The Study On Construction Machanics Of Overcrossing Existing Line Multi-section Soft Rock Tunnel

With the development of China's urbanization process, development and utilization of underground space is essential. In the use of underground space, rail transit system as the infrastructure is a very important aspect. In rail transit construction, the newly-built tunnel will inevitably overlap with the existing tunnel. Therefore, it is necessary to study it.This article studied the construction mechanics for Chongqing Jianxinpo tunnel across the existing tunnel sections, advancing analysis for surrounding rock and structure's stress and deformation through theoretical analysis, numerical simulation, monitoring, verifying the construction method's rationality through the comparison of numerical simulation and monitoring.The main results listed as follows:1. The state of the primary stress, the stress field after tunnel excavation, the state of surrounding rock pressure were analyzed, it also explained the factors to influence surrounding rock stability : one is the internal factors, for example, geological and geological structure,initial ground stress ,mechanic characteristics of rock mass; the other is external factors, for example, engineering factors and time facrors.2. According to status of the newly-built tunnel through the existing tunnel, FLAC numerical simulation was carried out, analysis on variation of surrounding rock stress and displacement was made, important conclusions were obtained: crossover phase of the tunnel surrounding rock tensile stress changes affected by the excavation significantly, the peak tensile stress of excavation section was significantly higher than that of non-excavated section; the peak tensile stress near the crossing even higher than that in the acrossing,which explained the reinforcement of exsiting tunnel near the crossing was more important than the crossing; reinforcement of the existing tunnel before excavation was more necessary especially of near the excavation section,which could contain and suppress the deformation of the stress concentration areas effectively, on the other hand, crossover phase of the tunnel step by step excavation could also control and avoid too big stress concentration in crossover phase of the existing tunnel surrounding rock.3. Through field monitoring, important data was got and analyzed, the variation of surface subsidence, surrounding rock pressure, the convergence, crown settlement and steel grid stress were found, the results shows: in the construction of small-spacing tunnel in soft surrounding rock, the lower parts'costruction would have effect on the upper, the influence between the left tunnel and the right tunnel would also not be to ignore, in the construction, the reinforcemen of upper part timbering is necessary, especially the reinforcement on the middle rock pillar of the tunnel, which could prevent too large perturbation affectted by excavation for this part of rock,and it would maintain the stability of the structure.4. The results of the existing tunnel's monitoring shows that: excavation of the newly-built tunnel did not exert a profound influence on the existing tunnel and the existing tunnel was stable through the period of construction.5. The comparion of numerical simulation and monitoring shows: simulated values and mersured values has some degree differece,but their variation was consistent, which proved that the numerical simulation was reliable and the construction method is reasonable.