Tunnel collapse in weak rock masses occurs when the support pressure given by the installed support system is not adequate to sustain the weight of broken rock resulting from the excavation. Further, installation of support system will encounter great difficulty when the maximum allowable limit of strain before setting up the support is exceeded. This paper presents the results of numerical simulations in computer code FLAC to integrate the use of ground reaction curve and equilibrium strain approach for designing tunnel support system in a non-circular excavation. The equilibrium strain is used as one of the design tools because the final radial deformation of a supported tunnel occurs at the equilibrium between the support and the deforming ground, not at the time of support installation. The approach is demonstrated on a large cross-section road tunnel that is excavated through weak rock masses with varying qualities. Results from the FLAC model show that when the time to install support system is correctly estimated at the equilibrium strain εeq of 1%, the amount of vertical deformation and the extent of the plastic zone around the tunnel decrease significantly. This correct estimation is made possible with the help from the ground reaction curve (GRC). Moreover, as shown by the support capacity diagrams, the induced bending moment, axial and shear loads in the tunnel lining are well inside the strength envelopes of the support system with factor of safety > 1.5. Results from this research indicate that the integrated use of GRC and equilibrium strain approach can be used as a tool to achieve a reliable tunnel support design.
Designing a tunnel support system in weak rock masses must cope with the estimation of support pressure that is needed to stabilize the tunnel face and the newly-opened room behind the face. In this situation, collapse of the opening occurs when the support pressure given by the installed support system is not adequate to sustain the weight of broken rock resulting from the excavation. In particular, installation of support system will encounter great difficulty when the maximum allowable limit of strain before setting up the support is exceeded.
The so-called convergence-confinement method (CCM) has been a standard practice for evaluating the displacement behavior of a tunnel and to determine the required support pressure to control the convergence of the tunnel wall (Carranza-Torres and Fairhurst, 2000; Vlachopoulos and Diederichs, 2009; Prassetyo, 2017; Prassetyo and Gutierrez, 2018). The CCM consists of the ground reaction curve (GRC), the support reaction curve (SRC), and the longitudinal displacement curve (LDP). The GRC represents the relationship between the increasing radial displacement of the tunnel wall ur and the decreasing internal support pressure pi. The SRC represents the relationship between the increasing support pressure ps and the increasing radial displacement of the support us. The last component, the LDP, represents the radial displacement occurring along the longitudinal axis of the unsupported tunnel.