Research On Deformation And Mechanical Behavior Of The New Construction Metro Crossing Under The Existing Metro In Loess Area

With the development of the underground space in urban and the constant improvement of underground transport network, interchange between metro lines will become very common approaching engineering. The approaching engineering is always of the complicated construction conditions and the existing metro lines have strict security control standards. So, the new construction metro crossing under the existing metro is always of the higher difficulty of construction, complicated construction technology and high security risk, etc. For loess area in Xi’an, due to various loads from outside its nature will change and it will occur large deformation, which would be a greater impact on the existing metro and threaten the stability of the existing metro structures. It is also a major problem faced by the Xi’an metro crossing project. The main purpose of this paper is to research the deformation and mechanical behavior of the new construction metro crossing under the existing metro in loess area by numerical simulation. There are the main conclusions:(1)The construction of the new metro will cause a certain degree of influence on the existing metro and the existing metro structure occur mainly vertical deformation; the existing metro structure is overall settlement. In fact, monitoring point selection will directly affect the monitoring measurement accuracy and reliability.(2) The existing metro has reinforcement action for the surrounding rock, and the effect on deformation is more obvious with the depth increasing. On both sides of the metro there is a trend of soil movement departure existing metro. So, there will be slack stratum zones and a width of compaction zones in the distance on the two sides of the existing metro.(3) The form of the existing metro structure stress is wavy. Within a certain range on both sides of the new metro axis, the existing metro’s invert is in tension and the dome is in compression. The stress is opposite in the distance. The maximum stress appears on new metro axis section and the safety of the existing metro structure depends on the maximum tension stress.(4)Based on the partition of the structure stress and the similar projects’, Xi’an existing approaching engineering’s control standards, and domestic norms and local standards, we determine the structure of the existing metro sedimentation control standards and partition indicators. Meanwhile, the structural settlement deformation is partitioned and there are four partitions. They are danger area, alarm area, warning area and safe area.(5)When the beneath spacing is 0.1D, the new construction metro crossing under the existing metro for the construction according to conventional construction methods are unreliable and the 0.2D is relatively safe. So, in fact if it is unavoidable that we must adopt the very small beneath spacing to cross under the existing metro, we should select the excavation method which can control the settlement effectively and pre-reinforcement measures which has enough stiffness and reinforcement range.