Experimental investigation on biaxial mechanical properties of laminated rock and floor heave mechanism of tunnel based on DIC

Floor heave at the bottom of a tunnel is a common defect during tunnel service. To study the deformation behavior and failure characteristics of the floor, laminated sandstone specimens with precracked U-shaped grooves were prepared, and a series of biaxial compression tests were carried out in the laboratory. Two typical floor structures of a tunnel (with and without inverts) and four lateral stress levels (σ2 = 0, 5, 10 and 15 MPa) were considered. The effects of lateral stress and inverted structure on the deformation and failure characteristics of the floor were investigated. The displacement, strain evolution and failure extent of the floor were analyzed based on the digital image correlation (DIC) technique. The testing results show that the stress concentration at the U-shaped grooves of the specimens can be effectively reduced by the inverted structure. The specimen with an invert can bear more axial deformation and axial stress than that without an invert. The V-shaped notch failure behavior occurred near the tunnel sidewalls for the specimen without an invert, and it occurred in the middle of the tunnel floor for the specimen with an invert. The failure intensity of the V-shaped notch was positively related to the lateral stresses. With the increasing of axial stress, the heave displacement at the tunnel floor gradually increased, while the settlement displacement of the sidewall showed a trend of first increasing and then decreasing. The tensile strain concentration area of the floor undergoes a process of increasing and then decreasing and finally transforms to compressive strain. The failure characteristics of the sandstone specimen observed in the laboratory tests are consistent with the phenomena in real tunnel engineering. In addition, the deformation and failure mechanism of the tunnel floor was analyzed. The key points for the treatment of tunnel floor heave are put forward based on the testing results.