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This article was downloaded by: [University of California, San Francisco] On: 19 December 2014, At: 02:21 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Marine Georesources & Geotechnology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/umgt20 Hydraulic Significance of Fractured Zones in Subsea Tunnels Jong-Ho Shin a , Kyu-Cheol Choi b , Jae-Ung Yoon a & Young-Jin Shin c a Department of Civil Engineering , Konkuk University , Seoul , Korea b Dodam E&C Co. Ltd. , Seoul , Korea c Department Civil Works Division , Samsung C&T , Seoul , Korea Published online: 25 May 2011. To cite this article: Jong-Ho Shin , Kyu-Cheol Choi , Jae-Ung Yoon & Young-Jin Shin (2011) Hydraulic Significance of Fractured Zones in Subsea Tunnels, Marine Georesources & Geotechnology, 29:3, 230-247, DOI: 10.1080/1064119X.2011.555712 To link to this article: http://dx.doi.org/10.1080/1064119X.2011.555712 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms- and-conditions

Hydraulic Significance of Fractured Zones in Subsea Tunnels

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Page 1: Hydraulic Significance of Fractured Zones in Subsea Tunnels

This article was downloaded by: [University of California, San Francisco]On: 19 December 2014, At: 02:21Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Marine Georesources & GeotechnologyPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/umgt20

Hydraulic Significance of FracturedZones in Subsea TunnelsJong-Ho Shin a , Kyu-Cheol Choi b , Jae-Ung Yoon a & Young-Jin Shin ca Department of Civil Engineering , Konkuk University , Seoul , Koreab Dodam E&C Co. Ltd. , Seoul , Koreac Department Civil Works Division , Samsung C&T , Seoul , KoreaPublished online: 25 May 2011.

To cite this article: Jong-Ho Shin , Kyu-Cheol Choi , Jae-Ung Yoon & Young-Jin Shin (2011) HydraulicSignificance of Fractured Zones in Subsea Tunnels, Marine Georesources & Geotechnology, 29:3,230-247, DOI: 10.1080/1064119X.2011.555712

To link to this article: http://dx.doi.org/10.1080/1064119X.2011.555712

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Hydraulic Significance of Fractured Zones in Subsea Tunnels

Hydraulic Significance of Fractured Zonesin Subsea Tunnels

JONG-HO SHIN1, KYU-CHEOL CHOI2,JAE-UNG YOON1, AND YOUNG-JIN SHIN3

1Department of Civil Engineering, Konkuk University, Seoul, Korea2Dodam E&C Co. Ltd., Seoul, Korea3Department Civil Works Division, Samsung C&T, Seoul, Korea

This article presents the effect of fractured zones on subsea tunnels. Generally, asubsea tunnel is designed to support high water pressure so that the hydraulic con-dition is of fundamental interest. In cases where fractured zones exist along the tun-nel route, attention should be paid to it at the design stage. However, there is notmuch information about the hydraulic influences on the fractured zones. Pore waterpressure and inflow rates are the main hydraulic factors to be considered in thedesign. In this article, the hydraulic effects on the fractured zones of a subsea tunnelunder construction and during operation have been investigated using a numericalmethod and small scale model tests. The Vardø tunnel, which is a subsea tunnel inNorway (Grønhaug and Lynnegerg, 1984), is used for the numerical modeling.Based on the analysis results, the significance of fractured zones in subsea tunnelsis identified.

Keywords fractured zone, hydraulic behavior, numerical method, subsea, tunnel

Introduction

Subsea tunneling has lately become a subject of special interest. Fresh bedrock hasnormally been preferred because of the high water pressure environments. In the casewhere the tunnel heads are faced with fractured zones, a large amount of inflow intothe excavation may occur and cause collapse due to scouring and high water pressureon the lining. Such an example can be found in the experience of the construction ofthe Norwegian subsea tunnels. In this sense, the effect of inflow rate and water press-ure due to a fractured zone is of fundamental importance on construction and duringoperation in subsea tunneling. Several studies have been conducted in order to inves-tigate the hydraulic behavior of underwater tunneling so far. The effect of waterpressure on the tunnel lining has been studied (Curtis et al. 1976; Ward and Pender1981; Atkinson and Mair 1983; O’Rourke 1984; Fernandez 1994). Design factors forsubsea tunnels have been reviewed (Kirkland 1986; Kitamura 1986; Eisenstein 1994).

Received 26 April 2010; accepted 9 December 2010.This work was supported by the Korea Science and Research Foundation under the

R01-2008-000-20109-0 Project. The support is gratefully acknowledged.Address correspondence to Young-Jin Shin, Department Civil Works Division, Samsung

C&T, Seoul, Korea. E-mail: [email protected]

Marine Georesources and Geotechnology, 29:230–247, 2011Copyright # Taylor & Francis Group, LLCISSN: 1064-119X print=1521-0618 onlineDOI: 10.1080/1064119X.2011.555712

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Stability of hard rock subsea tunnels has been analyzed (Olsen and Blindheim 1989;Dahlø and Nilsen 1992; Nilsen 1994; Nilsen and Palmstrøm 2001). This article buildson the previous work. The Vardø tunnel, which is a subsea tunnel in Norway(Grønhaug and Lynneberg 1984), and a hypothetical circular tunnel are used forthe numerical modeling in this article. The effect of the fractured zone on the hydraulicbehavior of subsea tunnels under construction and during operation has been inves-tigated and compared using numerical methods and small scale model tests. Waterpressure and inflow rate are the main focus. A number of numerical analyses havebeen carried out based on the following conditions: deep tunnel and groundwater flowwith steady-state. A finite element program Midas=GTS (Midas Information Tech-nology Co. 2005) has been used for all the analyses. The software is a multi-purposeFEM that can address 2- and 3-dimensional continuum problems with coupledgroundwater flow and ground deformations. Several analysis cases have been add-ressed for examining hydraulic behaviors such as (1) the effect of the fractured zone,(2) the effect of relative permeability, and (3) the effect of the hydraulic boundary.

Numerical Modeling of Subsea Tunnels

During tunneling works, excavation causes changes in effective stress as well as waterpressure distribution. Therefore, a coupled analysis is generally appropriate in anumerical analysis, where both water pressure and effective stress are considered.However, only the seepage analysis has been considered in this article, and hasfocused on the hydraulic behavior. The governing equation utilized (Strack 1989)is represented as:

@

@xkx

@H

@x

� �þ @

@yky

@H

@y

� �þ @

@zkz

@H

@z

� �þQ ¼ @h

@tð1Þ

where, H: total head, kx, ky, kz: permeability in x, y and z directions, Q: external flowrate, h: volumetric water content and t: time.

The Galerkin method, which is the finite element method utilized in this article,is represented as:

½K �fHg þ ½M�fHgt ¼ fQg ð2Þ

where, fHg: vectors of total head, [K]: permeability matrix, [M]: mass matrix andfQg: external flow rate vector.

Equation (2) represents transient as well as steady flow behavior. However, thisarticle is only concerned with steady state flow. The hydraulic boundary of the tun-nel was prescribed as full drainage, and was then gradually changed to non-drainage.A lining model considering relative permeability between ground and lining was usedas shown in Figure 1 (Shin et al. 2002). The finite element program Midas=GTS(Midas Information Technology Co. 2005) was used.

Hydraulic Significance of Fractured Zones during Construction atTunnel Heading

The main features of subsea tunnels that differ from those of land tunnels are thehigh water pressure condition or the high seepage gradient condition. The high water

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pressures reduce effective stresses around the tunnel, resulting in lower arching. Thehigh seepage gradients may result in critical inflows of water in the ground(Eisenstein 1994). These flow behaviors around the subsea tunnels are governedby the hydraulic conditions and hydraulic properties of materials involved in theseepage routes such as the ground, lining, and drainage system (Shin et al. 2009).Therefore, if fractured zones exist, they may influence the hydraulic behavior ofthe subsea tunnel. It is because the permeability of the fracture zone is significantlyhigher than the surrounding bedrock that the groundwater may flow through thiszone. Additionally, if a tunnel lining runs through the fractured zone, the waterpressure may be concentrated on the lining. Figure 2 presents diagrams of sea waterinflow when a tunnel passes through a fractured zone. In section (a), the water press-ure is concentrated on some part of the lining, while in section (b) the fractured zonelies across the entire tunnel face and the inflow of sea water towards the tunnel facemay cause instability of the tunnel.

Figure 2. Diagram of sea water inflow through fractured zones.

Figure 1. Tunnel lining model considering permeability (Shin et al. 2002).

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In the subsea tunnelings in Norway, several serious instability cases happenedduring subsea tunneling, and even a total collapse occurred due to its fractured zone(Olsen and Blindheim 1989; Dahlø and Nilsen 1992; Nilsen 1994; Nilsen andPalmstrøm 2001). At Ellingsøy road tunnel, the fault zone contained swelling clay.The water-bearing fissures caused a cave-in reaching up to about 7m above the tun-nel crown. At Vardø tunnel, a similar cave-in with propagation about 7m above thetunnel crown occurred (Nilsen 1994). In most of these cases, extensive dislocationsand fractured zones had been found. In this article, the effect of the fractured zoneon a subsea tunnel under construction and during operation has been investigatedand compared using the numerical method. A 3-dimentional tunnel seepage analysisrunning though ground with a fractured zone lying in front of a tunnel face was con-ducted and the mesh was generated based on the Vardø subsea tunnel (Grønhaugand Lynnegerg 1984). Homogeneous, isotropic continuum medium were assumedfor the fractured zone and the bedrock. The hydraulic effect due to the fracturedzone has been analyzed by comparing the water pressure development and the inflowrate in the sections directly effected to the fractured zone and not affected by it. Thesections affected by the fractured zone indicate the sections close to tunnel face run-ning through the fractured zone and vice versa. They make a definite difference dueto the fractured zone effect. Figure 3 presents the longitudinal section of the Vardøtunnel (Grønhaug and Lynnegerg, 1984).

Model Tunnel

The model tunnel considered in this study is shown in Figure 4. The tunnel is ahorse-shoe type, the same as the Vardø tunnel case, which is 8m high and 13m widewith a lining thickness of 0.5m, as shown in Figure 4. The mesh used was establishedas 400m wide and 130m high, and extended to 560m longitudinally to eliminate theinfluence of boundaries. The fractured zone with an angle of 30� was applied on thebasis of the Vardø tunnel case. The Mohr-Coulomb model and elastic model wereadopted for the modeling of ground and lining behavior, respectively. Figure 4shows the diagrams of cross and longitudinal sections of the modeling. The perme-abilities used are indicated in Figure 4(a).

Figure 3. Longitudinal section of Vardø tunnel (Grønhaug and Lynnegerg 1984).

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Analysis Cases

In order to identify the hydraulic behavior of the subsea tunnel passing through thefractured zone, 5 cases with water levels ranging from 20m to 100m were analyzed(see Figure 5(a)). The depth of tunnel was fixed at 50m below the ground surface.

Figure 4. Mesh used.

Figure 5. Tunnel profile.

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Table

1.Aanalysiscase

(Constructionstage)

Waterlevel

(m)

Cover

depth

(m)

Permeability

ratio(k

l=ks)

20

40

60

80

100

50

1C50W20K1

C50W40K1

C50W60K1

C50W80K1

C50W100K1

0.1

C50W20K0.1

C50W40K0.1

C50W60K0.1

C50W80K0.1

C50W100K0.1

0.01

C50W20K0.01

C50W40K0.01

C50W60K0.01

C50W80K0.01

C50W100K0.01

0.001

C50W20K0.001

C50W40K0.001

C50W60K0.001

C50W80K0.001

C50W100K0.001

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The initial hydrostatic condition was assumed in the ground while the tunnel wasprescribed as a zero water pressure condition (free discharge). In the seepage analy-sis, the permeability for the ground and lining were prescribed. Constant per-meability is assumed in this article. Analysis cases with lining=ground permeability(kl=ks) of 1, 0.1, 0.01 and 0.001 were considered, while the unlined tunnel face main-tained during construction (see Figure 5(b)). These are used for simulating thedeterioration of hydraulic performance in the drainage system and consequentchanges of water pressure on the lining, which can actually occur in real tunnels.Table 1 shows the analysis cases and symbols presenting the depth of soil cover fromthe tunnel crown (C), the water level from the seabed surface (W), and the lining=ground permeability (K).

Results

Figures 6 and 7 show the water pressure via distance from the tunnel face. InFigure 6(a), when the lining=ground permeability (kl=ks) is unity (fully drained con-dition), the maximum water pressure is around 380 kPa at the vicinity of the tunnelface running through the fractured zone. As the water level rises from 20m to 100m,the maximum water pressure changes from 380 kPa to around 800 kPa (seeFigures 6(a) and 7(a)). Water pressure around the tunnel face radically developsdue to the rush of sea water inside the tunnel, and these sections can be regardedas an area affected by the fractured zone. The water pressure shows drastic reductionafter 30m from the tunnel face, and then gradually converges after 50m. Thus thesections over 50m away from the tunnel face can be considered as areas not affectedby the fractured zone. In these sections, seepage forces develop rather than waterpressure. However, as the performance of the lining drainage system deteriorates,where the permeability (kl=ks) reaches 0.01 (partially drained condition), the maxi-mum water pressure increases to around 600 kPa and 1200 kPa (see Figures 6(b)and 7(b)), which gets very close to the hydrostatic water pressure. The hydraulicbehavior approaches non-drainage with the lowering of permeability and theresidual water pressure develops at sections that are both affected and not affected

Figure 6. The water pressure distribution (C50W20).

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by the fractured zone. Therefore, it can be said that the water pressure acting on thelining significantly increases with the rise of water level and deterioration of drainageperformance. Particularly in the location affected by the fractured zone, the waterpressure on the lining is much higher than the water pressure not affected by the frac-tured zone in the fully drained case. However, in partially drained or non-drainageconditions, the effect of the fractured zone disappears.

Figure 8 shows the normalized water pressure in relation to the lining=groundpermeability (kl=ks) according to the distances from the fractured zone. Figure 8(a)presents the distribution of water pressure not affected by the fractured zone forminga stretched S-shaped curve. In the fully drained case (kl=ks¼ 1), less than 5% hydro-static water pressure acts on the lining. The water pressure, however, rises toapproximately 80% hydrostatic water pressure as the drainage performance is dete-riorated (kl=ks¼ 0.001). In the long term, the deterioration of drainage performancebrings about increased water pressure that can cause instability of the lining struc-ture. On the other hand, Figure 8(b) shows the distribution of normalized waterpressure affected by the fractured zone, and there is about 50% hydrostatic water

Figure 8. Normalized water pressure distribution.

Figure 7. The water pressure distribution (C50W100).

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pressure to the lining at the tunnel crown, even though the drainage works well at theearlier stages (kl=ks¼ 1). As the drainage performance worsens, the water pressureconverges to about 80% of the hydrostatic water pressure (kl=ks¼ 0.001), similarlyto the case of Figure 8(a). Thus, it can be mentioned that the lining=ground per-meability (kl=ks) as well as the existence of the fractured zone have a considerableinfluence on the development of water pressure on the tunnel lining and the effectincreases when the drainage performs well. Especially, according to Figures 6(a)and 7(a), about 45% hydrostatic water pressure can be reduced if the fractured zoneis predicted and the appropriate waterproof methods for the fractured zone areapplied. Hence comprehensive geological investigation and extensive mapping dur-ing tunneling are recommended. Based on the results of the investigation, the frac-tured zone can be identified and supported.

Figure 9(a) presents the amount of inflow with respect to the permeability atthe section not affected by the fractured zone (at 250m from the tunnel face), whileFigure 9(b) shows the amount of inflow at the section directly affected by the frac-tured zone (at 3m of the tunnel face). The distribution of the inflow shown inFigure 9 is of the inverse shaped curve compared to the water pressure distributioncurve shown in Figure 8. In other words, the amount of inflow is also affected bythe fractured zone similarly to the water pressure developments. Additionally, thewater level and the lining=ground permeability (kl=ks) affect the amount of inflow.According to Figure 9(a), the maximum inflow difference due to water level changeis around 2 times while the lining=ground permeability (kl=ks) makes it around 4times difference. Because of this view, the radius of the grouting zone was ration-ally optimized in the Seikan tunnel (Kitamura 1986). However, the existence of afractured zone makes it around 10 times difference (See figures 9(a) and 9(b)).Therefore, it can be learned that the presence of the fractured zone has a more sig-nificant effect on the amount of inflow than other factors such as permeability orwater depth. In terms of the construction of the subsea tunnel, the presence of thefractured zone may drastically increase the inflow of sea water, the occurrence ofan eruption or high water pressure, thus influencing the stability of the tunnel.Particularly when the support capacity of the bedrock is hardly expected and per-meability is considerably higher in the fractured zone, in-depth investigation andwaterproofing reinforcement such as pre-drainage holes or pre-grouting shouldbe considered.

Figure 9. The flow rate distribution.

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Hydraulic Significance of Fractured Zones during Operation

The hydraulic behavior of the fractured zone has significant effects not only on theconstruction stage but also on the long-term operation of subsea tunnels. In the casesof the Seikan tunnel in Japan or the Channel tunnel between UK and France, vari-ous hydraulic measures had been conducted in order to control the water pressure onthe lining caused by the high depth condition during construction (Kirkland 1986;Kitamura 1986). Even though measures had been attempted at the constructionstages, problems related to the groundwater were still reported during the operation.Hence, in this article, a parametric study has been conducted so that the hydraulicbehavior of the subsea tunnel currently working has been examined.

Model Tunnel

A circular tunnel with the diameter of 7.16m was set up and placed at 100m belowthe surface, which is the typical shape of subsea tunnel. The software program usedfor analysis is Midas=GTS (Midas Information Technology Co. 2005). A 2-dimen-sional seepage analyses was conducted. The fractured zone and bedrock have beenassumed as homogeneous, isotropic medium. Figure 10 presents the model used inthe analysis. To eliminate the influence of the boundary condition, the width ofthe ground model was set up as 400m, 50 times wider than the diameter of the tun-nel, and the depth below the tunnel was set up as 100m, 10 times deeper than thetunnel diameter. Fractured zone 1(J1) and fractured zone 2(J2), shown inFigure 10, have been considered. Fractured zone 1(J1) is located from the leftshoulder part (315�) and the lower part of the tunnel (180�) when the tunnel crownis the origin. Fractured zone 2(J2) lies across the crown (0�) and left invert part(225�). The tunnel lining system used in this study is comprised of primary lining,drainage layer and secondary lining, as shown in Figure 11(a). The external waterpressure on primary lining (p0) and the internal water pressure between the drainagelayer and the secondary lining (pi) develop depending on the drainage condition. The

Figure 10. The model utilized in 2-dimensional analysis.

Fractured Zones in Subsea Tunnels 239

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lining model considering the secondary lining was used for the parametric study isshown in Figure 11(b) (KISTEC 2007).

Analysis Cases

In order to identify the hydraulic behavior of the subsea tunnel passing through thefractured zone during long-term operation, parametric studies with joint=groundpermeability (kj=ks) of 1, 10, 100, and 1,000 were examined (see Table 2). Hydraulicconditions at the tunnel have been divided into: 1) peripheral drainage, and 2)drain-hole drainage, shown in Figure 12, and water levels of 20m, 60m, and100m were taken into account. Hydraulic conductivity used in this analysis is shownin Figure 10. The locations and number of fractured zones have been considered in 3

Table 2. Analysis case (Operation stage)

Water level (m)

Permeability ratio (kj=ks) 20 60 100

J1 1 J1W20K1 J1W60K1 J1W100K110 J1W20K10 J1W60K10 J1W100K10100 J1W20K100 J1W60K100 J1W100K1001000 J1W20K1000 J1W60K1000 J1W100K1000

J2 1 J2W20K1 J2W60K1 J2W100K110 J2W20K10 J2W60K10 J2W100K10100 J2W20K100 J2W60K100 J2W100K1001000 J2W20K1000 J2W60K1000 J2W100K1000

J12 1 J12W20K1 J12W60K1 J12W100K110 J12W20K10 J12W60K10 J12W100K10100 J12W20K100 J12W60K100 J12W100K1001000 J12W20K1000 J12W60K1000 J12W100K1000

Figure 11. The water pressure development on linings.

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different cases, namely J1, J2 and J12. Table 2 shows analysis cases and symbols.These are also used in a legend of the result interpretation. Symbols represent frac-tured zone 1 (J1), fractured zone 2 (J2), fractured zone 1 and 2 (J12), water level (W)and permeability of joint=ground (K).

The Peripheral Drainage

The peripheral drainage is the hypothetical state in which the entire tunnel perimeteris assumed to be a drainage membrane. It is generally used for the numerical analysisof a fully drained tunnel. The water pressure of the primary lining (p0) and theamount of inflow under the peripheral drainage have been investigated.Figure 13(a) shows the distribution of water pressure in the J1 condition when thewater level is 20m (W20). As the permeability of the fractured zone graduallyincreases, the water pressure acting on the lining also rises. At points where the tun-nel lining interacts with the fractured zone (180� and 315�), the concentration of seawater flow takes place due to a relatively higher permeability in comparison to thebedrock and the tunnel lining such that the amount of inflow increases. Thus, a con-siderably higher water pressure can be found in these places. Approximately 30%hydrostatic water pressure is applied to the lining until kj=ks¼ 100 and when itreaches kj=ks¼ 1000, the water pressure increases to 58% hydrostatic water pressure.Figure 13(b) represents the distribution of water pressure in the J2 condition whenthe water level is 20m (W20). In addition to the different positioning of the fracturedzone, it shows very similar results with J1. In the case of J12, fractured zone 1 andfractured zone 2 are installed together and these are placed at 0�, 180�, 225� and315�. While 58% of hydrostatic water pressure is generated for J1 or J2, it increasesto 65% in J12, as shown in Figure 13(c). The 7% difference, even though slight,shows that the number of fractured zones can also influence the water pressure devel-opment. Figure 13(d) shows the amounts of inflow under the peripheral drainage inthe cases of J1, J2 and J12 (W20 and W100). While the amounts of inflow are similarin the cases of J1 and J2, the inflow of J12 is higher by a maximum 150% comparedto J1 and J2. A similarity can be found at the inflow trend and the water pressuredevelopment. The change is gradual until the permeability (kj=ks) reaches 100, andthen when it reaches 1000, the amount of inflow rises up rapidly. Therefore, it canbe stated that the permeability also has a considerable effect on the amount of inflow

Figure 12. Hydraulic boundary of tunnel.

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so that the control of permeability in the fractured zone seems to be instrumental tothe control of inflow amount inside the tunnel.

The Drain-Hole Drainage

In general, peripheral drainage is an assumption for the tunnel design. However, in asubsea tunnel, where a high water pressure condition occurs, especially when con-sidering the effect of a fractured zone, such an assumption is not appropriate. Thedrain-hole drainage is more realistic. The drain-holes are installed at the tunnelinvert and the groundwater inside the tunnel is therefore drained out only at theinvert. In order to simulate such behavior, the drain-holes are installed at the loca-tions of 135� and 225�, as seen in Figure 12(b). The water pressure on the lining andthe amount of inflow through the drain-hole drainage have been investigated. InFigure 14(a), normalized water pressure of the primary lining (p0) is shown. Similarto the peripheral drainage (see Figure 13(a)), the water pressure increases when it

Figure 13. Results for peripheral drainage.

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reaches 180� and 315� where the tunnel meets the fractured zone. Furthermore,Figure 14(b) shows the internal water pressure applied on the secondary lining(pi). Because the drainage layer is installed between the primary and secondary liningand the hydraulic boundary of the drainage holes, the water pressure is rendered zeroat 135� and 225� of the lining. The gap between the external water pressure and inter-nal water pressure is the net water pressure, described in Figure 14(c). If the joint=ground permeability (kj=ks) is within the range of 1–10, the water pressures inducedare similar to each other, but if it increases to the range of 100–1000, the water press-ure abruptly increases to about 50% of the hydrostatic water pressure. The J2 casepresents a somewhat different net water pressure compared to that of J1, and this

Figure 14. Results for drain-hole drainage.

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figure reaches approximately 70% of the hydrostatic water pressure, as seen inFigure 14(d). This is because the locations of the fractured zone and drainage holecoincide at the location of 225�. The groundwater running through the fracturedzone therefore directly flows out to the drainage hole. It can be mentioned at thispoint that relative positioning of the fractured zone and drainage hole is one ofthe factors that affects the water pressure development. In the case of J12, amaximum of 80% hydrostatic water pressure is produced at the lining adjoining tothe fractured zones due to the effect of two fractured zones running through the tun-nel (see Figure 14(e)). Figure 14(f) shows the amount of inflow into the tunnel underthe drain-hole drainage. Unlike the peripheral drainage, drainage just occursthrough the drain-holes. It is shown that the amount of inflow at the drain-holes caseappears to be smaller than that peripheral case. However, as the amount of inflowconcentrates in the drain-holes, a relatively higher water pressure develops comparedto the peripheral drainage (see Figures 13(a) and 14(a)). In the case of the peripheraldrainage, the water pressure reaches a maximum of 60% hydrostatic water pressure,while presenting a maximum 80% hydrostatic water pressure at drain-hole drainage.

Discussions

A model test considering the J1 fractured case with drain-hole drainage has beenconducted in order to validate the numerical analyses. Figure 15 shows the schematicdiagram of the model test. The exterior box is made of a 15mm thick acryl plate inconsideration of high water pressure development and the size of the boxis 400mm� 150mm� 670mm. The ground in the box is made of plaster and thefractured zone is simplified as a 5mm groove, so that this test presents the case of

Figure 15. The schematic diagram of the model test.

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permeability of joint is much higher than that of ground (kj=ks). The water levelinside the box is controlled by means of the air pressure on the water surface. A frac-tured zone is placed at 42� from the crown in a counter-clockwise direction. A drain-age membrane is attached around the tunnel and a drain-hole is installed at thetunnel invert. The water pressures are monitored at 5 points around the tunnel,and the amount of inflow into the tunnel is measured. A set of equipment for thetest is comprised of a measuring system, a test model, an air regulator, and an aircompressor. The air compressor has a maximum capacity of 1MPa, and an air regu-lator can control the air from the compressor to a maximum of 1 kPa. In the test, thetotal head was fixed as 0.4m at the tunnel crown. The water pressure applied on thetunnel was measured while increasing the air pressure in the range of 0, 5, 10, 15, and20 kPa. The amount of inflow was measured using a micro chronometer and a cyl-inder. The water pressure distribution on the tunnel lining was measured at 5 check-ing points, as shown in Figure 16(a). The results show the same trend as thenumerical results. The water pressure reaches the highest level at the fracture zonedue to concentration of inflow (see Figure 14(a) and 16(a)). It is rendered zero atthe drain-hole because of free drainage (see Figures 14(b) and 16(a)). This behaviorbecomes clearer when the water level increases, showing the effect of the fracturedzone. The inflow amount inside the tunnel also increases with the rise of water level,as shown in Figures 14(f) and 16(b). Based on the results from numerical method andmodel test, it can be concluded that the hydraulic behavior of subsea tunnels is affec-ted by many factors. Among them, this revealed that the most influencing factor is afractured zone that is characterized by the relative permeability, the orientation, andthe number of fractures. This coincides with the fact that the main stability problemsin subsea tunneling were associated with zones of weakness (Nilsen 1994). Therefore,it should be emphasized that stability problems can be prevented by comprehensivegeological investigations of the site. Mapping during tunneling, probe drilling, andgeophysical testing may be carried out during construction. The hydraulic optimiza-tion by controlling the amount of inflow and water pressure acting on the liningshould be also considered during operation (Shin et al. 2009).

Conclusions

In this article, the hydraulic behavior of a subsea tunnel that is intersected by a frac-tured zone has been studied. Major considerations are the water pressure on the

Figure 16. Results of model test.

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lining and the amount of water flow inside the tunnel. Numerical analyses in con-sideration of different hydraulic conditions have been conducted. A 3-dimensionalseepage analyses based on an actual tunnel collapse have been performed and ana-lyzed in order to simulate the hydraulic effects of fractured zones during construc-tion. The hydraulic behavior of the subsea tunnel during operation has also beenexamined by 2-dimensional analysis in consideration of the location and distributionof the fractured zones. The numerical analyses have been validated by laboratoryscale model tests. It can be concluded that the hydraulic affect of the fractured zoneis significant in the construction and long-term management of a subsea tunnel.Detailed results are as follows:

1. The presence of fractured zones in a subsea tunnel may induce the inflow of seawater, an eruption, or high water pressure, thus influencing the stability of thetunnel.

2. Therefore, the tunnel design considering a fractured zone is necessary in subseatunneling. An economic design by optimization of the distribution of water press-ure and the inflow amount is recommended, and the effect of long-term hydraulicbehavior, such as deterioration of the drainage system, must be considered.

3. The water pressure on the lining and the amount of inflow are in inverse pro-portion. When significant amount of inflow occurs, a small water pressure devel-ops on the lining causing the increase in the seepage forces. Meanwhile, when theamount of inflow decreases, the residual water pressure may rise.

4. The numerical results present that the fractured zone, the water level, as well asthe relative permeability (kl=ks) affect the hydraulic behavior. Among theseeffects, the fractured zone itself has the most effect on the behavior. When thedrainage performance deteriorates, the fractured zone effect diminishes.

5. The amount of inflow into the tunnel increases linearly as the water depthincreases, showing that the inflow rate is inversely proportional to the distributionof water pressure. The presence of the fractured zone changes the hydraulic beha-vior due to surge of water flow.

6. With respect to the hydraulic boundary of the tunnel during operation, the frac-tured zone increases the water pressure development in both peripheral anddrain-hole drainage cases. The water pressure in peripheral drainage reaches amaximum of 60% hydrostatic water pressure, while presenting a maximum of80% hydrostatic water pressure in drain-hole drainage. This is because theamount of inflow concentrates in the drain-holes.

References

Atkinson, J. H. and R. J. Mair. 1983. Loads on leaking and watertight tunnel linings, sewersand buried pipes due to groundwater. Technical note: Geotechnique 33(3): 341–344.

Curtis, D. J. and A. M. Muir Wood. 1976. The circular tunnel in elastic ground. Geotechnique26(1): 231–237.

Dahlø, T. S. and B. Nilsen. 1992. Stability and rock cover of Norwegian hard rock sub-seatunnels. Norwegian Subsea Tunneling (8): 27–34.

Eisenstein, Z. D. 1994. Large subsea tunnels and the progress of tunneling technology. Tunnel-ing and Underground Space Technology 9(3): 283–192.

Fernandez, G. 1994. Behaviour of pressure tunnels and guidelines for liner design. Journal ofGeotechnical Engineering 120(10): 1768–1791.

246 J.-H. Shin et al.

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Grønhaug, A. and T. E. Lynneberg. 1984. The Vardø subsea tunnel – a low cost project?Proceedings of International Symposium on Low Cost Road Tunnels, Tapir Publishers:185–203.

Kirkland, C. J. 1986. The proposed design of the English Channel tunnel. Tunneling andUnderground Space Technology 1: 271–282.

KISTEC (Korea Infrastructure Safety and Technology Corporation). 2007. Safety Evaluationand reinforcement of tunnels under residual pore-water pressure. Internal Report (inKorea).

Kitamura, A. 1986. Technical development for the Seikan tunnel. Tunneling and UndergroundSpace Technology 1: 431–450.

Midas Information Technology Co. 2005. Geotechnical and tunneling analysis system andUser’s manual.

Nilsen, B. 1994. Analysis of potential cave–in from fault zones in hard rock subsea tunnels.Rock Mechanics and Rock Engineering 27(2): 63–75.

Nilsen, B. and A. Palmstrøm. 2001. Stability and water leakage of hard rock Subsea Tunnels.Modern Tunneling Science and Technology, Proceedings of International Symposium,Kyoto: 497–502.

Olsen, A. B. and O. T. Blindheim. 1989. Prevention is better than cure. Tunnels and Tunnelling20(9): 41–44.

O’Rourke, T. D. 1984. Guidelines for Tunnel Lining Design. American Society of Civil Engi-neers. New York: 42–43.

Shin, J. H., T. I. Addenbrooke, and D. M. Potts. 2002. A numerical study of the effect ofground water movement on long-term tunnel behaviour. Geotechnique 52(6): 391–403.

Shin, H. S., D. J. Yoon, S. E. Chae, and J. H. Shin. 2009. Effective control of pore water pres-sures on tunnel linings using pin-hole drain method. Tunneling and Underground SpaceTechnology 24: 555–561.

Strack, D. I. 1989. Groundwater Mechanics. New Jersey: Prentice Hall.Ward, W. H. and M. J. Pender. 1981. Tunneling in soft ground-general report. Proceedings

of the 10th International Conference of Soil Mechanics and Foundation Engineering,Stockholm, 4: 261–275.

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