Field immersion tests on tunnel in large-thickness collapsible loess

The stability of tunnel structure in large-thickness collapsible loess strata will be threatened by the potential collapse deformation of loess. In order to explore the mechanism of tunnel lining structure influenced by collapse deformation of the surrounding rock, field immersion tests are conducted on tunnel construction site of collapsible loess with large thickness. A series of parameters such as the volumetric water content of soil at different depths, collapse deformation of strata, settlement of tunnel foundation, lateral displacement of soil layer and stress condition of tunnel structure are measured in the infiltration process. The collapse deformation characteristics, infiltration laws of the loess site and mechanical change rules of tunnel lining structure are emphatically investigated. The results show that the depth of water infiltration at this site is much deeper than that at the natural loess site due to the greater permeability of the surrounding rock caused by tunnel excavation disturbance. The interaction between tunnel structure and surrounding rock is changed by the collapse of surrounding rock during water infiltration, such as the significant increase of the extrusion force at tunnel side wall and the vertical pressure at two flanks of tunnel inverted arch. Meanwhile, the subgrade reaction in the middle of tunnel inverted arch restrains the sedimentation of this area. A longitudinal crack formed in the middle of tunnel inverted arch is under the impact of differential settlement. In addition, with the increase of water infiltration depth, the collapse deformation of loess stratum is released layer after layer, and it includes the vertical and horizontal deformations. The collapsible loess site with large thickness in mountainous area has the complex characteristics of stratum soils, which is reflected by the tremendous difference of collapsible deformation in the immersion scope.