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Seismic Isolation of Tunnel Lining A Case Study of the Gavoshan

Tunnel in the Morvarid Fault in IRAN

K. Keshvarian M.1, M. R. Chenaghlou

2, M. Emami Tabrizi

2, S. Vahdani

3

1Mahab Ghods Consulting Engineering Company, Iran

2Civil Engineering Faculty, Sahand University of Technology, Tabriz, Iran

3Technical Faculty, Tehran University, Tehran, Iran

ABSTRACT

Strike slip faults are more wide spread around the world than the other kinds of faults. Therefore,huge structures such as dams, tunnels and power plants are more likely to meet these faults. When the

fault is active, special treatment should be considered. In order to analyze the shape and amount of

fault rupture effect on tunnel lining, it was necessary to build an accurate tunnel model. This case

study concerns Gavoshan tunnel in west of Iran, which is a twenty kilometers, horseshoe shaped,

water supply tunnel by 4.2 meters diameter. The tunnel crosses the Morvarid fault, which is an active

strike slip. The Tunnel is modeled by Finite Element Method, using ANSYS software. First, the fault

rupture has imposed and failure zone in lining and rock is calculated. In the second step, lining

isolation is achieved by considering a gap between lining and rock mass. The last step is filling the

gap with rubber-based material and study the effect of this material over lining behavior during

earthquake. Finally, a suitable solution and some comments for the protection of lining in seismic

zone are suggested.

1. INTRODUCTION

Seismic isolation of tunnel lining has been investigated in the literature and some methods have been

proposed [1, 2, 3].

Extra excavation and filling the gap between lining and rock with bituminous based material [1],

using silicon based material as the gap filler, using soft back packing material [2], seismic isolation

by improving the rock characteristics with consolidation grouting [3] are some of the features in

which researchers have used.

In above mentioned methods, the main challenge was to decrease the effect of earthquake propagating

wave and fault rupture effect had been neglected. But there are limited studies on the fault rupture

effects [2, 4]. In this paper, taking advantage of ASCE suggested procedures [4], lining behavior has

been studied under fault rupture then effect of extra excavation for seismic isolation with and without

the presence of rubber fillers, has been studied by accurate modeling.

2. PROJECT DESCRIPTION

In order to accurately study the fault rupture effect on concrete lining stability, a finite element

numerical model was constructed according to one real situation. Because of different conditions and

multiple choices available for analysis, finding an applicable and expandable form to commonsituations, was difficult. With attention to considerable amount of strike slip faults around the world

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in comparison with other kind of faults, a strike slip fault was chosen. Between general shapes of

openings, a modified horseshoe section selected according to the type of rock mass and method of

excavation.

Gavoshan water supply Tunnel was chosen for study. This Tunnel passes through Sanandaj-Sirjan

geological zone, which is one of the complicated tectonic structures in Iran. It pipes drinkable water

to Kermanshah city. The tunnel passes through one of the young branches of Zagros fault that is

called Morvarid. The Morvarid fault is strike slip and the fault zone length is about 120 meters. The

connection zone of the tunnel and fault was considerably long to permit the lining to accommodate

the huge amount of shear and displacement imposed by fault rupture. By these specifications this case

has been chosen for modeling and analysis.

3. REQUIREMENTS OF MODELING THE LINING UNDER FAULT RUPTURE

Stresses imposed by excavation and overburden dead load of rock, are supposed as initial condition of

the problem, at rupture time. In order to calculate the initial stresses in different points of lining, the

FLAC software is used. Type and characteristics of rock mass existing in tunnel passing through faultzone [5], is given in table (1).

Table (1): Rock mass characteristics of Gavoshan tunnel in fault zoneSaturated

Density

(kg/m3)

Dry Density

(kg/m3)

Cohesion

(MPa)

Angle of

Internal

Friction (Deg.)

Poison Ratio Modulus of

Elasticity

(GPa)

Rock Type

2720 2690 0.25 28 0.2 4 Limestone Shale

The rock mass in highly sheared area, because of alteration and numerous amounts of joints existing,

can be represented as a continuum with equivalent rock mass properties [6]. One of the important

factors in accurate estimation of stress distribution in rock is to synchronize the time of definition of

rock supports in the model and real situation on site. In this case panes theory has been used [7].

Shotcrete and rock bolts of tunnels Support system have been considered in the modeling and after

definition of support system, analysis has been continued to reach the balance point. In figure (1) the

Finite Difference model made by FLAC (Fast Lagrangian Analysis of the Continua) is shown.

Figure 1: Under ground space using FLAC Figure 2: Lining rupture modeled by SAP2000

In order to analyze the lining failure zones, Finite element method has been applied. Taking

advantage of two continuous and discrete models, and using two different soft wares (Ansys andSAP2000 Nonlinear) the problem has been analyzed and compared. The discrete model is a three

Spring

Line

Crown

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dimensional and made by combination of shell elements for lining and nonlinear links (springs) for

rock mass by the aid of SAP2000 Nonlinear (Figure 2). The stiffness of springs are determined by

applying a unit displacement along the same direction of spring stiffness to a 11 meter rock model.

The continuous model is made using the ANSYS (figure 3) and by the solid elements for concrete

lining and rock mass and contact elements for interaction between lining and rock. More details havebeen mentioned in research project [8].

Figure 3: Lining model under influence of fault rupture made by ANSYS

In Figure 4a and 4b the results of two models (continuous and discrete) are shown at the invert, crown

and spring line of lining. Comparison of two graphs shows good agreement in results of discrete and

continuous model. The results of continuous model would be used as the base for seismic isolation

study [8].

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 10 20 30 40 50 60 70

Tunnel length(m)

RupturedeformationofLining

ux-invert

ux - crown

ux - spring line

Figures 4a: Displacement curves in horizontal axis direction (x) along tunnel axis under effect of

fault rupture, Analyzed by Sap2000 (Discrete model)

(m)

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-4.00E-02

-2.00E-02

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

0 10 20 30 40 50 60 70

Tunnel length(m)

LiningRuptureDeformation

UX - Spring line

UX - Invert

UX - Crown

4. ANALYSIS THE COMPOSITE TUNNEL LINING

After determining the length of lining failed during rupture, the next step is to isolate tunnel lining in

that segment. The main purpose of extra excavation is avoiding contact or development of direct

pressure of lining over the rock, therefore the amount of extra excavation is a function of fault rupture

rate (Figure 5). An alternate is to fill the gap between lining and rock by special rubber based

material. The main reason for this job is to transmit the fault rupture energy. In first case (Gap)

although lining is taken apart from rock in over excavated zone, but at the ending point of the extra

excavation considerable amount of energy would be transmitted to the rock and this energy

concentration could be very dangerous. In other side the stability of rock over the linings crown,

when losses the second stage lining (at time of rupture displacement) would make problems. But

using rubber isolators (Figure 6), gradual transmission of energy and forces are occurred.

Figure 5: GAP Isolated model Figure 6: Rubber isolated model

Figures 4b: Displacement curves in horizontal direction (x) along tunnel axis under effect of fault

rupture, Analyzed by Ansys (Continuous model)

(m)

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5. MODELING THE TUNNEL

5.1 Loading

In addition to dead load related with over burden rock load and induced stresses resulting from

excavation, which computed by FLAC, another load is the seismic load induced by fault rupture.Taking advantage from Wells and Coppersmith suggested relations [9]; the amount of fault

displacement is calculated according to geological situation of the project.

Earthquake hazard potential of site region has been reported 6.4 Richters. According to fault type,

the maximum probable displacement is 18 centimeters, which occurs strike slip. The fault rupture is

applied to model in displacement control method, because considering the displacement control

method. According to the symmetric nature of problem half of the real form of the structure is

modeled.

5.2 Elements

The solid material such as Rock, Shotcrete and Concrete are modeled by solid elements. Thefollowing assumptions have been made for the modeling of material and element.

1-The Dracker Prager behavior parameter is chosen for modeling of rock mass behavior.

2-The Mooney Rivelin rules are applied for defining the rubber based material behavior.

3-Lining and rock interaction is modeled by the aid of contact elements and surface to surface contact

type which is one of the most accurate kinds of contact analysis methods. In tables 2 and 3 Behavior

parameters belonging to concrete and steel reinforcement are shown. Rubber material specification is

reported in Table 4.

Table 2: Reinforced concrete behavior parameters

Compressive Strength Reinforced Concrete

Unit Weight

Unreinforced

Concrete Unit Weight

Poisson Ratio

25 Mpa 24 kN/m3

23 kN/m3

0.17

Table 3: Reinforcement characteristics

Allowable Stress Ultimate Stress Yield Stress Steel Type

160 Mpa

500 Mpa 300 Mpa

A-II

Table 4: Rubber material specifications

Density Shear Modulus Poisson Ratio Mooney Rivelin

First Constant

Mooney Rivelin

Second Constant

590 kg/m3

940 kPa 0.49 293 kPa 177 kPa

5.3 Modeling the gap between lining and rock

In the first step the length of the failed lining during fault rupture is predicted. Then equal to the same

length a gap between rock and lining according to the strike slip fault movement is considered.

Considering theses parameters diameter of extra excavation in model was chosen one meter more

than outer diameter of the lining (Figure 5).

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-500

-400

-300

-200

-100

0

100

200

300

0 50 100 150 200 250 300

Tunnel Section

Clockwise

Moment

Without isolatio

Gap Isolation

Thin Long Isola

Thick Short Iso

Thick Long Isol

Figure 10: Distribution of flexural moment in five cases

In Figures 10, the axial force and flexural moment reach to the maximum amount in spring line, butFigure 9 shows that the maximum of shear force are appeared in crown and invert of tunnel.

Overall result of Figures 9 and 10 shows that relative to unprotected case, the thick long rubber

coating shows the most efficient force reduction and in this condition the minimum stress exists in the

tunnel section. Amount of stresses in the lining sections satisfy ACI 318 provisions.

7. LINING DISPLACEMENTS DURING FAULT RUPTURE

In this section lateral displacement of lining parallel to fault rupture direction vector in all the

length of tunnel is monitored (Figure 11). In order to analyze that does the strike slip nature

of fault impose the displacement in shear direction, therefore in this section the displacementin lining and rock at the point, which the spring line is under contact pressure by rock, is

monitored (Figure 12). Figure 11 and 12 show that the least amount of displacement in lining

and rock occurs at the time of using the thick long rubber.

-4.00E-02

-2.00E-02

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

0 10 20 30 40 50 60 70

Tunnel length(m)

LiningDisplace

ment

Without isolation

Gap Isolation

Thin Long Rubber Isolator

Thick Short Rubber Isolator

Thick Long Rubber Isolator

0o

90 o

270 o

180 o

Figure 11: Displacement in lining imposed by fault rupture in spring line

(kN

.m)

(m)

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Transfered Displacement into Rock

in Different Isolator Types

-5.00E-03

0.00E+00

5.00E-03

1.00E-02

1.50E-02

2.00E-02

2.50E-02

3.00E-02

3.50E-02

0 10 20 30 40 50 60 70

Tunnel Lenght(m)

LiningDisplacement

Bare Tunnel

Gap Isolation

Thick Long Rubber

Thick short Rubber

Thin Long Rubber

Figure 12: Displacement in rock imposed by fault rupture in spring line

8. CONCLUSION

Composite linings of rubber and concrete, in case of suitable design and execution can efficiently

absorb the energy released by Earthquakes. It is possible to reduce considerably the effect of

earthquakes using rubber isolators between lining and rock. The results of investigations for the

design of Gavoshan tunnel in crossing point by MORVARID Fault, extra excavation by 4 meters

width and 20 meters long and injection of rubber material between rock and lining is suggested .

REFERENCES

ASCE, 2001, Seismic Design of Underground Nuclear Waste Repositories.Bouvard-leconet A. & Colombet G. & Esteulle F., 1993, Ouvrages souterrains: conception-realistion-

entretien, France; [translated by Behnia, A. H.; 1998, Tehran university pub., Iran].

Brown E.T., 1987, Analytical and computational methods in engineering rock mechanics, Allen &

Unwin Ltd, London, UK.

Keshvarian K., 2003, Seismic isolation of tunnels, M.Sc. thesis, Sahand University of Technology,

Tabriz, Iran.

Kim & Konagai, 2001, Seismic Isolation of Tunnels Covered with Coating Material, Tunneling and

under Ground Space Technology, Vol.15, No.4.

Kojima K. & Kawabata A., 2001,Optimization of Soil Improvement For Seismic Isolation Design of

Urban Tunnels, Modern Tunneling Science and Technology, Japan.

Mahab ghodss consulting engineers, 1997, Second phase study of Gavoshan water conveyance tunnel

and related access tunnels, Geological and Rock mechanics Report.Wang J.N., 1993, Seismic Design of Tunnels, Parsons Brinckerhoff Inc, New York.

Wells and Coppersmith, 1994, Assessment Fault Rupture Hazard, BSSA.

(m)

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