Investigation of energy evolution process of rock mass during deep tunnel excavation based on elasto-viscoplastic damage model and time-dependent energy indices

Understanding the energy evolution process of rock mass during tunnel excavation is crucial for revealing the potential of deep geological hazards. A series of numerical simulations for continuous tunneling were conducted based on an elasto-viscoplastic damage model, and time-dependent energy indices, strain energy density (SED) and energy dissipation rate (EDR) were introduced to analyze the spatio-temporal evolutionary process of energy accumulation and release. Several new insights on rockburst and large squeezing deformation were subsequently proposed from an energy perspective. The influences of excavation rate and pilot tunnel on energy evolution were further discussed. Results indicate that deep tunnel excavation generally involves an energy accumulation process (a SED peak) and two energy release processes (two EDR peaks); the energy of rock mass exhibits a temporal evolution pattern of "release-accumulation-re-release" and a spatial migration pattern of "deep-shallow-deep." Initial energy release, energy accumulation, and secondary energy release are induced by excavation unloading effect, elastic stress redistribution, and viscoplastic strain accumulation. Rockbursts exhibit different spatio-temporal characteristics; two EDR peaks appear shortly before and after excavation, indicating the potential of instantaneous rockbursts at excavation face and delayed rockbursts at sidewalls, respectively. Reducing excavation rate helps to lower the SED peak and EDR peaks, thus mitigating the rockburst potential; transient unloading can cause a more sudden and significant initial energy release, obviously increasing the risk of instantaneous rockbursts. Pilot tunnel contributes to energy pre-concentration of rock mass ahead of excavation face, thereby decreasing energy peak upon secondary tunneling. The findings of this study provide theoretical basis for evaluation and control of rockburst and large deformation in deep tunnels.