Deformation Evolution and Failure Mechanism of Monoclinic and Soft-Hard Interbedded Strata: Study of Muzhailing Tunnel

The large deformation issue in soft rock mass is a worldwide difficulty that has puzzled tunnel engineering for a century and a half. Previous studies have mainly focused on a large deformation mechanism, prediction, and control of soft rock with single lithology, while there are limited studies on the tunneling-induced large deformation in monoclinic and soft-hard interbedded rock strata. This paper studies the deformation law, evolution process, and failure mechanism of a tunnel excavated in monoclinic and soft-hard interbedded rock mass by onsite measurements in the No. 3 inclined shaft of the Muzhailing Tunnel, which is a key control project of the Lanzhou-Haikou National Expressway. The achieved results indicated that the tunnel deformation was characterized by significant asymmetry, the maximum deformation in the cross section was observed to occur in the normal direction of the bedding plane, and the side of the larger deformation was consistent with the dip direction of the strata. The vertical displacements were greater than the horizontal displacements except for the right foot of the middle bench, and the primary lining at the upper left of the tunnel had intruded into the space of the secondary lining and had to be demolished and reconstructed. The displacement speed and amount mostly reached the peak in the construction stage of the middle bench, and its displacement amount accounted for about half of the total displacement. The spatiotemporal curves of large deformation could be divided into four stages according to the excavation time and the distance from the tunnel face. Two types of exponential functions were used to fit and predict the tunnel displacement, which displayed good applicability. The disaster evolution process of large deformations was summarized into five stages on a macro scale: premise, gestation, development (including four substages), occurrence, and treatment of large deformation. The failure mechanism of monoclinic and soft-hard interbedded rock mass is mainly the bending-tensile failure of thin-layered strata.