Study on displacement mode and structural failure of jointed surrounding rock of shallow buried tunnel under tectonic stress

Under the action of tectonic stress, the stress release during the construction of shallow buried massive rock tunnel often leads to the change of surrounding rock displacement mode. Based on the phenomenon of upward movement of the ground surface during the tunnel excavation of Qingdao metro, combined with the distribution characteristics of surrounding rock structural plane, a new block test model is constructed, and it is clear that the displacement mode of surrounding rock of shallow buried tunnel deflects upward under the action of tectonic stress, but there is a 1.2-3.5 cm collapse area on the surface of surrounding rock of the tunnel. The numerical simulation shows that with the increase of tectonic stress level, the displacement mode of surrounding rock of shallow buried tunnel develops from the settlement type at 0 MPa to the rise and fall type at 3 MPa, and finally becomes the rise type when the tectonic stress exceeds 7 MPa. Through theoretical analysis, it is found that the strength of surrounding rock bearing structure under the action of prestressed bolt support is mainly related to the diameter, length and prestress value of the bolt, as well as the spacing between rows of bolts. Based on the fact that the shallow buried tunnel of Qingdao Metro is in the displacement mode of both rise and fall, increasing the bolt length is conducive to enhancing the synergy between the rock surface and the deep. After field verification, the surrounding rock deformation of the tunnel is controlled within the range of 2-5 mm. The research results provide a basis for the selection and control of surrounding rock support parameters under similar geological conditions.