Downloaded from ascelibrary.org by Chengdu University of ...hgycg.cdut.edu.cn/data/upload/1563439804108.pdfFigure 1 shows the portal section of a mountain tunnel according to China

Failure Mechanism of Tunnel Portal during Strong Earthquakes

X. Zhao1, 2, T. B. Li2, L. J. Tao3, S. Hou3, L. Y. Li1 and W. G. Qiu4

1Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Post Code.100124, Beijing; PH&FAX(86-10) 67391931; email: [email protected] 2State Key laboratory of Geohazard Prevention and Geoenvironment Protection, Dongsan Rd. 1, Chenghua District, Chengdu City, China 3Institute for Geotechnical Engineering and Underground Construction, Beijing University of Technology, Post Code.100124, Beijing, China; email: [email protected] 4Key Laboratory of Transportation Tunnel Engineering, Ministry of Education, School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China.

ABSTRACT

Tunnel portal section is one of the most easily damaged parts in mountain tunnel during strong earthquake. Tunnel opening is the only part exposed outside the surrounding rock. Failures of this part may block roads, which greatly influences the transportation of the local area. Examples are studied in this paper focus on failures on tunnel portals. Damage types are categorized. According to these damage types, influencing factors and mechanism of these damages are analyzed including thickness of cover layers, interfaces and discontinuities, construction methods. Suggestions of seismic resistance and reduction methods are proposed regarding these influencing factors. INTRODUCTION

Tunnels are underground structures which have long been assumed to have good earthquake-resistance during earthquakes as they are constrained by surrounding rock. But recent site investigations and research have shown that tunnel portal sections and tunnel body run across displaced faults are the two most possible parts of a tunnel that may be damaged severely during earthquakes especially strong earth quakes.

Tunnels portals are the only part exposed outside the surrounding rock. Figure 1 shows the portal section of a mountain tunnel according to China code for design of road tunnels, in which B is the width of tunnel excavation. It could be seen from this figure that by constructing the tunnel portal section, the original slope is disturbed, also the cover of the tunnel is very thin, which makes this part rather unstable compare with tunnel body. If failure happens on this part during an earthquake the road pass would be blocked or totally cut, which greatly influences the

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International Efforts in Lifeline Earthquake Engineering

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transportation of the local area. So that it is important to guarantee the stability of this section. Also, understanding on interacting mechanism of tunnel and slope helps repairmen and retrofit of this section. Thus, scholars and engineers could be able to work out new codes in designing this part of tunnel which is influenced by slopes.

1-2B

Gallary

Tunnel Portal Tunnel Entrance

Tunnel Portal Section Tunnel Body

Transitional Section

Figure 1. Tunnel Portal Section (China Code for Design of Road Tunnel, 2004).

DAMAGES OF TUNNEL PORTAL FAILURES

There are many ways in researching tunnel seismic performance and their damages during earthquake. Site investigations after an earthquake are an important way in collecting practical data. Many scholars carried out such investigations in different country, which provide typical examples of damaged portal area of tunnels. Some of these examples are listed in Table.1. Table 1. Examples of Tunnel Portal Failures.

Earthquakes Year Magnitude Damages

Wenchuan 2008 8.0 10 of 11 tunnels on Duwen highway are damaged on the portal section (Li, 2008).

Hsinchu-Taichung

1935 7.1 Some tunnels of the 8 old Sanyi tunnels were damaged at the portals (Hwang and Lu, 2006) .

Chi-Chi 1999 7.3 16 tunnels were found portal failure and 3 tunnels collapsed due to slope failure out of 57 tunnels (Wang, et al., 2001).

TYPES OF TUNNEL PORTAL FAILURES

By analyzing the examples collected, tunnel portal failures can be categorized as follows: Slope damage including collapse, slide, rockfall, cracks and deformation; Portal structure damages including collapse, cracks and deformation; Tunnel lining damage including cracks, collapse and deformations.

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Deformation on slopes. Slope may deform during earthquakes, though it is not failed but deformation of slopes would result in unbalanced pressure on tunnel structure, which lead to structure deformation. Slope collapse around tunnel opening. Collapse around tunnel openings are very severe damages as the debris would block ways, even bury the opening of the tunnels.

Figure 2. Failure of tunnel slope at Beichuan.

Figure 3. Rock fall at the entrance of Zagunao Tunnel.

As shown in Figure 2, the tunnel slope is collapsed during Wenchuan

Earthquake and the whole tunnel opening is buried under broken rocks, which result in way blocking. By strong seismic action, the front slope of Longdongzi Tunnel is severely damaged, too. The slope slides, rocks rush down the mountains and buried the right tunnel entrance (Zhou et al., 2010). Tunnel on No.149 part of Qingshui line during Chi-Chi earthquake is one of this typical failure, too (Wang et al., 2001).

Sometimes rock fall happens, though the whole slope is still stable, big blocks of rocks would also cut roads. As shown in Figure 3, during Wenchuan Earthquake, Zagunao Tunnel is blocked.

Deformation on tunnel portal structures. In such cases, deformation is formed on tunnel portal structure and lining. Though the slopes are still standing, but cracks have formed and are found. In Hsinchu-Taichung earthquake, serious cracks and

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deformation occurred to the portal structures and to most of the linings. But, there were no collapse of the tunnels reported, which meant that the tunnels were repairable (Hwang and Lu, 2006). In Wenchuan earthquake, 11 tunnels were investigated on Duwen high way, 4 of them met with cracks on portal structure of on the slopes and cracks on Taoguan Tunnel portal is as wide as 50cm (Li, 2008).

Deformation on tunnel lining. Except for tunnel portal structure, tunnel lining may be deformed due to deformation of slopes. In some cases tunnel lining were deformed that cracks and spalling were found at the near-portal area of. Compare with slope deformation, linning deformation are less important. MECHANISM AND INFLUENCING FACTORS OF PORTAL FAILURES

By analyzing the examples, tunnel portal sections are easily to be damaged are closely related to their characteristics both in engineering geology and structure. There are many influencing factors and the mechanism of failure is related to these factors which could be concluded as follows: Thickness of cover layer. In common cases the cover on a tunnel around the tunnel portal section is often thin. In general, if a tunnel is not connected with special weak geological conditions including faults and soft layers, the amount and degree of damage decreases while the thinness of tunnel cover increases. Sharma (1991) concluded that while the cover of tunnels is over 50m in thickness, seismic damages would be alleviated. For tunnel portal sections, the cover is always under this limitation, as shown in figure 1. In this section, as the cover layer is thin, restraint from surrounding rock is weak, which is similar to the effects of risen the density of lining material, that the seismic inertial force on the lining is also raised, which may exceed the limitation of the material’s capacity(Wang et al., 2003). Also, those rocks are always loose and weathered in this area which is not conducive to its stability.

As in these cases, slope-structure interaction is one of the key factors to study. During earthquake, the sliding zone of a slope forms. This zone influences the tunnel structure very much. While the zone passes the tunnel lining, the lining would deform such as crack and bending directly under the shear deformation of the surrounding soil and rock. And the deformation of tunnel structure would further facilitate the failure of slope. If a clear sliding zone is not formed during the earthquake, deformation may act in the same way, but with less damage degree with the inclination of failure in surrounding soil and rock.

There is also one kind of damage need to be discussed is that, sometimes very high slopes fail from high mountains. This is not related to slope-structure interaction but only to slope itself. Collapse, slopes sliding and rock falls are sometimes of this kind. By analyzing these problems, slope can be set as the studying object, but the influences of the debris should be studied as they would also damage tunnel portal structure severely and would block way.



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Interfaces and discontinuities. Interfaces between surrounding rock and structures such tunnel portal and tunnel lining exists in this section. Also, there are free surfaces of front slope and side slopes around the tunnel opening. While earthquake happens, seismic wave reflects and refracts on these interfaces and surfaces, the acceleration are amplified according to that of the bed rock which result in big deformation in surrounding rock and structure. Construction methods. In constructing tunnel portals, original slopes are always cut in traditional constructing methods, and then portal structures are built. This is the so called traditional method of “early out and late in”. By this method the slopes at the entrance at the top of the hill were cut and were less stable; the lining was damaged because of the slope instability during the earthquake (Chen, 2011). Even by modern construction methods which emphasis on protecting the slope stabilities, slopes are inevitability disturbed and in tunnel excavating which results in the second stress field may induce instability of slopes. In this procedure, if the slope is disturbed to certain extend, hidden danger may be left and slope failure may happen in future. But by site investigation, that new tunnel portal structures such pipe shaped tunnel portals, are more likely to be intact compare to the tradition wall type tunnel portals (Li, 2008). These methods are always connected with the concept of “early out and early in” which is a rule in the Chinese tunneling code, which would be preferable. METHODS OF SEISMIC RESISTANCE AND REDUCTION

By analyzing the examples, some methods of seismic resistance and reduction could be suggested here. Construction methods. Adopt new type tunnel portals, whose construction protects the original stability of slopes. In constructing this part, work should be careful to keep the tunnel slope stable. Seismic reduction methods. As damages of tunnel portal section is related to the stiffness difference between tunnel lining, tunnel portal and surrounding rock, measures of reducing reflection and refraction between these interfaces should be considered. Soft layers of rubber or other materials may be emplaced between the first lining and the second lining or between the lining and rocks. Seismic resistance methods. For weak rock surrounding tunnel structures, reinforcement measures should be taken, bolts or grouting can be used to unite the shrouding rock and the tunnel portal structures as a whole and improve the mechanical performance or rock. It is concluded by Ji (2009) that grouting improves the seismic resistance of tunnels during Wenchuan earthauake. On slopes, prestressed rock bolts and grid should used to keep the stability. Example show that at tunnel entrance of Longdongzi tunnel, during Wenchuan earthquake, the slope with prestressed bolts and reinforced concrete grid is kept well in stability while the bridge



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nearby is damaged severely (Zhou, 2010). This example shown that, reinforcement is very efficiency on this slope during strong earthquake. CONCLUSION

Tunnels portals are the only part exposed outside the surrounding rock, which is one of the most easily damaged parts in mountain tunnel during strong earthquake. Examples are studied in this paper and conclusions are:

1) The main damage types in this section can be categorized as Deformation on slopes，deformation on tunnel portal structures and slope collapse around tunnel opening. Among these damages, slope collapse is the most sever disaster and is hard to repair.

2) Tunnel portal sections are easily to be damaged are closely related to their characteristics both in engineering geology and structure. The influencing factors in this section are thickness of cover layer, Interfaces and discontinuities and construction methods. Mechanisms of failure are closely related to these factors.

3) For reducing the damages, engineering performance could be improved by

selecting Construction methods and adopt seismic reduction methods and seismic resistance methods. These methods have been proved to be efficient by practice. ACKNOWLEDGEMENTS

This study is sponsored by the National Natural Science Foundation of China (41202221, 51038009), State Key laboratory of Geohazard Prevention and Geoenvironment Protection (SKLGP2012K002) and Beijing Municipal Commission of Education (KM201210005027). REFERENCES Chen Z., Shi C. and Yuan Y., (2011). “Damage characteristics and influence factors of

mountain tunnels under strong earthquakes.” Natural Hazards (online), 21.Aug

Chongqing Institute of Transportation Research and Design. 2004. China code for design of road tunnel: 168. Beijing: China Communications Press .

Huang J.H. and Lu C.C. (2007). “Seismic capacity assessment of old Sanyi railway tunnels,” Tunneling and Underground Space Technology, 22, 433-449. (In Chinese)

Ji, S. W., Tang, Y. J., and Hu, D, G. (2009). “Analysis of typical seismic damages of highways in Wenchuan Earthquake-induced hazard areas in Sichuan Province.” Chinese Journal of Rock Mechanics and Engineering, 28(6), 1250-1260. (In Chinese)



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Li, T. (2008). “Failure characteristics and influence factor analysis of mountain tunnels at epicenter zones of great Wenchuan earthquake.” Journal of Engineering Geology, 16(6), 742-750. (In Chinese)

Sharma, S., and Judd, W. R. (1991). “Underground opening damage from earthquakes” Engineering Geology, 30(3-4), 263-276.

Wang W. L., Wang T. T. and Su T. T., etc. (2001). “Assessment of damage in mountain tunnels due to the Taiwan Chi-Chi Earthquake. ” Tunneling and Underground Space Technology, (16), 133-150

Wang, X. Y., Liu, W, N., and Zhang, M. (2003). “Study on the Categorization and Mechanism of Seismic Damage of Underground Structures.” China Safety Science Journal, 13(11), 55-58. (In Chinese)

Zhou, D. P., Zhang, J. J., and Tang, Y. (2010). “Seismic damage analysis of road slopes in Wenchuan Earthquake.” Chinese Journal of Rock Mechanics and Engineering, 29(3), 565-576. (In Chinese)



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Downloaded from ascelibrary.org by Chengdu University of ...hgycg.cdut.edu.cn/data/upload/1563439804108.pdfFigure 1 shows the portal section of a mountain tunnel according to China