DAMAGES OF URBAN TtJNNELS DUE TO THE SOUTHERN HYOGO EARTHQtJAKEoF JANtJARY 17, 1995 AND THE EVALUATION OF SEISMIC ISOLATION EFFECT
TAKEYASU SUZUKI

TechnicalResearch and Development Institute, Kumagai GumiCo･, Ltd,
1043 0nigakubo, Tsukuba‑shi, 300‑22 IBARAKI, JAPANABSTRACTA subway station was collapsed due to the southern Hyogo earthquake of January 17, 1995･ This paper
presents a typical damage of a subway tunnel due to this quake and introduces two different seismic isolation
methods effective to highly improve seismic safety of underground structures･ Earthquake response
simulations are carried out, modeling the most severely damaged subway station, in order to detect the major

cause of the damage･ The simulation indicates that the damage would be mainly orlglnated from the horizontal

earthquake motion･ Earthquake response analysesare also carried out, applying two seismic isolation methods
to the subway station model･ Then, the effectiveness and the validity of these methodsare exhibited･

KEYWORD S

Earthquake damage; Urbantunnels; Numerical simulation; Seismic isolation method; Effect evaluation

INTRODU CTION

In the momlng Of January 17, 1995, a terrible earthquake motion hit the Hanshin area of J叩an, Centerlng at

Robe clty･ One of the subway stations in Robe clty COmpletely collapsed and several kilometers of the general
tunnel section and several other subway stations were more or less damaged by this earthquake･ Even though

the underground sewer tunnel in Mexico city was damaged due to the Michoacan earthquake of 1985 (Tamura
et al., 1986), it was not a destructive one. The collapse of the subway station in Kobe city was the first severe
damage that subways have ever experienced.

It is true that underground structures are safer than ground facilities. This damage indicates, however, that a
destructive case may possibly occur when an earthquake ground motion stronger than we have never estimatedstrikes a lifeline. The safety of important underground lifelines which must fulfil its function in emergency

should be maintained even if a large earthquake occurred.
The author have proposed the seismic isolation system f♭r urban tunnels (Suzuki, 1990), in which the tunnel

structure is covered by a thin soft isolation layer･ Several functions of the seismic isolation layer are clarified
through numericalanalyses as well as verification tests uslng a Shaking table･ In this paper, another method forseismic isolation f♭r underground tunnels is presented, in which a seismic isolation mbber with no damplng lS

fixed between a ceiling slaband a center column for a tunnel with a rectangular cross‑section.

In this paper, a typical damage of subway stations due to the southern Hyogo earthquake of January 17, 1995

are demonstrated first. Then, the main cause of the damage is deduced by numericalsimulations, modeling thedamaged station･ Next, the two seismic isolation methods are applied to the tunnel model and numericalsimulationsare carried out. Then, the effectiveness and necesslty Of the application of seismic isolation
methods to underground structuresare discussed.

DAMAGES OF SUBWAY TUNNELS
Subways in Kobe cityand generals of damagesinsubway tunnels
There are two subway lines in Robe city. One is the Kobe city subwayruns in the northernpart of the city･Another is the Kobe express railway whichruns in the southem part. They cross each other at Nagata･ The

former was constructed around 1980, the latter was in 1960S. Both of the subway lines were constructed bymeans of the open‑cut‑methodand the shapeof a tunnel crossISeCtion is rectangular with columns atthe centerof a tummel. Fig. I illustrates main transportation lines in Kobe city1nCluding subway lines.
Earthquake damages occurred in both subway lines, Coverlng a few kilometers in length. One of the typicaldamages is the shearcollapse of center columns, especially at station platforms with wide width. Collapses ofcenter columnsarealso seen in the generaltunnel section, where the width of the tunnel is shorterthanthat at
stations. Longitudinal cracks were seen on side walls and transverse cracks are seen both on afloor slab andon a ceiling slab as well as on side walls in generaltunnel section. The severest damage occurred atthe Daikai
station is briefly surrmarizedinthe followlng Paragraph.

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Damages occurred at a subway station

The severest damage ∝curred at the Daikai station, the location of which is polnted out in Fig･1･ At thisstation, over 14 center columns were completely collapsed and the ceiling slab bent down to the floor slab. On
the ground su血ce right over the collapsed portion, a large subsidence with almost 80 m in length, 20 m inwidth,and 1 m in depth at maximum, took place as shown in Photo･1.
Fig･2 illustrates仙e schematic representation of the damage ∝cu汀ed at the platfbm The collapse of center

columnsare mainly classified into the followlng three patternS･ Photo･2 shows the first collapse pattem, i･e･,
complete collapse･ In this pattem, columns with a cross‑section of 1 m by O･4 m are completely collapsedand
main reinfわrcement bars are bent out to whichever northem or southem side. Photo.3 shows the second

pattem in which collapse due to shear force took place at the bottom or the top of colurrms･ Photo･4 showsthe
third pattern, on the contrary, in which columns are buckled at the bottom or the top subjected to the axial
force･ Judging from the distribution of each patternof collapse illustrated in Fig･2, the main cause of thedamage can be estimated as fbuows:Photo.

Subsidence of the pavementoccu汀ed over the subway station

Photo.2 Complete collapse of center columns(the first pattem of collapse)

Photo.3 Shear collapse of center columns(the second pattem of collapse)(the third pattem of collapse)

In the first step, a large relative displacement is provoked due tothe strong horizontal ground motion and shear
cracks of center columns take place at the bottom and the top of columns. Cracks due to the bending moment
are developed on side walls as well orlglnated血.om the shear defわrmation of the tunnel cross‑section, which

accelerates the collapse of center columns. The second collapse pattern appears in this step. Generally

speaking, center colurrms support the heavy earth pressure by their axial force, the axial force of which is muchhigher than that supported by side walls in the tunnel cross‑section with a large widthlikethis station. Due to alack of support by center columns, the downward deformation of the ceiling slab is accelerated and the cracks
take place there. The soil coverlng the tunnel also defbms in company with the slab defbmation. Then, thetensile crackpenetratlng from the tunnel to the ground surface developed, which can be clearly seen in Photo. 1.
The ceiling slab collapses and drops down to the floor slab, crashing center columns as shown in Photo.2.
Large axial force having been supported by a sound center column shifted to the neighboring column
successively, which forces the column being buckled in a form classified into the third collapse patternrepresented by Photo.4.

NUMERICAL SIMULATIONS OF THE DAMAGED SUBWAY STATION
Earthquake response analyses of the damaged subway tunnel were conducted, in order to detect the main causeof the damage･ In the analyses, the cross‑section of the subway station and su血ce soil deposits were modeledroughly uslng 21dimensional linearfinite elements･ The profile and dynamic properties of soil deposits used in
the analysesare shown in Fig･3 as well as the location of the subway station･ They are glVen from the logglng

data conducted near the subway station (Geological Survey of Japan, 1983). The soil is replaced byquadrilateral or triangular plane elements and the tunnel is replaced by beam elements with unit depth,
respectively･ The dimensions in thickness for a ceiling slab, afloor slab and side walls are 0.8 m, 0.85 m andO･7 m, respectively･ Since RC‑center columns with a crossISeCtion of I m by O･4 mare arranged in an intervalof 3･5 m in the tunnel axial direction as shown in Fig･2, the stiffness of beam elements for a center column isdete‑ined so as to replace the column into a successive thn wall with a cross‑sectional area equlValent to thecolumn･ The input motions for the analyses both in the horizontal and vertical directions were glVen from l‑dimensional earthquake response analyses based on the multiple reflection theory, by the deconvolution to thebase rock at the site from the recorded motion at the JMA Kobe station which is located on the outcrop nearthesite･ Fig･4 shows the time histories of the input motion･ The maximum accelerations of the input motions
were 667 gals and 301 gals in the hodzontal and the vertical directions, respectively･(kgTfcm2) 蛭2

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Three cases of earthquake response analyses were conducted according to the input condition: ( I) Case‑ I in
which both horizontal and vertical motions are input; (2) Case‑2 in which only horizontal motion is input; and(3) Case‑3 in which only vertical motion is input. Comparison on the result of the three cases of analyses areshown in Fig･5 for shear force on a center column and for bending moment on a side wall, representing tlme

histories of intemal forces. The result of comparison can be summarized in the followlngS･
The axial force of the center column was orlglnated mainly from the vertical vibration of the surface ground･The value of which, however, is much smaller than the value generated by the static earth pressure･ Therefore,

the collapse of center columns can not be explained only by the vertical ground vibration Most of the shear
force generated at the center column, on the contrary, lS Orlglnated from the horizontal ground vibration asshown in Fig.5(a). The value of which reaches the level of the average allowable shear strength of concrete･
The tunnel model used here is rough and does not necessarily represent the damaged subway･ These results

indicates, however, that the main cause of the damage would be the horizontal earthquake motion･ Most of thebending moment and the shear force generated on side walls are orlglnated from the horizontal earthquake

motion, represented by Fig.5(b). Even a large part of axial force at the bottom of side walls is also govemedby the horizontal motion･ The values of moment and shear fわrce both at the top and the bottom of side walls∬e much higher than their allowable levels.

Because the stiffness of surface soil deposits is comparatively high,the relative displacement betweenthe
ceiling and floor slabs is almost identical with that of soil deposits at that depth. Though the maximum relative
displacement between the ceiling and floor slabs analyzed was as small as 2 cm, it is considered to be large

enough to defbm a tunnel compulsorily to a extent so as to be disintegrated as shown in the photographs.

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SEISMIC ISOLATION METHODS FOR UNDERGROUND STRUCTURES
Methods using seismic isolation laver covering a tunnel bodv
The seismic isolation method effective to highly improve seismic safety of underground structures have beenpresented by the authorand the author et al. (Suzuki, 1990, Suzukiand Tamura, 1995)･ In the method, thesoft isolation layer is formed between the tunnel and the surrounding soil as illustrated in Fig･6 The material

properties necessary for the isolation layerare (1) low shearmodulus, say, 5･O kgf/cm2 and (2) high Poisson's

ratio almost equals to 0.5. The latter property is indispensable tomimimize the ground settlement overthetunnel. The materialshould be capable of large sheardeformation and it should be highly wateHesistent･Suzuki proposed this seismic isolation systemand verified its effectiveness through model vibration tests usinga shaking table (Suzuki, 1990). One of the material for an isolation layer proposed by the authors isrubber‑like materialmainly composed of liquified silicone‑rubber (Suzuki et al. , 1995), which can be easily filled up

the tale voidinshield driving by injection fromaninside of the tunnel,thenthe seismic isolation system can beestablished even for shield‑driven tunnels as well as other urban tunnels constructed by the open‑cut method.
The isolation effect of the method wasalready discussed by the author for shield‑driven tunnels with a roundcross‑section. Then, high seismic isolation effect in the tunnel axial deformation and comparatively low
isolation e飴ct in the cross‑sectional defbmation are shown (Suzuki and Tamura, 1995). In this paper, theseismic isolation effect of the method applied to the tunnel witha rectangular cross‑section is examined byearthquake response analysis for a case that this method was applied to the Daikaistation, focusslng On a CrOSS‑

sectionaldefomation during earthquakes.

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Method aT)I)lvinE a device for sheardefomation absorption to the top of a center column
The method proposed in this paragraph is a simple one in which a device for shear deformation absorptlOn isapplied between a center colurrm and a ceiling slab. Because this typeof tunnel supports a high axial force bycenter columns in a static condition, the collapse of center columns due to an earthquake leads tothe strategic
damage as described in the previous chapter. If the center column is maintained in a sound condition, thecomplete couapse like the case in the Daikai station could not have occurred, even if the side walls weredamaged･ Therefore, this method is effective to a crossISeCtional deformation during earthquakes. Fig.6(b)shows the schematic diagram ofanexample of the device. The device needs high sheardeformation capacity
and high capacity in axial force, but it does not need any damplng. Therubber material is constrained its
lateral deformation by steelrings covenng therubber. These rlngS are Stratifiedand they canslide each otherwith a low friction･ The cost of the device is much lower thanthat of the laminated high damplngrubber
usually adopted for seismic isolation of bridges and buildings. The device can be applied to a previously
constmcted tumel as well as a new one.

EVALUATION OF THE EFFECTIVENESS OF SEISMIC ISOLATION METHOD

Effectiveness of seismic isolation laver

Earthquake response analysis was conducted for a case, in which the seismic isolation layer with shear
modulus of 5 kgf/cm2 is formed coverlng the subway station with 10 cm in thickness. Two‑directional
earthquake motions in Fig･3are Input in the analysIS･ The case of this analysts is named here as Case‑4 todifferentiate thisanalysis fromthe previous cases･ The result of the analysis is compared withthat of Case‑1 inwhich no seismic isolation is made.
Fig･7 compares the time histories of tunnel intemalforces generated atthe bottom oftheright side wall between

the two cases･ The maximum values in axial force, shear forceand bending moment were reduced to almost ahalf bythe application of the seismic isolation layer, which denotes that the seismic isolation effect uslng thismethod is considerably high in case of cross‑sectional defbmation of a rectangular tunnel･ The extent of

reduction in intemalforces is remarkable especially at the main part of the time histories･ Then, high isolation
effect is attained even when the early stage of the mainpart ofanearthquake motion･Fig･8 illustrates the comparison of the distribution for intemalforce of a tunnel between two cases of analyses,

at the time when the vertically accumulated shearstrain of the surface ground reaches a maximum value. The
scale is 10 times enlarged for a center column･ The comparison denotes that the reduction in intemalforces dueto the isolation layer is large at comer parts of a tunnel where a large intemalforce is concentrated. However,the intemal forces on center columns are little reduced by applying an isolation layer, because the relative
displacement between the ceiling and floor slabs is not largely reduced･ー‥一一mOt isolated‑ isolated

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In order to clarifythe mechanism of the seismic isolation,the shearstress of the isolation layerand surrounding
twothin soil elements witha thickness of 30 and 60 cm, respectively, are illustrated in Fig･9･ In the figure, thedistribution of shearstress for three thin layersinCase‑4 is compared with that in Case‑1･ All of the scales forshearstressinthe figure are identical.When the isolation layer is not applied (Case‑ I), shearstress of the flrStlayer around a tunnel is extremely large and the distribution forms for three layersarealmost coincident witheach other.When the isolation layer is applied, on the contrary, shearstress of the first layer (isolation layer) is
extremely smal1･ Shear stress of the second layer is also reduced to a certain extentand the distribution foms
for the outer two layersare largely different from those in Case‑ 1 ･ The difference in shearstress distribution

between two cases denotesthatthe effect of seismic isolation layer on the reduction of intemalforce of a tunnel
is mainly explained by a drastic reduction of shear force actlng around a tunnel which compulsorily deforms atunnel.

Effectiveness of isolationrubber 1)laced at the top of a center column

Earthquake response analysis was conducted for a case, in which the seismic isolationrubber of 30 cm in outer
diameter and 20 cm in height is placed onthe top of a center column･ The shearmodulus of rubber surroundedby steelring plates is rlXed to 5 kgf/cm2and that of Poisson's ratio is O･5･ Two‑directionalearthquake motionsin Fig･3are Input in theanalysis･ This case of analysis is named as Case‑5 here･ As a result of theanalysis,
shear fわrce at the bottom and at the top of a center column were reduced to 1/1 1 and 1/3, respectively andbending momentthere were reduced to I/330and 1/2, respectively, in comparison withthose in Case‑ l･Whena center column is newly constmcted, a large reduction of shear fわrce and bending moment of a center colunm

canbe attained by placing such devices at both ends of a colurrm.

CONCLUS IONSThis paper presents the typical damage of subway stations occurred due to the southern‑Hyogo earthquake ofJanuary 17, 1995. Based on the damage investlgation and numerical simulations f♭r the damaged subway
station, discussion on the main cause of the damage was made. Then, two seismic isolation methods fわr

underground structures were introduced. Finally, the methods were applied to the damaged station innumerical simulations and the effectiveness of the two methods on the reduction of tunnel internal force wasdiscussed. Conclusions derived in this paper are as follows:( I) Main cause of the subway damage was estimated to be the horizontal earthquake ground motion.

(2) The method to apply a seismic isolation layer covering a tunnel is highly effective to the reduction ofintemal force due to cross‑sectional deformation during earthquakes for a tunnel with a rectangular cross‑section, though it cannot contribute to the reduction of shearforceand bending moment of a center colurrm.

(3) The effect of seismic isolation layer on the reduction of intemal force is mainly originated from a drastic cutof shear force actingaround a tunnel.

(4) The method to placeanisolation device with no damping on the top of a center colurrm is effective to reducecolumn's shear fわrce and bending moment. The collapse of a center column leads to a destmctive damage,since lt generally supports high earth pressure. Therefore, the application of thistype of device with a low costis practical.

REFEREN CE S

Geological Survey of Japan ( 1983). Geology in Kobe area.
Suzuki, T･ ( 1990)･ A study on the evaluation of earthquake resistance and an approach to the development ofseismic isolation system regarding shield‑driven tunnels, Thesis of Doc. Eng., Univ. of Tokyo.
Suzuki, T･ and Tamura, C. (1995). Proposal of a seismic isolation structures fわr urban tunnels and themethod to evaluate its isolation effect. Proc. JSCE, 525m‑33, 275‑295.
Suzuki, T･ et al･ (1995)･ Experimental study on the silicone‑based material f♭r seismic isolation system fわrurban tunnels, Kumagai Technical Research Report, 54, 63‑70･

Tamura, C･ et al･ (1986)･ Damages due to the 1985 Mexico earthquake. Jour. JSCE, 5, 79‑85.