propagate in the ground and lose energy becauseof dispersion and ground resistance, causing tunnels to
be under greater seismic forces if they are closer to thedisplaced fault zone or the epicenter. Additionally,when seismic waves reach the ground surface, theyrelease energy due to reflection or refraction, and thustunnels near the surface, and especially those nearslope faces, will absorb a greater seismic energy.
5.2. Condition of the surrounding ground
Most mountain tunnels run through very hardground, and a few tunnels pass through the displacedfault zone and fractured zones. Seismic waves propa-gate faster in hard and dense materials, and thus lessenergy will be released at places where the tunnels liein ground that is harder than the tunnel structure,meaning that such tunnels will tend to deform with theground and suffer less damage. On the other hand, ifthe tunnels lie in relatively weaker ground they willabsorb larger amounts of energy and thus suffer greaterdamage. Concrete linings can particularly be damaged
Ž .Fig. 12. Damage pattern � wall deformation. a sketch of inwardŽ .deformation of sidewall; b inward deformation of sidewalls in the
adit of No. 1 San-I railway tunnel.
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150148
Ž .Fig. 13. Damage pattern � cracks nearby the opening. a sketch ofŽ .the cracks nearby the opening; b lining collapse occurred above the
refuge of No. 1 San-I railway tunnel.
easily by ground displacement or ground squeeze wheresoft and hard grounds meet, as soft and hard groundsbehave differently during earthquakes.
Any unfavorable events such as cave-in or collapseduring tunneling would extend the plastic zone aroundthe tunnel, weaken the surrounding rock and causeexcessive vibration when seismic waves pass through. Inaddition, if the ground has previously experienced ver-tical stress from loosening, plastic stress owing tosqueezing, inclined stress or any other weakeningprocesses, tunnels in these areas will suffer greaterdamage to their concrete linings during an earthquake.
5.3. Seismic capacity of the tunnel
The seismic capacity of tunnels can be assessed bystudying the amount of damage sustained, a higherseismic capacity implies the less substantial the da-mages should be. Based on a general review of the 57tunnels investigated, the seismic capacities of tunnelsdepend on structural arrangements such as cross-sec-tions and refuge openings, the presence of linings andinverts, the presence of lining reinforcements, liningthickness, and any unusual conditions such as porous
structures, presence of cavities and serious concretedeterioration in the linings.
6. Concluding remarks
Among the 57 tunnels investigated in central Taiwan,49 tunnels suffered various degrees of damage after theChi-Chi earthquake. The most and often serious da-mages were found on the east of the Chelungpu fault
Ž .line hanging wall while damages on the footwall andother areas suffered less. The most severely damagedtunnel sections in the hanging wall are those close tosurface slopes or portal openings, while sections with athick overburden generally suffered less. Nevertheless,however badly the tunnels were damaged, they re-mained relatively unscathed when compared to surfacestructures.
The extent of damage to tunnel linings was influ-enced by the position of the tunnels in relation to faultzones, ground conditions, and closeness to the epicen-ter and surface slopes. Additionally, the presence andtype of lining and lining reinforcement, and any un-usual condition in the linings are also important influ-ence factors. It is difficult in a short time to gather thebasic data of a damaged tunnel, especially informationon ground conditions, support structure designs andunusual construction events immediately after anearthquake. It is deemed necessary to establish adatabase of basic information on existing tunnel struc-tures and damage assessment.
To prevent slope failures and fault displacementsfrom damaging tunnels, efforts should be made toavoid passing through active faults and avoid placingtunnels too close to slope faces when planning futuretunnels.
The effects of earthquakes on mountain tunnels haveseldom been investigated. Up to now, no establishedmethods can be employed for assessing and evaluatingtunnel stability during earthquakes, and design codesfor earthquake protection in tunneling are lacking. Toensure the functionality of existing tunnels and enablefuture tunnels to withstand earthquakes, further inves-tigation of the above topics is necessary.
Acknowledgements
The Highway Bureau, MOTC, the Taiwan RailwayAdministration and RSEA Engineering Corporationare appreciated for providing valuable information onthe tunnels, helping in on site investigation, and assist-ing in other various tasks.
( )W.L. Wang et al. � Tunnelling and Underground Space Technology 16 2001 133�150 149
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