# TONGJI UNIVERSITY THE 5TH JAPAN-CHINA JOINT SEMINAR FOR THE GRADUATE STUDENTS 1 ANALYSIS OF BACK CAVITY IN HIGHWAY TUNNEL Authors: J.F. Wang & H.W. Huang.

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TONGJI UNIVERSITY THE 5TH JAPAN-CHINA JOINT SEMINAR FOR THE GRADUATE STUDENTS 1 ANALYSIS OF BACK CAVITY IN HIGHWAY TUNNEL Authors: J.F. Wang & H.W. Huang & X.Y. Xie Geotechnical Engineering Department, Tongji University, China

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 2 OUTLINE Problems 1 Hazard of cavity 2 Mechanical Analysis of Cavity 3 Case study And Discussion 4 Conclusion 5

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 3 PROBLEMS

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 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.

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 5 PROBLEMS On November 28, 1999, an accident in the Rebunhama Tunnel of the Muroran Main line Figure 1. Hazard of cavity

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 6 HAZARD OF CAVITY  water leakage ， freezing damage and reinforcement corrosion may appear.  The surrounding rocks will relax and deform, then spalling may happens.  The stress state of the secondary lining and surrounding rocks will change, the cracks occurs more easily on the top of the secondary lining.

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 7 Mechanical Analysis of Cavity (a) (b) (c) Figure 2. Schematic diagram of internal force of the secondary lining

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 8 Mechanical Analysis of Cavity In figure 2: q 1: the sum of surrounding rock pressure q w and the arch’s deadweight q z ； q 2 ： the lateral pressure of the surrounding rock The equation of force method is Where is redundant force.  secondary lining Internal Force without cavity

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 9 Mechanical Analysis of Cavity The bending moment of vertical wall BC(AD) and the arch: Part BC(AD): Part CE(DE):

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 10 Mechanical Analysis of Cavity In part CE(DE), suppose, We can find:,, And

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 11 Mechanical Analysis of Cavity Then we can get the redundant force R x, and so the bending moment of any section ： The axial force, shear force and bending moment of the skewback: the 1/4 section: the crown:

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 12 When there is cavity behind the lining, the surrounding rock pressure of the crown is zero, so Mechanical Analysis of Cavity  secondary lining Internal Force with cavity,the lateral pressure of the arch ring on the is: height h1 is zero, which means q2 is also zero, so,

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 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

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 14 Mechanical Analysis of Cavity The internal force of 1/4 section

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 15 CASE STUDY A road tunnel with a height h=6.79m, h 0 =-1.13m, h 1 =2.07m, h 2 =4.72m,R=7.88m,l=11m,, the thickness of the crown d d =50cm, the vertical wall thickness d 1 =40cm, concrete mark C 20, the surrounding rock classification is Ⅲ, q 1 =115.2KN/m 2, q 2 =0.3 q w =34.56KN/m 2.the results is shown in table 1.

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 16 Section location Without cavity R x (KN)N(KN)Q(KN)M (KN·m) Crown91.680326.3420-323.214 1/4 section400.581435.261-354.938 skewback687.068670.371817.700 With cavity R x (KN)N(KN)Q(KN) M (KN·m) Crown-55.728107.3950146.817 1/4 section113.09173.152374.659 skewback56.1553.685121.935 Table 1. Section internal force results CASE STUDY

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 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 2 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. DISCUSSION

The 5th China-Japan Joint Seminar For the Graduate Students in Civil Engineering 18 CONCLUSION 3 Some treatment measures must be taken to relieve the damage caused by back cavity. 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 2 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.

TONGJI UNIVERSITY THE 5TH JAPAN-CHINA JOINT SEMINAR FOR THE GRADUATE STUDENTS 19

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