Stability Analysis Of Double-Hole Parallel Tunnel Undercrossing Highway With Dynamic Load

With the rapid development of urbanization, the form of urban traffic has become complex and varied. Urban subways and highways built quickly in the first and second-tier cities, which bring many problems about overlapping construction. This paper takes Chongqing rail circle line engineering which contain under-crossing the inner ring highway between two station as the research background, and use the Flac3 D numerical software with integrating the theory with practice. Analysis the construction stability of the highway with cars dynamic load which has double hole for subway tunnel under it, and have regularity study mainly on three factors: different depth, double hole interval space and asynchronous excavation. The theory of broken zone is also introduced to the further analysis of the plastic zone along the metro tunnel after excavation, which will guiding the similar construction projects. The main research results are as followed:①The stability of double-hole subway tunnel with dynamic load undercrossing highway is affected by multiple factors, and some factors are mutually contradictory. For the depth of tunnels, with the depth increases surface subsidence was not unchangeable decrease or increase, however, there is a turning point. The maximum settlement value of the road first increases with the increase of the depth then decreases after the turning point. The maximum and minimum principal stress of surrounding rock increases with the depth of tunnels first and then decreases. The subsidence and convergence of the tunnels showed a gradual decreasing trend with the increase depth.②The spacing of undercrossing double-hole parallel tunnel also affect the stability of surrounding rock after excavation and it’s an important factor in the settlement of the road. With holes tunnel spacing increases, the two holes are superimposed and disturbance will gradually decrease, that means the greater the spacing, the greater the volume of settler roads, the larger scope of the road by excavation disturb, and after applying the dynamic load to the overlying pavement, the larger road subsidence. However, the smaller holes tunnel spacing, the greater mutual disturbance in excavation process, and also increase sedimentation pavement. Therefore, holes tunnel spacing changes affect settlement on the road is interacted by two opposing factors. At the same time, with increasing spacing of the holes, two-hole excavation mutual disturbance weakened, its cave settlement and surrounding rock convergence and the maximum and minimum principal stress showed a decreasing trend.③Double tunnel excavated respectively is long duration, and low efficiency. Therefore it should be excavated at the same time but asynchronous.From the research results, the space of the two-hole working face in excavation to the deposition on the overlying pavement is almost zero, but for the stability of the holes, the tunnel excavated in advance is disturbed once again and stress redistribution because of the underwent tunnel’s excavation, therefore the value of the settlement of the tunnel excavated in advance is slightly larger than underwent tunnel. With the increasing asynchronous distance of two-hole the mutual perturbations decreases gradually, and closer to the single-hole excavated stress contours, a reasonable choice of asynchronous excavation spacing is very important.④Use AHP to analysis depth, spacing, spacing asynchronous three factors on the road excavation settlement and subsidence cave vaults weight analysis. The results prove that was depth the largest weight, for 0.6025 to the impact of the settlement on the road; and the spacing of the impact of the settlement on the cave maximum weights for 0.8275; asynchronous excavating face hole spacing on the road and Settlement of weights is very small, almost can be ignored.⑤Introducing the broken zone theory to quantitative analysis the excavated tunnels’ stability of surrounding rock, and the use of the Kastner formula into Flac3 D for calculation, and showing the range of broken zone in the model, when the depth of 10 meters, the maximum thickness of broken zone was 1.386 m. According to the project experience, the maximum thickness of broken zone of sandy shale was 1.1-2.1 m, so the calculated results works.