Spalling mechanism of plain concrete linings in mountain tunnels under loosening loads

Spalling and block shedding of linings are typical and hazardous defects during the service period of mountain tunnels. However, limited research has been conducted on the crack propagation and spalling mechanisms of plain concrete linings with intersecting cracks under loosening load conditions. Based on Linear Elastic Fracture Mechanics, 1:15 scaled model test of plain concrete linings were carried out which using Pingyan Channal as the prototype. A coupled computational framework based on ABAQUS-FRANC3D was established to investigate the crack evolution and spalling formation mechanism of cracked tunnel linings under loosening loads. The results show that: (1) Under loosening loads, the deformation of cracked tunnel linings evolves through three stages, namely initial development, rapid deformation growth and structural softening instability. As the initial depth of longitudinal cracks at the crown increases from 1/3D to 2/3D and 3/3D, the structural stiffness is reduced by approximately 9%-24%, 12%-29% and 11%-29% across three stages, respectively. (2) Spalling mainly occurs at the crown and shoulder, predominantly governed by longitudinal through-cracks. The crack propagation through the lining thickness exhibits a three-stage pattern characterized by “radial extension-deflection-transverse propagation.” The radial and transverse stages are dominated by Mode I fracture, while the deflection stage is governed by a mixed-mode involving both Mode I and Mode Ⅱ mechanisms. (3) Two fundamental types of lining spalling are proposed, namely tensile-induced spalling and compressive-induced spalling. Both show lateral crack deflection before failure, corresponding to crown settlement thresholds of about 50 mm and 100 mm. The stage from crack deflection to reaching these settlement levels can be treated as a key period for spalling risk warning and inspection.