Experimental study on the stress of loess tunnel lining structure under cyclic loading and unloading

Surface cracks are prone to occur in shallow buried large cross-section loess tunnels. The lining structure undergoes repeated loading and unloading actions under the periodic infiltration of seasonal rainfall or agricultural irrigation. It leads to serious structural diseases. In view of this, the stress law of the lining structure of large-section loess tunnels under cyclic loading-unloading is explored in this article. And the mechanical response characteristics and failure mechanism of the lining structure under different load peaks and cyclic loading times is analyzed. The research results indicate that the lining structure exhibits three response modes under cyclic loading and unloading: elastic deformation, progressive failure, and direct cracking. The external loads are between less than 53.2%, 79.8%~100%, and greater than the loading failure value, respectively. Under the elastic deformation mode, the internal force value is stable. The axial force ranges below -932 kN, and the bending moment ranges from -233 kN•m to 230 kN•m. During the progressive failure stage, cracks appear on the surface after multiple loading and unloading cycles. The minimum safety factor decreases from 0.34 to 0.42 to below 0.23. In the crack propagation mode, stress concentration occurs in the cracked parts of the structure during loading and unloading, and the cracks rapidly expand. The internal force value of the un-cracked area remains stable.