| Transportation infrastructure has been constructed rapidly in China.Double-arch tunnels have been widely used in highways built in karst areas because of their more convenient connections with bridges, smoother alignment and less land occupation.However construction of double-arch tunnels is very complicated for its large span and complexity of surrounding rock stress status.Taken Shanglongmei double-arched tunnel in Baise-Jingxi expressway as an example, influence of location, size and orientation of karst caves on the middle wall has been studied by using numerical simulation method in both two and three dimension in this paper,and the limit length of middle wall under different karst conditions has been proposed.The results are as follows:(1) Numerical models considering the location of karst cave have been established. Based on these numerical models, influence of different locations on the displacement of middle wall of double-arched tunnel has been analyzed.The results show that the tunnel has largest horizontal displacement when the karst cave is located at the top of the wall, followed by the result when it’s situated at top of each tunnel,and the horizontal displacement is much smaller when the cave is seated below the wall, and a minimum result can be obtained when the cave is settled at the left or right side of the wall. Meanwhile the bottom of tunnel has a largest uplift when cave location is at the top of the wall, and a large settlement when cave appears beneath the wall. Moreover other cave locations have slight influence on the vertical displacement of the middle wall.(2)Numerical models considering the size of karst cave have been established. Based on these numerical models, influence of different sizes on the displacement of the middle wall of double-arched tunnel has been analyzed.The results show that the larger the size of kasrt cave is, the greater the horizontal displacement is, and the greater the rise value of the middle wall is. At the same time, the smaller the size of the karst cave is, the greater the sedimentation value of the middle wall is.(3)Numerical models considering the direction of karst cave have been established. Based on these numerical models, influence of different directions of karst caves on the displacement and press of middle wall has been analyzed.The results show that influence of the karst cave in the Y direction on the horizontal displacement of middle wall is the largest.The second is Z axial direction.X axial affect least but influence of karst cave in the X direction on the sedimentation value of middle wall is the largest and influence of karst cave in the Y direction on the rise value of middle wall is the largest.(4)Numerical models considering buried depth of tunnel have been established. According to these numerical models, the relationship between the limit length of the middle wall and the slope which under different conditions has been analyzed.Results show that, under the circumstances that a big karst cave is located at the top of the middle wall, as well as with the the single span of double-arch tunnel is 10.75m, the width of middle wall is 3.5m, the limit length of middle wall is 60/tana(m),(a is the slope angle).Given this particular practical work, Strength Criterion is recommended to judge the stability of double-arch tunnel.Under the circumstances that the karst is intensively developed, as well as with the the single span of double-arch tunnel is 10.75m, the width of middle wall is 3.5m, the limit length of middle wall is 57/tana(m),(a is the slope angle).Given this particular practical work, Displacement Criterion is recommended to judge the stability of double-arch tunnel. |
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