Dilation angle variations in plastic zone around tunnels in rocks-constant or variable dilation parameter

Dilation angle is a significant parameter needed for numerical simulation of tunnels. Even though dilation parameter is physically variable and dependent on confinement and experienced shear plastic strain based on the existing dilation models, numerical simulations of tunnels and underground openings with constant dilation parameter usually lead to satisfactory results in practical use. This study aims to find out why constant dilation angle is enough under practical conditions to simulate numerically tunnels and underground excavations in spite of the fact that dilation angle is variable in laboratory and experimental scale. With this aim, this work studies how mobilized dilation angle varies in a plastic zone surrounding a tunnel. For the circular tunnel under uniform in situ stress field, the stepwise finite difference approximation analytical solution considering strain softening rock mass behavior with mobilized dilation angle was used to study how mobilized dilation angle varies in plastic zone around tunnel under very different conditions. In practical conditions determined in this study, dilative behavior of all over the plastic zone around the tunnel can be approximated to constant dilation angle in the middle region of the plastic zone. Moreover, the plastic zone displacements for mobilized and constant dilation angle models are compared with each other. Further investigation under more general non-uniform in situ stress conditions and non-circular tunnels is performed by using the commercial finite difference software to numerically simulate the Mine-by experimental tunnel of AECL (Atomic Energy of Canada Limited) and the arched tunnel. Although the Mine-by and arched tunnels were numerically simulated based on the mobilized dilation angle model, the variability associated with dilation angle around the simulated Mine-by and arched tunnels is insignificant, and dilation angle is approximately constant in the plastic zone.