RESEARCH AND DEVELOPMENT ON THE SEISMIC ISOLATION SYSTEMAPPLIED TO URBAN TtJNNELS(PARTl2: EFFECTS OF SEISMIC ISOLATION AND SEISMIC DESIGN)

Tsutomu TANAKAl)and Takeyasu SUZUKI2)

1 ) Director, CentralResearchand Development Department, 0rientalConsultants Co･ , Ltd･

2) Manager, Earthquake Engineering Research Group,TeclmicalResearchand
Development Institute, Kumagai Gumi Co., Ltd･

ABSTRACTTechniques for improvlng Seismic safety of urbantunnels have hitherto been limited to thosefor increaslng the flexibility of tunnel structures by applyingflexible joints and segments･

However, since such techniques are insufficient for ensurlng Seismic safety of undergroundlifelines in urbanareas when strong earthquakes occur, there has been a demand for developlng anew, highly reliable technique, especially for protecting tunnel sections where seismic strain isconcentrated locally. Given this situation,aninnovative joint research project between the Public

Works Research hstitute and private companies was commenced to develop a seismic isolation
system to be applied to urban tunnels. This paper will describe the results of analyses andexperiments conducted for validating the effectiveness of seismic isolation, as well as thedevelopment of design methods.

INTRODUCTION

Several reseamhers have been investlgating the application of a seismic isolation system tounderground structures sincearound 1988, and the effectiveness of seismic isolation has been
confirmed by numerical analyses and laboratory experimentsl)‑5).Asshown in Fig･ 1, thefundamentalprlnCiple of the seismic isolation system for underground structures is to cut off thetransmission of ground strain by isolating a tunnel body from the peripheralsoil by forrmng a thin,softlayeraround the tunnel.

In the case of axialor bending deformation of a tunnel that occurs in the longitudinaldirection

due to earthquakes, tunnel sectional forces canbe effectively reduced when a seismic isolation

Ground Surface

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Su血ce

Soil

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Cicular Tunnel

Rect弧gular Tunnel(Shield Tunneling Method)(Cut‑and‑Cover Method)

DiluviumFig. 1 Schematic disgram to demonstrate the seismic isolation system for urban tunnels
layer is applied to a tunnel section where ground strain is concentrated locally･ This is not only

because the ground strain transmission to the tunnel body is cut off, but because the tunnel strain
in the isolated section is dispersed.
In a similar vein, when cross‑sectional defわrmation of a tunnel occurs, the results ofnumerical analyses have shown that the seismic isolation system helps to reduce the sectional

forces concentrated at the comer portions in a tunnel with a rectangularcrossISeCtion constructedby the cut‑and‑cover method, thus contributing greatly to the reduction of sectional forces at theupper and lower slabs and side walls of such a tunne16),7).

Meanwhile, earthquake responseanalyses were conducted to examine the effectiveness of theapplication of seismic isolation system to a subway tunnel that was severely damaged by the

Hyogoken‑nanbu earthquake of 19956). In the case of tunnels with a circularcross‑section suchas shield‑driven tunnels, however, there was a report that the application of seismic isolation was
not so effective in reducing sectional forces during cross‑sectional deformation due toea血quakes8).

Since there has been no systematic research on the seismic isolation system, with which athin seismic isolation layer is fomed covenng a tunnel body, studies on this system as describedabove, development of materials for the seismic isolation layer,and validation tests for developlng
construction methods have been conducted separately by various institutions8),9). Thus, acomprehensive three‑year project for clarifying the seismic isolation mechanism, and fordeveloplng Seismic isolation materials, application methods, and seismic design, was started inJuly 1995 as a joint research project between the Public Works Research Institute of the Ministryof Construction and 17 private‑sector companies in Japan. This paper will examine the results ofthis joint research, focuslng On the results obtained by analyses and experiments conducted forvalidating the effectiveness of seismic isolation, describe the results of a study on the developmentof design methods for seismic isolation tunnels, and presents the flow of the design methods.

EFFECTS OF SEISMIC ISOLATION

Longitudinal tunnel deformation during earthquakes
This chapter will examine the results of numericalanalyses conducted for identifying the effectof reducing tunnel sectional fわrces by applying seis血c isolation to shield‑driven tunnels･ The

sectional forces acting on a tunnel in the longitudinal direction can be reduced by applying aseismic isolation layer to a section that becomes subject to the concentration of the maximum local

ground strain. Such a layer is formed by injecting and filling the seismic isolation materials
developed by the joint research, whichare soft with a shearmodulus of around 3 kgucm2, into thetail void that is created during shield driving, and which is usually 5 to 10 cm thick･ In thischapter, the reduction of sectional fわrces and the behavior of the isolated tunnel section are
described in detail fわr seismic isolation that is applied to a shield‑driven tunnel section locatedundemeath a caisson‑type quay.

The longitudinaltunnel sectionand the surrounding ground structure selected for the earthquake
responseanalysesare illustrated at the top of Fig･ 2･ The numerical models used in the analyses∬e analogous to those proposed by Tamura et al･10), in which a mass‑sprlng System and a beam‑spnng system are used fわr the ground and the tunnel, respectively･ The ground model was madein consideration of the fundamentalto the third shear vibration modes, uslnganeXtended quasi‑

two‑dimensionalmodell 1) which is capable of handling thefundamentalto the N‑th shearvibration
modes･ The quay was considered a ground element with concrete properties･ The soil was
considered a linear material, whose stiffness had been reduced in reference to the initial value

obtained beforehand by a one‑dimensional equivalent linear analysis based on multiple reflection

theory･ Assuming that flexible segments absorb the displacement, extremely weak sprlngS Wereused at a Junction connectingthe tunneland shaftS･
Two shield‑driven tunnels composed of reinfわrced concrete segments of 5･4 m in outer

diameter and 22.5 cm in thickness were used, one with and the other without seismic isolation.

Theyare considered a non‑linearelement with stiffness that changes according to either tensile orcompressive defomation･

The results oftheanalysesare summarized in (a) through (e) in Fig･ 2･ As shown in Fig･ 2 (a),the maximum values of tunnel displacementarealmost constant for the two tunnels regardless ofseismic isolation; however, the gradient of the displacement distribution of the tunnel section
located undemeath the quay in the case of isolation is more gentle than in the case of non‑isolation･

The maximum axial force distributions in Fig. 2 (b)and (C) show that the seismic isolation layer
reduced the axial force markedly to 1/10 for the compressive defomationand 1/5 for the tensile
defbmation. In addition, both the bending moment and the shear fわrce decreased remarkably as

shown in Fig. 2 (d)and (e). Thus, theseanalyses showed that the application of seismic isolation
to a tunnel section where ground strain is concentrated is highly effective･ Fig･ 3 shows the time‑dependent response of axial strain at a tunnel section where the tunnel sectional force hits the

maximum for the two tunnels, one with and one without seismic isolation.

Seismic isolation at a JunCtionwith a shaft
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layer is fomed, Covenng a tunnel section measunngaround 10 m from the shaft, for isolating the血nb 9dAl‑uOSS!t!〇 12 qlt29tu9pun t9uunl u9^!Jp‑pl印s t2 JOJ S9SATt!tn 9SuOds9J 9耳enbt即t!9 Lq p9u叩qO ̀st9uunl p911!lOS!‑uOu
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tunnel section from both its peripheral soil and the shaft body･ Numerical analyses wereconducted, uslng a Shield‑driven tunnel composed of reinfわrced concrete segments, 5･05 m inouter diameter and 25 cm in thickness, to examine the effect of the seismic isolation layer in

reducing the sectional forces at such a JunCtion･ The model used was an axISymmetric finite

element model (EASIT) developed in this joint research12),13). Statistically loading the entire

model with inertialforces corresponding to O･3 G of unifom horizontalearthquake acceleration,

three cases wereanalyzed: ( 1) seismic isolation was applied to a ten‑meter‑long sectionaround theshaft; (2) no seismic isolation was applied; (3) aflexible segment was installed on the first ringfrom the shaft. Fig. 4 compares the tunnel axial force distributions of these three cases･ Despite

the ability of theflexible segment to absorb large displacement, the effect of tunnel sectional force
reduction reached only to the vicinity of theflexible segment. This is because theflexible segment

failed to gather the tunnel strain completely due to the shearresistance of the peripheralsoil actlng

on the outer surface of other tunnel segments nearby. When a seismic isolation layer was applied,however, the tunnel maximum axial fわrce decreased up to 1/3, since the tunnel strain could be

dispersed toward the shaft effectively･ In addition, unlike the technique which uses aflexible

segment, the seismic isolation system is highly reliable because there is no mechmiCaluncertainty･
Cross･sectionaldeformation of tunnels by the cut‑and･Cover method
In the case of cross‑sectionaldefomation of a tunnel with a rectangularcross‑section, which isusually constructed by the cut‑and‑cover method, great sectional forces are concentrated at thecomer portions and the upper and lower edges of slabs and side walls during ea仙quakes･

Therefore., model vibration tests were conducted to verifythat such sectional forces canbe reducedby coverlng the tunnel body with a seismic isolation layer that isolates the tunnel from its
peripheralsoil.
Photo 1 shows a view of installing model tunnels made of acrylic resins in a soil chamber･ A
tunnel model covered with a siliconerubber isolation layerandanother tunnel model withoutanyseismic isolation device were placed end to end on a vibration table,and oscillation was applied ina direction perpendicularto the tunnel axis. Then, the tunnel strain, as well as the shearstressand

the nomalstress acting on the outer surface of the tunnel bodies, were measured. Photo 2 showsa generalview of this vibration test.
Fig. 5 illustrates one of the test results, companng the bending moment distribution of tunnels

withand without seismic isolation. As shown in the figure, the seismic isolation layer decreased
the bending moment up to I/3, which is concentrated to the comer portions of the tunnel. The

shear stress actlng On the outer surface of the tunnel was decreased to 1/10 0r less by the seismic
isolation layer, which was proven to be the most important factor reducing the tunnel sectional
forces. Fig. 6 illustrates a shearstress distribution of the ground surrounding the tunnel obtainedby the earthquake response analyses6). This is rather simi1arto the ground strainaround a cavity
without lining, since the seismic isolation layer that isolates the tunnel body from the peripheral
soil changed the shearstress distributionaround the tunnel greatly.

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DEVELOPMENT OF SEISMIC DESIGN FOR ISOLATED TUNNELS

Factors affecting the seismic isolation effects(a) Stiffness of a seismic isolation layer

As discussed in the previous chapter, seismic isolation takes effect when a seismic isolation
layer is formed uslng materials with shear modulus of around 3 kgf/cm2･
To identify the range of shearmodulus, in which seismic isolation takes effect in the crossI

sectionaldefomation during earthquakes, numeriCalanalyses were conducted based on the seismic
deformation method using a finite element model (will be described later)･ Fig･ 8 shows a modelof a large box culvert (illustrated in Fig. 7) and a subsurface ground･ Fig･ 9 summarizes the
maximum sectional fわrce ratios obtained at a side wall of two tunnels, one with and the otherwithout seismic isolation, in which MノMo denotes the ratio of the bending moment; WQo, of theshear force; S/So, of the axial force;and the horizontal axis Gm/Gg, the shearmodulus ratio of an

isolation material to a subsurface deposit12)･Asshown in Fig･ 9, the tunnel sectional forces
decrease, the smaller the shear modulus of the materials becomes,and the effect of sectional force
reduction is considerable when the ratio becomes 1/100.

Meanwhile, a shield‑driven tunnel constructed in the subsurface ground composed of two soil
deposits with different soil impedances bounded by a verticalgeological boundary, was analyzed

uslnganaXISymmetric finite element model to identify the seismic isolation effect in the case of
longitudinal deformation･ Fig･ 10 surrmarizes the results of the analysis in which the shear

modulus of an isolation layer was used as a parameter13)･ As shown in the figure, the seismic
isolation materialreduced the tunnel sectional forces considerably, when the shearmodulus was as

low as 3 kgf/cm2, which is equivalent toaround 1/50 the peripheral ground･When the shear modulus ratio ofanisolation materialis as low as 1/1000, the natural period oftunnel resonance vibration shifts to a low frequency range, and the sectional force distribution

becomes different from that usually observed under static conditions･ Thus, the shear modulus ofanisolation materialshould be around 1/100 that of the peripheralground･

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Fig. 13Comparison of bending moment distributionsamong four casesﾂ(2) Poisson‑〜 ratio or an isolation layer

Assumlng that Poisson.s ratio of an isolation layer may be affecting the effect of seismic
isolation, since a seismic isolation layer is defomed greatly when the tunnel body is defomed in
the cross‑sectional direction, the authors conducted analyses based on the defわrmation methoduslng the same model shown in Fig･ 8, uslng Poisson‑s ratio as a para腕terFig･ 1 1 shows theresult of analyses on sectional forces at a side wall･ It was confirmed, however, Poisson's ratio of

anisolation layer did not affect the seismic isolation effect･(3) Arrangement and thickness or an isolation layer

The effect of thearrangement and the thickness of an isolation layer on seismic isolation was
examined, focuslng On the cross‑sectionaldeformation of a tunnel with a rectangularcross‑section･
Four cases of arrangement (Fig. 12) wereanalyzed using a tunnel model shown in Fig･ 813)･ The

fundamental case is a tunnel covered by an isolation later with a thickness of 10 cm･ Companng
the bending moment distributions of the four cases in Fig･ 13, one cannotice that the thickness ofanisolation layer scarcely affects the tunnel sectional forces within the range of thickness used in

the analyses, and that the seismic isolation layer does not manifest its maximum effect unless it isinstalled covenngthe entire outer surface of the tunnel.

The same effect in the case of longitudinaldeformation of a shield‑driven tunnel constructed in
the subsurface ground described above was also analyzed uslng an aXISymmetric finite element

model･ Fig･ 14 illustrates a summary of theanalyses in which the thickness ofanisolation layer isused as a parameter13). The figure indicates that the effect of the thickness ofanisolation layer onseismic isolation is negligible.

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Procedures and the flow of design(1) Static design
The static design fわr isolated tunnels is conducted in the same manner as that f♭r non‑isolated

tunnels.

(2) Seismic design of the crossISeCtionaltunnel deformation
A丘nite element model as shown in Fig. 8 is used in the seismic design, so that the soil‑

structure interaction canbe taken into consideration accurately. Although it is desired to conductdynamiCanalysis using such a model for a precise output, a static method was used for simplifying
the design methods on the assumption that thefundamentaltunnel structure is designed without
sufficient geologicaldata.

Since the defomation of underground structures is govemed by the ground deformation, it isnecessary to generate the ground deformation, when a tunnel sectional force reaches its maximum
value, by a static method･ It is supposed that the ground vibration can be represented only by afundamental mode of shear vibration, which is appropriate in many cases･ In the case of aresonant vibration, thetimethat the ground displacement reaches its maximum value is coincident
with the time that the ground acceleration reaches its maximum value. Therefore, the maximum
vertical distribution of horizontal ground displacement can be obtained by loading the fわrce ofinertia, which corresponds to the acceleration distribution forfundamental shearvibration, to thefinite element model. The force of inertia canbe given by equation (I).

F(Z) ‑讐A(Z)
A(Z) =￠(Z) β SA

(1)where, Z: dep血F(Z): force of inertia at depthZ
Ⅵ(Z): weight per unit volume of soil at dep血z

g: gravitationalacceleration

A(Z): absolute acceleration at depthZや(Z): modalvector at depth z

P: participation factor of thefundamentalshearvibration mode
SA: absolute acceleration response vector at a naturalperiod offundamentalmode ofground vibration

The authors named this method the "deformation method based on ground force of inertia/. 12)(3) Seismic design on the longitudinal tunnel deformation
Seismic isolation in the longitudinal direction is conducted by applying a seismic isolation

layer to a section where the tunnel strain is concentrated locally, such as a Junction with a shaftand

anarea with abrupt transition of geological conditions. Thus, when conducting a seismic design,
earthquake response analyses on the subsurface ground of a tunnel, uslng Such numeriCalmodels

as a mass‑spnng systemand a 2‑D finite element model,are conducted in the first place, followed
by a detem血ation of the section where seismic isolation should be applied based on the tunnel

sectional forces obtained bytheanalyses.
As shown in Fig･ 15, tumel strain in an isolated reglOn is dispersed and becomes unifbm dueto the redistribution of tunnel strain. The divergence of tunnel strain becomes remarkable, thesmaller the shear modulus of an isolation layer becomes, and the larger the tunnel stiffness

becomes･ Such a tunnel strain redistribution is conflrmed by staticanalyses uslnganaXISymmetricfimite element model, a simplified three‑dimensional finite element model, or a three‑dimensional
finite element model.

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CONCLtJDING REMARKSThis paper has briefly described the results of numericalanalysesand model vibration tests

for identifying the effectiveness of seismic isolation applied to tunnels, as well as the study on theseismic design of tunnels･ It was proved that adequate application of a seismic isolation layer
provides reduction of tunnel sectional forces at a section where the ground conditions change

abruptly,and that the sectional force reduction at a joint section with shafts by the seismic isolation
system is fargreater thanthat by the technique to useflexible segments, both of which have been

proven prormslng for practicaluse･ Given that the basic flow of the seismic design of tunnels has
been consolidated, the authors intend to publish the results into a manualof design methods, apply
the developed technology to practical construction works of seismic isolation tunnels to help

prevent earthquake‑related damage to lifelines･

ACKNOWLEDGMENTS
This study was conducted as part of the joint research between the fhblic Works ResearchInstitute of the Ministry of Constmction, the Public Works Research Center, and 17 prlVate
companies, under the title of一一The Development of Seismic Isolation System fわr Underground

Structures.''The authors would like to express their deep gratitude to the members of the joint

research program for their wan cooperationand helpful suggestions and discussions･
REFERENCES
1) Suzuki, T.: Model Vibration Tests on the Seismic Isolation Strucure f♭r a Shield‑driven

Tunnel, Proc. 20th Earthquake Engineering Symposium, JSCE, pp.565‑568, 1989･ (in Japanese)2) Unami, K. and Suzuki, T.: A Study on the Seismic lsolation Structure fわr a Shield‑driven

Tunnel (Confimation of Isolation Efects by FEM), Proc. 44th AnnualMeeting of JSCE, Vol･ 1,
pp.1204‑1205, 1990. (in Japanese)3) Suzuki, T. and Unami, K.: A Study on the Seismic Isolation Structure fわr a Shield‑driven

Tunnel (Application to Ea血quake Response Analyses), Proc. 44th Annual Meeting of JSCE,

Vol.1, pp.1206‑1207, 1990. (in Japanese)
4) Kawashima et al.: The Development of Seismic Design fわr Underground Stmctures, Joint

Research Report No.29, PWRI, pp.208‑235, 1989. (in Japanese)5) Takeuchi M. et al.: Experimental Study on the Sectional Force Reduction in Cross‑sectional
Defomation of Shield‑driven Tunnels due to Earthquakes, Proc. JSCE, No.483m‑26, 1994･ (inJapanese)6) Suzuki, T.: Damages of UrbanTunnels due to the Southem Hyogo Earthquake of January 17,

1995and the Evaluation of Seismic Isolation Effect, CD‑ROM of the Eleventh World Conerenceon Earthquake Engineenng, Acapulco, Mexico, 1996.
7) Otsuka H., Hoshikuma, ∫. and K. Nagaya: A Fundamental Study on Innovative Menshin

System for Underground Structures, Proc. 6th U.S.‑Japan Workshop on Earthq. Dis. Prevent.
for Lifeline Systems, Osaka, pp.427‑436, 1995.

8) Suzuki, T. and Tamura C.: Proposal of a Seismic Isolation Structure for Shield‑driven Tunnels
and the Method to Evaluate Its Isolation Effect, Proc. JSCE, No.525m‑33, pp.275‑285, 1995. (inJapanese)9) Suzuki, T. et al.: Experimental Study on the Silicone‑based Material fわr Seismic Isolation

System for Urban Tunnels, Proc. JSCE, No.534NI‑30, pp.69‑76, 1996. (in Japanese)10) Tamura C･, S･ Okamoto and M. Hamada: Dynamic Behavior of a Submerged Tunnel dudng
Earthquakes, Report of Institute of industrial Science, University of Tokyo, Vol.24, No.5, 1975.

1 1) Suzuki, T.: The Extended Quasi‑three‑dimensional Ground Model for Irregularly Bounded
Surface Ground, Structural Englneerlng侶arthquake Englneerlng, Vol.9, No. 1, pp.2ls‑32S,

1992.
12) PWRI: Joint Research Report on the Development of Seismic Isolation System fわr

Undergroud Structures (PART‑ 1), 1996.

13) PWRI: Joint Research Report on the Development of Seismic Isolation System for
Undergroud Stmctures (PART‑2), 1997.