Study On The Influence And Control Of Metro Box Tunnel Structure’s Longitudinal Deformation Induced By Above Unloading

Metro box tunnel has been applied to some extents in Guangzhou metro’s construction, which has great advantages of clear force-bearing, large structural stiffness, good property of bending and torsional and convenient for construction etc. However, when outside constructions like above unloading happens, defects such as metro structure’s larger uplift replacement, damage of waterproof structure will emerge on account of the shallow conditions and weakness of deformation joints and waterproof structure. Moreover, accidents due to lack of anti-floating will happen, bringing great safety risks to metro operation.So far, aiming at open-cut box tunnel using a common row pile support, the researches on above-unloading induced longitudinal deformation of metro box tunnel are relatively few. Previous studies clearly show that set of deformation joints of box tunnel and capping beam or not has great influences on tunnel’s longitudinal deformation, but the studies were only remained in shallow degree and more works need to be done. Besides, the standards don’t regulate metro box tunnel’s deformation monitoring indexes and control value. It is urgent to properly put forward metro box tunnel’s deformation indexes and control value for the safety of structure.For this purpose, depending on one two-span box tunnel in Guangzhou Metro, laws of longitudinal deformation under different above unloading conditions are studied systematically by using Load-Structure Method. In order to guide the engineering practice, the indexes and values suitable for structural safety of box tunnel are put forward and the unloading depth in control indexes are calculated.The achievements are concluded as follows:(1) Depending on design files of one two-span box tunnel’s project and using Load-Structure Method, the box tunnel’s numerical model is established in MIDAS/FEA based on rational hypothesis of simplified box tunnel structure.(2) Through numerical analysis of above unloading and based on the box tunnel’s model, the laws of longitudinal deformation indexes under different unloading conditions are studied, for example, tunnel’s longitudinal deformation, settlement difference between sides of deformation joints, the destruction degree of roof etc. Besides, the paper analyses the influence of deformation joint by having it or not.(3) Taking the usual anti-floating measure by setting capping beam in box tunnel into consideration, a simulation method of capping beam is presented and analyses it’s influences on tunnel’s longitudinal deformation.(4) According to the control indexes of metro tunnel’s safety deformation, the suitable indexes and values are proposed for box tunnel, some of which convenient and reasonable are provided to calculate the controlling depth under different unloading conditions that can guide the engineering practice.