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Analysis of the Characteristics and Mechanism of Lining Cracking for a Tunnel Group in Schist
(1 School of Architecture and Engineering, Hebei University, Baoding 071002; 2 Gansu CSCEC Municipal Engineering Investigation and Design Institute, Lanzhou 730030; 3 Center for Hydrogeology and Environmental Geology, China Geological Survey, Baoding 071051)
Abstract Cracking in the secondary lining of tunnels in schist was found to be common when a survey of second? ary lining integrity was conducted for the tunnels of the Gucheng-Zhuxi expressway in Hubei province. To scientifically solve the problem of cracking, the following characteristics were summarized based on an investigation of the 48 tunnels along this line: 1) it is very common to see cracking in the secondary lining of tunnels in schist; 2) cracks are mainly located at the shallow-buried entrance and exit sections; 3) numbers of cracks in the first excavated tunnel are higher than that in the tunnel excavated later; 4) the cracking pattern is mainly oblique; 5) it is easier for the secondary lining to crack at places where large deformation and collapse occur; 6) and cracking occurs 5 months after the completion of the secondary lining. Based on the above experience, laboratory tests and site investigation, the mechanism and reasons for cracking of the secondary lining were studied and it was concluded that the schist is often soft when water is encountered due to its low strength; it is difficult to complete an arch at the shallow-buried section of the entrance; asymmetrical loading is very common; the settlements at the tunnel invert are uneven; and the rock is affected by underground water and the creep effect. The following suggestions and measures are presented: the grade of surrounding rock will be adjusted; much attention should be paid to construction at the entrance and the design of middle rock pillar stability of tunnels with small spaces.
Abstract：
Cracking in the secondary lining of tunnels in schist was found to be common when a survey of second? ary lining integrity was conducted for the tunnels of the Gucheng-Zhuxi expressway in Hubei province. To scientifically solve the problem of cracking, the following characteristics were summarized based on an investigation of the 48 tunnels along this line: 1) it is very common to see cracking in the secondary lining of tunnels in schist; 2) cracks are mainly located at the shallow-buried entrance and exit sections; 3) numbers of cracks in the first excavated tunnel are higher than that in the tunnel excavated later; 4) the cracking pattern is mainly oblique; 5) it is easier for the secondary lining to crack at places where large deformation and collapse occur; 6) and cracking occurs 5 months after the completion of the secondary lining. Based on the above experience, laboratory tests and site investigation, the mechanism and reasons for cracking of the secondary lining were studied and it was concluded that the schist is often soft when water is encountered due to its low strength; it is difficult to complete an arch at the shallow-buried section of the entrance; asymmetrical loading is very common; the settlements at the tunnel invert are uneven; and the rock is affected by underground water and the creep effect. The following suggestions and measures are presented: the grade of surrounding rock will be adjusted; much attention should be paid to construction at the entrance and the design of middle rock pillar stability of tunnels with small spaces.
2017, Vol. 54 