ANALYSIS OF BACK CAVITY IN HIGHWAY TUNNEL

1. ANALYSIS OF BACK CAVITY IN HIGHWAY TUNNEL Authors: J.F. Wang & H.W. Huang & X.Y. Xie Geotechnical Engineering Department, Tongji University, China

2. Problems 1 Hazard of cavity 2 Mechanical Analysis of Cavity Conclusion 3 5 Case study And Discussion 4 OUTLINE

3. PROBLEMS

4. PROBLEMS • In a sampling inspection on 4 highway tunnels: 29 sites are inspected, among which the cavity exist in 15 sites, the average height of the cavity is 14.5cm, the highest is 79cm.

5. Figure 1. Hazard of cavity On November 28, 1999, an accident in the Rebunhama Tunnel of the Muroran Main line PROBLEMS

6. HAZARD OF CAVITY • The stress state of the secondary lining and surrounding rocks will change, the cracks occurs more easily on the top of the secondary lining. • water leakage，freezing damage and reinforcement corrosion may appear . • The surrounding rocks will relax and deform, then spalling may happens.

7. Mechanical Analysis of Cavity (b) (a) (c) Figure 2. Schematic diagram of internal force of the secondary lining

8. Mechanical Analysis of Cavity • secondary lining Internal Force without cavity In figure 2: q1:the sum of surrounding rock pressure qw and the arch’s deadweight qz；q2： the lateral pressure of the surrounding rock The equation of force method is is redundant force. Where

9. Mechanical Analysis of Cavity The bending moment of vertical wall BC(AD) and the arch: Part BC(AD): Part CE(DE):

10. And Mechanical Analysis of Cavity In part CE(DE), suppose , , , We can find:

11. the skewback: the crown: the 1/4 section: Mechanical Analysis of Cavity Then we can get the redundant force Rx, and so the bending moment of any section： The axial force, shear force and bending moment of

12. Mechanical Analysis of Cavity • secondary lining Internal Force with cavity When there is cavity behind the lining, the surrounding rock pressure of the crown is zero, so ,the lateral pressure of the arch ring on the , height h1 is zero, which means q2 is also zero, so is:

13. Mechanical Analysis of Cavity So we can get the axial force, shear force and bending moment of the crown The internal force of skewback

14. Mechanical Analysis of Cavity The internal force of 1/4 section

15. CASE STUDY A road tunnel with a height h=6.79m, h0=-1.13m, h1=2.07m, h2=4.72m,R=7.88m,l=11m, , the thickness of the crown dd=50cm, the vertical wall thickness d1=40cm, concrete mark C20, the surrounding rock classification is Ⅲ, q1=115.2KN/m2, q2=0.3 qw=34.56KN/m2 .the results is shown in table 1.

16. CASE STUDY Table 1. Section internal force results

17. From the results we can find: The bending moment of the crown is sagging moment, which means the upper part of the lining is under tension, but there is no steel bar in it, that may result in leakage and erosion of the reinforcement, and then the lining may be destroyed. 1 DISCUSSION When there is back cavity, the surrounding rock’s supporting and protection is missing, so its relaxation and deformation will increase, and then the surrounding rock may lose its stability, flaking away and breaking. So prevention and treatment of back cavity must be taken. 2

18. Some treatment measures must be taken to relieve the damage caused by back cavity. 3 The bigger the cavity is, the more unfavourable of the lining stress; The worse the surrounding rock is, the more effect of the cavity’s size on lining internal force. 2 CONCLUSION When there are cavities behind the tunnel lining, the stress conditions of the secondary lining and surrounding rocks will change, and so cracks, leakage, sudden collapse may happen. 1

19. Thank You !