RESEARCH AND DEVELOPMENT ON THE SEISMIC ISOLATION SYSTEMAPPLIED TO URBAN TtJNNELS (PART･1: DEVELOPMENT OF SEISMIC
ISOLATION MATERIALS AND CONSTRUCTION METHODS)

Takeyasu StJZUKIl) and Tsutomu TANAKA2)

1) Manager, Earthquake Engineering Research Group, TechmiCalResearchand
Development hstitute, Kumagai Gumi Co･, Ltd･
2) Director, CentralReseaDChand Development Department, 0rientalConsultants Co･, Ltd･

ABSTRACT

Techmiques for improying seismic safety of urbantunnels have hitherto been limited tothosefor increaslng the flexibility of tunnel structures by applyingflexible jolntSand segments･

However, Since such techniques are insufficient for ensurlng Seismic safety of undergroundhfelines in urbanareas when strong earthquakes occur, there has been a demand for developing a

new, highly reliable technique, especially for protecting tunnel sections where seismic strain isconcentrated locally･ Giventhis situation,aninnovative joint research project between the Public

Works Research Instituteand private companies was clommenced to develop a seismic isolation
system to be applied to urbantunnels. This paper describes the tunnel sections where seismic
isolation canbe applied effectively, seismic isolation materials developed bythe joint research,andvalidation experiments conducted to conr1rmthe feasibility of the methods for applying thesematerials.

INTRODUCTION

Severalresearchers have been investigatlng the application of a seismic isolation system tounderground structures since around 1988,and the effectiveness of seismic isolation has been

conFlrmed by numericalanalyses and laboratory experimentsl)15).Asshown in Fig. 1, thefundamentalpnnciple of the seismic isolation system for underground structures is to cut off thetransmission of ground strain by isoladng a tunnel body from the peripheralsoil by forrmng athin,so允 layer around血e mmel.

Inthe case of axialor bending deformation of a tunnelthat occursinthe longitudinaldirection
due to earthquakes, tunnel sectional forces canbe effectively reduced when a seismic isolation

■●
●

●■

/

Ground Surface

Isolation Layer Surface Soil Deposit

Cicular Tunnel

Rectangular Tunnel(Shield Tunne払ng Method)(Cut‑and‑cover Method)

Diluvium

Fig･ 1 Schematic disgramto 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 fわrces at theupperand lower slabsand 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 ea血quake of 19956)･ 也 the case of tunnels with a circular cross‑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 crossISeCtional deformation due toearthquakes8).

Since there has been no systematic research on the seismic isolation system, with which athin seismic isolation layer is fbmed coverlng a tunnel body, studies on this system as describedabove, development of materials for the seismic isolation layer,and validation tests for developing
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 fhblic Works Research Institute of the Ministryof Constmction and 17 prlVate‑Sector COmpanies in Japan. This paper will examine the results ofthis joint research, focuslng On the tunnel sections where the application of the seismic isolation iseffective, as well as the development of seismic isolation materialsand validation tests conducted

for confiming the feasibilityof application methods.

TUNNEL SECTIONS AND THE MECHANISM OF
THE SEISMIC ISOLATION SYSTEM

The tunnel sections where seismic isolation is considered to be e飴ctive are (1) sections

where geological conditions change abruptly and (2) junctions with different structures, such asvertical shafts, and sections with sharp structural variations･ The sections where geologicalconditions change abruptly can be further classified into several categories: (a) boundaries

between softand stiff soil deposits; (b) sections nearfault or fracture zones; (C) sections where thethickness of subsurface layers changes abruptly due to irregular boundaries with bedrock; and (d)sections where the predominant frequency of the surface ground changes abruptly due to artificial
fillingand construction of structures as shown in Fig･ 2･Fig･ 3 illustrates the distribution of longitudinal ground strain of the su血ce groundcomposed of two types of soil deposits with different soil impedances bounded by a verticalgeologicalboundary, which is from a study on the boundary between softand stiff soil deposits

where abrupt change in geologiCalconditions is commonly observed･Asshown in Fig･ 3, themore consplCuOuS the impedance contrast of the le氏and right soil deposits becomes, the more

intense the localconcentration of longitudinalstrain on the soil boundary becomes･ This means, intum, that tunnels constructed in such places become subject to excessive ground strain transmitted
through the tunnel structure during earthquakes･ Fig･ 4 is a schematic diagram that illustrates thee飴ct of the seismic isolation applied to a section where the ground strain is concentrated･ As

shown in the figure, the seismic isolation layer canisolate the tunnel section from the surrounding
ground, where ground strain is concentrated over a certainarea･ Idealistically, the values of thestrain withinthe isolated section, whichare detemined by the tunnel stiffnessand the displacement

responses of the ground on both leftandright sides of the isolated section, should be constant･ In

actuality, however, peak values of tunnel strain appearat the sections where strain is concentrated

because smalllSCale ground strain is transmitted from the surrounding ground to the tunnel due tothe slight sti仇IeSS Of the seismic isolation layer･ Thus, the mnnel sections where seismic isolation

is appliedand the shearmodulus of seismic isolation materials should be detemined according to
the tunnel strain distribution.
In addition to the concentration of ground strain, a tunnel section located close to a shaft

which is connected to the tunnel becomes subject to the concentration of tunnel strain during

earthquakes, since such a junction is a structuralboundary between the tunnel and the shaftwhose

naturalvibration characteristicsare different from those of the tunnel body. Accordingly, such a

section requlreS a Seismic isolation layer for isolating the tunnel from both the peripheral soiland
the shaft body･ Fig･ 5 is a schematic diagram showlng the effect of seismic isolation applied to atunnel‑shaftjunction of a shield‑driven tunnel, along with a case in which aflexible segment isinstalled at a junction for comparison.Although the flexible segment provides high displacement
absorption as shown in the figure, the distance in which the tunnel sectional force reduction take's
effect is short in the case of non‑isolation, since tunnel strain cannot be concentrated to the flexible
segment due to the shear resistance from the peripheral soil around the segment neighboring theflexible segment. On the other hand, with the seismic isolation system,the sectional forces canbe
reduced effectively in accordance with theareas covered by the seismic isolation layer, which ismore reliablethantheflexible segment becausethere is no mechanicaluncertainty.

(a) Boundary between softanf stiffsoil
deposits formedina sedimentary(b) Section near fault or fracture zones

prOC e SS(d) Junction with a vertical shaft(C) Section where the thickness of sub‑surface layers changes abruptly due to(e) Transition of naturalperiods of sub‑surface ground due to filling andirregular boundaries with bedrock

construction of a structure

Fig.2 Sections wherethe application of seismic isolation is effective2㈱x lOjSoftSoil:V.q1=100m/see++StiffSoil:V.q2

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Seism cZsota ionLayerisotatedTun el StrainDstribu onf ･DEVELOPMENT OF SEISMIC ISOLATION MATERIALS

A totalof five different seismic isolation materials were developed, three for shield‑driven
tunnelsand two for tunnels bythe cut‑and‑Cover method･ They provide highly effective seismic
isolation, meetthe criteria of propertiesthat are considered indispensable in the ordinary static
design of these tunnels (such as watertightnessand ground settlement prevention),and have theperformancethat is required for construCtingthese tunnels.
Despite slight differences due tothe type of tunnel (shield‑driven tunnelsand tunnels

constructed by the cut‑and‑cover method), themin properties required for seismic isolationmaterials canbe summarized as follows10):(1) Low shear modulus(2) Highsheardeformability(3) Highdurab山tyand long‑ten stability(4) Watertightness(5) Extremely small volumetric changes atinjection,vulcanization,andthereafter(6) Transportal)ilityby pumpinginaliquid state withoutany materialseparation(7) High filling‑upperfomance(8) No dilution by groundwater(9) No cont鮎ninants

Seismic Isolation Materials for shield･driven tunnels
A seismic isolation layer for shield‑driven tunnels is formed by injectingand filling upan
isolation materialtothe tail void generated during shield driving work. The properties (4) through(9) listed above arealso required of the materials for丘11ingthe tail void in ordinary shield driving.

The seismic isolation materials developed inthe joint research are outlined below:
AsDhalt‑based Materialrroa Doro Kogvo K. K.I

The base materialofthe asphalt‑based materialis amixture of asphalt emulsionand high‑early‑strengthportland cement. Whenmixed witha high water‑absotbing polymer, it gelates in 10
to 30 seconds, gradually hardens due tothe hydration of the cement, then finally becomes a solid

mass about one monthafterinjection.
Urethane‑based Material(Asahi Denka Kogvo KKI

The base materialofthe urethane‑based materialis isocyanate.When amixture offly ash
and polyor is added as a hardenerandmixed, it starts tovulcanize in severalhours,and becomes
urethane elastomer withlow elastic modulusand high deformabilityone day after curing. The

addition of a specialPolyor whenmixing withthe hardener canincrease its viscosityup to 100
poISeinoneminute afterinjection, which stabilizesthe excavated soil fan of the tail void.
Silicone‑based Material(Shin‑Etsu ChemiCalCo.. Ltd.l
The base materialofthe silicone‑based materialis composed mainly of silicone oilandfly
ash･When a hardener ismixedin, itvulcanizes in about three hours a氏erinjection,and becomes arubber‑like solidinone day･ Reacting to silica contained inthe base material, polyether contained

inthe hardener increasesthe viscosityup to 200 poise in about 10 seconds,and stabilizesthe
excavated soil face of the tail void.

Seismic Isolation Materials for Tunnels by the Cut･and‑Cover Method
Inthe case of tunnels constructed bythe cut‑and‑cover method, seismic isolation materials
are not injected into the earth, unlike the case withshield‑driven tunnels･ Since the space isnarrow, where the seismic isolation system work is conducted, the materials must be wellcontrived. The seismic isolation materials developed inthe joint researchare outlined below:
Liquid Rubber rTokaiRubber Industries. Ltd.1

The base materialofthe liquidrubber is amixture of polybutadiene‑type1iquidrubberandasphalt･When polyisocyanate ismixed in as a hardener, this liquidrubber becomes arubberllike
solid in one血y. Even when血e job site is not spacious enough, such as when applied to仙e sidewalls of tunnels constructed bythe cut‑and‑cover method, thismiformrubber‑like solid canbe
fomed by uslng molds.

Precast Rubber Panel (Sumitomo Rubber Industries. Ltd.)The precastrubber panel is made by solidifyingrubber tips made of shredded tires using

urethane prepolymer resin. Coated by urethane resin,the panel surface is waterproof. This is aprecast product produced in a factory,that is shaped into panels of a certain size to facilitate
handling. Adhered to tunnel walls at job sites,the panels forma seismic isolation layer.
Hollow Cylindrical Cyclic Shear Tests

The seismic isolation layer is formed between a structureand itsperipheralSoil at a depth
anywhere from a few meters to tens of meters below the ground level. In addition to a static

constrained pressure generated underground,the layer becomes subject to compulsory shear
deformation during earthquakes in both the axialand cross‑sectionaldirections of the tunnel.

Therefore, dynamic properties of the seismic isolation materials should be measured by glVlng
cyclic shear loading under the same constrained conditions as underground. h consideration of

the frequency dependencyinthe materialproperties,the loading frequency should be simi1artothe
predominantfrequepcy of the surface soil deposit･ Thus, it was decided to measurethe dynamic
properties of seismlC isolation materials by conducting hollow cylindriCaldynamic sheartests
Photo 1), which are commonly conducted for testing dynamic shearproperties of soil materials.
The f陀quenCy Of cyclic loading used inthe tests is 1 Hz which corresponds tothe averagefrequency of the surface deposits. Starting witha small level,the shearamplitude was increased
graduallyuntil it reached 20%inthe finalstage. The rotationaldisplacement measurement methoduslng a laser displacement gauge was adopted, since the cyclic shear1ng Was tO be measured withsuch large shearamplitudesand at 1 Hz. To verifythe dependency of the constrained pressure ofmaterialproperties, simi1arcyclic loading tests weI℃ conducted uslng three different constrainedpressures, i･e･, 0･5, 1･5,and 3.5 kgf/cm2. Using 1 I sine waves, loading was applied cyclically tothe respective shearstrainamplitudes,and the shear modulusand the damplng factor were
calculated based onthe hysteresis loop of the stress‑strain relationship at the loth cycleandthereafter. The dynamic properties of the silicone‑based material, which is one of the mostcommonly used materials,are described below:
Figs･ 6and 7 show the strain‑dependent curves of shearmodulus Gand damping factor hi
respeCtively･ These figures indicate thatthe materialprovides a constant prope爪y regardless of thedep也where tumels are constmcted, since nei仙er G nor h presents弧y COnStrained pressure

dependency, desplte the factthatthe constrained pressure glVeninthe tests had a considerablylarge range of 0.5 to 3.5 kgf/cm2. Since the values of shearmodulus G of the materialarelt!Pa】!?tU

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constantly around 3 kg批m2 without any strain amplitude dependency (unlike the case with soilmaterials), G measured duringmicro strain may be used when the seismic isolation effect isevaluated inthe phase of design.

In addition, the values of damplng factor h werearound 2 % withoutany strain dependency･
When the seismic isolation system is applied to underground structures, no hysteresis damplng Of
the material is likely to occur. Thus, the seismic isolation materials developed in the joint researchwere proven stable even duringthe cyclic shearloading with largeamplitudes･

DEVELOPMENT OF SEISMIC ISOLATION METIIODSll)

In the case of shield‑ddven tunnels, a seismic isolation layer should be fわrmed securely

around a tunnel by injecting and filling a liquid isolation material into the tail void which is
generated during shield driving work, in the same manner as with the filling work conductedduring shield driving. It is thus necessary to produce prototypes of the equlPment for the rmxlng

and pumplng Systems for each seismic isolation materialto conduct expenments for confirrmng thefeasibilityof the respective methods. In the joint research, two feasibility tests were conducted toverifythe methods for applying the seismic isolation materials, i. e., injection into a model groundchamberand a trialinjection at actualtunneling sites.
Fig. 8 shows the time‑dependent variations of the viscoslty Of the silicone‑based material

after lqeCtion.Asshown in the figure, this material attains high viscosity immediately after
injection, fills up the tail void,and sustains the earth pressure. Ifanadditionalinjection is applied,

this material becomes thixotropic and provides a plastic flow that expedites thefilling.Asshownin Photo 2, a prototype injection facility was produced, and injection tests using a model soilchamber were conducted to verify the feasibilityand practicality of this experimentalfacility. The
chamber used for the injection tests is also used for testing the backfilling materials for shieldtunneling work; thus, the test results showed that silicone‑based materialinjection was as practicalas uslng backfilling materials. In addition, a simi1artest wasalso conducted for the urethane‑basedmaterialuslng the same chamber.
Photo 3 shows a trial叫ection of the silicone‑based material into the tail void at an actual

shield tunneling site. The materialwas Injected into the tail void of actualearth with groundwater,and sampling cores from grout holes that had been drilled in segment pieces beforehand conflrmedthat a seismic isolation layer withthe target properties and thickness was formed. The same trial
injection was conducted for the asphalt‑based material, which was also confirmed to be highly
feasible.

Inthe case of the liquidrubber applied to tunnels by the cut‑and‑cover method, it is necessary
to confim its mlXlng and vulcanization characteristics. This material was placed between a

concrete sheet pile and a wooden fom uslng a Prototype Injection facility.Asa result, it was
confined that a seismic isolation layer with a uniformproperty was fomed on the concrete wallas shown in Photo 4･AnOther prototype was produced to test the precastrubber panel,and it was
confirmed that a seismic isolation layer was formed satisfactorily as shown in Photo 5, as the panel
adhered to a concrete sheet pile with sufficient strength.

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■
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A black panel stuck to a concrete sheet
panel in山e left hand side is the seismic

isoladon layer made of血e liquid mbber.

The liquid mbber is being叫eCted into aformin theright hand side.

Photo.5 Tests for the precast rubber panel
The precastrubber panel is adhered tothe concrete sheet panel covered by arubber sheet for water‑proofing.

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0

5

10

15

20

25

30

Time (min)Fig･8 Representative time‑dependent variations of the viscosity of thesilicone‑based material

CONCLUDING REMARKSThis paper has briefly described the seismic isolation system for underground structures
focuslng On the tunnel sections where the application of the seismic isolation takes effect, thedevelopment of seismic isolation materials, and validation experiments conducted for conrlrrmng
the feasibility of the application methods. Five different seismic isolation materials applicable toshield‑driven tunnelsand tunnels by the cut‑and‑cover methods were developed, which have been
proven promslng f♭r practical use as a result of material and validation tests. Now the authors

intend to apply the developed technology to practical construction works to help prevent

earthquake‑related damage to lifelines.

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

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

research programfortheir 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 Ea血quake Engineering Symposium, JSCE, pp.565‑568, 1989･ (in Japanese)
2) Unami, K. and Suzuki, T.: A Study on the Seismic Isolation Stmcture fわr a Shield‑driven

Tunnel (Confirmation of Isolation Effects by FEM), Proc. 44thAnnualMeeting of JSCE, Vol･ 1,
pp.1204‑1205, 1990. (in Japanese)3) Suzuki, T. and Unami, K.: A Study on the Seismic Isolation Stmcture f♭r a Shield‑driven

Tunnel (Application to Ear山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♭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
Defbmation of Shield‑driven Tunnels due to Ea血quakes, Proc. JSCE, No.483/Ⅰ‑26, 1994･ (inJapanese)6) Suzuki, T･: Damages of Urban Tunnels 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, 1 996.
7) Otsuka H., Hoshikuma, ∫. and K. Nagaya: A Fundamental Study on Innovative Menshin
System f♭r Underground Structures, Proc･ 6th U･S･‑Japan Workshop on Ea血q･ 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.525/Ⅰ‑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) PWRI: Joint Research Report on the Development of the Seismic Isolation System for
Undergroud Stmctures (PART‑ 1 ), 1996.

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

PERSONAL CAREER
NAME :

Takeyasu Suzuki

POSITION :Manager
Earthquake Enginee‑g Research Group
Tech. Resear. 良 Develop. hst.

KumagaiGumiCo･, Ltd･
ADDRESS :1043, Onigakubo, Tsukuba‑shi,
300‑26 ⅡiARAKI, JAPANTeュ. 8 1‑298‑47‑7508

Fax.8 1 ‑298‑47‑7480

DATE OF BIRTH:May 16, 1956
EDtJCATION:

Bachelor of Engineering, Saitama University( 1 980)

Master ( 1982)and Doctor ( 199 1) Degrees of Engineering, Umiversityof Tokyo

MAJOR SUBJECT: Earthquake Engineering
h4AJOR AREA OF EXPERIENCE:
1982‑1983

Research Engineer, hstitute of Construction Technology, KumagaiGumi

1983‑1984

Visiting Research Personell, Department of Civil Enginering, Umiversityof Tokyo

1984‑1985

Research Engineer, Institute of Construction Teclmology, KumagaiGumi

1986‑1989

Visiting Research Personell, hstitute of IndustrialScience, Umiversityof Tokyo

1989‑1991

Research Engineer, TechmiCalReserchand Development hstitute (TRDI),

1991‑1995

KumagaiGumi
Assistant Manager, Civil Engineering Research Department, TRDI, KumagaiGumi

1995‑

Manager, Earthquake Engineerlng Research Group, TRDI, KumagaiGmi

TRAVEL ABROAD: USA, Mexico, England, Switzerland, France, Phillipine

MAJOR PtJBLICATIONS:"The Extended Quasi‑three‑dimensionalGround Model for Irregularly Bounded Surface Ground",StructuralEngineering侶arthquake Engineenng, Vol･9, No. 1 , pp.2 1 s‑32S, 1992.

''Application of Microtremor Measurement to the Estimation of Earthquake Ground Motions in

Kushiro City duringthe Kushirol0ki Earthquake of 15 January 1993'', Earthquake Engineeringand

StructuralDynamics, Vol.24, pp.595‑613, 1995.
日Proposalof a Seismic Isolation Structure for UrbanTunnelsand the Method to Evaluate Its lsoladon

Effect" (in Japanese), Jour. JSCE, No･525m‑33, pp.275‑285, 1995.

"ExperimentalStudy on the Silicone‑based Material for Seismic Isolation System for UrbanTunnels(in Japanese)I‑, Proc. JSCE, No.534/V1‑30, pp.69‑76, 1996.

''Damages of UrbanTunnels due to the SouthernHyogo Earthquake of January 17, 1995and the

Evaluation of Seismic Isolation Effect'', CD‑ROM of the Eleventh World Conerence on Earthquake
Engineenng, Acapulco, Mexico, 1996･