| As a special form of tunnel structure with large spans and complex stress states,double-arch tunnels are more prone to structural cracking than common single tunnels.The safety of asymmetric double-arch tunnels should be particularly paid more attention to.Therefore,the cracking characteristics,cracking mechanism,and bearing performance of cracked asymmetric double-arch tunnels are systematically analyzed and studied by model tests,numerical simulations,and theoretical analyses.The main contents and results of the research are as follows.(1)The cracking performance of asymmetric double-arch tunnels and three typical local cracking patterns under the influence of voids behind linings are presented.Considering the bidirectional internal forces of the linings,the local three-dimensional response and cracking mechanism of the tunnel structure under the influence of the void behind linings are revealed.For asymmetric double-arch tunnels without voids,the bottom of the middle wall cracks most seriously.The cracks occur intensively in the inside surface of the invert and vault,and the outside surface of the arch foot and sidewalls of both tunnels.The void-induced local cracking pattern can be mainly divided into three types: circumferential cracking pattern,closed cracking pattern,and longitudinal cracking pattern.The uneven earth pressure near the void is the key reason that causes the local cracking of the structure.(2)The effects of lining cracks on the asymmetric double-arch tunnels are revealed.The structural failure modes caused by cracks at different locations can be mainly divided into two types,i.e.,overall structural instability caused by local crushing or shear damage,which have significant brittle damage characteristics.The deformation process of cracked linings under load can be divided into the stable working stage with cracks and destabilization stage,and the local damage at the location of cracks is the dividing point of the two stages.In addition,the crack has a significant "truncation effect" on the transfer of the lining bending moment,and the bending moment near the crack is significantly reduced.The more cracks there are,the greater the influence range on the bending moment is.(3)A numerical model of cracked linings based on cohesive elements is developed,which overcomes the shortcoming that the existing beam-spring model can not consider the propagation of existing cracks and concrete damage.And the influence of lining cracks on the asymmetric double-arch tunnel structure is clarified.The lining cracks decrease the effective bearing area of the structure,and thus reduce the bending stiffness of the structure and leads to serious stress concentration of the cracking cross-section of the lining,which is the key reason for the local failure and the overall instability of the structure.(4)The deformation and failure modes of asymmetric double-arch tunnel lining structure under the influence of voids behind linings and the nearby cracks are revealed.The effect of cracks near the voids on the structure depends on their orientation and location.For longitudinal cracks near the void,when the cracks are far from the void boundary,the tunnel is destabilized by local crushing failure on the backside of the cracks.When the crack is located at the boundary of the void,the lining structure first experiences compressive damage on the back of the crack and eventually local shear damage along the crack section occurs.For the circumferential crack near the void,when the crack is located at the center of the void,the crack is prone to penetration.When the crack is located at the boundary of the void,the crack section is prone to local shear damage.(5)Based on an artificial neural network,the prediction model of the bearing capacity loss of asymmetric double-arch tunnels is established considering the characteristic parameters of voids behind the lining and cracks.And the influence of each characteristic parameter on the bearing capacity loss of the lining is evaluated. |
PDF Full Text Request