Disease characteristics and causes analysis of segments of shield tunnels in composite stratum during construction

The construction of shield tunnels in upper-soft and lower-hard composite strata is easy to cause cracking diseases of segments, and has a serious impact on their long-term safety. Based on a metro shield tunnel, a large number of field investigations are carried out for the segment cracks during the construction stage. The distribution laws and characteristics of cracking damage of linings are summarized. On this basis, causes for cracking of segments are analyzed by means of the extended finite element method. The results show that the scale of proportion of cracking forms in the shield tunnels from large to small is the spallings in the circumferential region, longitudinal cracks, and cracks at corners and edges. The first two can be described as the structural failure while the last one can be indicated as the material failure. The generation and expansion of longitudinal cracks, which are related to the jack thrust and the unevenness of contact surface, mostly distribute along the thrust interface of jacks. The propagation of cracks is a reciprocating process of energy accumulation and release with a step shape growth. The mechanism of fracture is tensile failure. The spallings in the circumferential region are related to slab staggering between the rings. The critical value of slab staggering for the spallings in the circumferential region is 8 mm, which is the same as the allowable radial displacement of mortise and tenon. During the construction period of shield tunnels with staggered joint assembly in upper-soft and lower-hard composite strata, the segments with tenon should be avoided and the depth of the grooves should be reduced.