Seismic Damage Mechanism Of Secondary Lining Of Mountain Tunnel

Tunnel engineering is an important part of transportation system. Accordingto its buried depth, tunnel can be divided into shallow-buried urban tunnel anddeep-buried mountain tunnel. It is widely accepted from past experiences thatmountain tunnels are considered to have higher seismic stability, therefore, thefollowing studies are limited. In the2008Wenchuan earthquke (M8.0), more than10mountain tunnels in service around the epicenter had been damaged seriouslyto need complicated repair and reinforcement. At present, the seismic-damagedmechanism on lining structure of mountain tunnel during earthquake has becamea difficult point in the discipline development. It restricted the quake-proof designof mountain tunnel structures. This paper aims at clarifying the seismic responseand damage-causing mechanism of secondary lining in deep-tunnels underearthquake. This is achieved by means of theoretical analysis methods andshaking table tests, as well as advanced numerical simulations with the systematicresearch. The results from the study were acquired as follows:1） According to the data from the on-site investigation，the modes ofearthquake damage to mountain tunnels located between Dujiangyan and Yingxiuhave been described in detail. It was found that the damage proportion ofsecondary lining in unreinforced concrete tunnels located in the poor geologicalconditions were large.2） By the equivalent simulation method, the attention was specifically onthe seismic damage mechanism of deep tunnels under earthquake when therewere voids behind the lining of tunnel crown or when the incident angle ofseismic wave are different, as well as when the tunnel was buried in differentground rigidity. From the numerical analyses, it was found that the voids abovethe lining have negative effect on the aseismic performance. Further more, it waslikely that the tunnel crown were raised up or wholly collapsed under earthquake.3） The analysis was applied to a real case, the Longxi tunnel located inSichuan. With poor geological conditions, a model experiment on the shakingtable was conducted to clarify the damage mechanism and assess tunnel aseismicperformance. It was revealed that actual earthquake damage was able to bereproduced by the model test, in which the seismic response sensitivity for thekey parts of lining changed under different loading conditions. Based on the testresults, the ultimate compressive strain in the C25concrete lining structure mayreach at450με.4） A dynamic compared experiment of reinforced concrete lining and plainconcrete lining were carried out on the shaking table. It’s essentially to verify theultimate response of unreinforced concrete lining is prone to collapse suddenlyduring the strong motion. Moreover, it has been experimentally and analyticallyconfirmed that the inclined crack between sidewall and foot of the lining linkedto the strong vertical component of earthquake motion. Subsequently, to improve the structural ductility along the tunnel’s transversal direction requires setting aminimum bending reinforcement ratio of2%which is useful for the quake-proofdesign of mountain tunnel structures.5） A numerical simulation was conducted to clarify the seismic responsemechanism of mountain tunnel along its lognitudinal direction. The results of theanalyses are shown that under earthquake the longitudinal peak tensile stress oftunnel was much higher than the ultimate tensile strength of concrete structurewhen the tunnel located at the junction between hard and soft strtum. It hasinterpreted the seismic damage-causing mechanism of ring cracks on thesecondary lining.