Study On Construction Mechanical Behavior Of Subway Passage Closely Under Existing Station In Sandy Cobble Stratum

With the continuous encryption of the underground line network of urban rail transit,the frequency of adjacent construction increases in underground engineering,and the adjacent types and ways become more and more complex.The net distance between the newly-built structure and the existing structure is becoming smaller,and even the adjacent condition with zero distance appears,which brings great challenges to engineering construction.This paper based on the newly-built underpass tunnel of Nijiaqiao station in Chengdu Metro Line 8,which zero-distance going under through metro station of Line 1,and carried out the study on the construction mechanic behavior of the existing structure and newly-built underpass tunnel by data investigation,theoretical analysis,numerical simulation and field test,the research results are as follows:(1)There are four adjacent ways involved in practical project,including orthogonal up-down tunnel,parallel up-down tunnel,parallel left-right tunnel,and parallel left-right(un-down)tunnel with 45 angle,the mechanical evolution law is studied in different net distance under the assumption that the surrounding rock is in elastic and elastoplastic state after excavation(2)Multi-partition excavation method is used in the construction of the newly-built underpass tunnel,and the excavation sequence of pilot hole and the demolition sequence of temporary lining are optimized respectively.The results show that: it is better to excavate the pilot hole corresponding to the central pillar of the existing station first.In terms of demolition sequence of temporary lining,construction of secondary liner first can effectively reduce the adverse impact caused by the demolition of temporary lining,but more time may spent in construction,when the safety and construction speed are taken into account,removing the temporary lining on both sides at the same time first,following by corresponding secondary liner construction,then removing the remaining temporary lining and constructing the remaining secondary liner is a better choice.(3)Under the influence of the unidirectional excavation(asymmetric excavation)of the newly-built underpass tunnel,the cross section of the existing station occurs torsional deformation with the construction of the newly-built underpass tunnel.At the beginning time of the construction,the settlement of the left track is greater than that of the right track,and the deformation of the cross section of the existing station appears counterclockwise torsion(the settlement of the starting side of the excavation is slightly larger).At the end time of the construction,the deformation of the cross section of the existing station appears clockwise torsion(the settlement of the starting side of the excavation is slightly smaller).(4)Under the influence of the existing structure,the mechanical behavior of the newly-built underpass tunnel is different from the traditional semi-infinite cavity problem,and the corresponding force and deformation is smaller compared with the single cavity condition.On the other hand,in the case of the same support parameters,carried out the comparison analysis on the calculation results between the partial excavation and the all in one excavation of the underpass tunnel: the maximum settlement of the underpass tunnel roof in partial excavation is about 78.21% of that in all in one excavation,and the maximum bending moment of the primary support is about 40.07% of that in all in one excavation,and the final state value is not always the maximum value with the influence of the partial excavation,which indicates there is strong process relation in the construction.