Confinement Effect on Operating Limits of TBM Based on a Theoretical Model for Rock Indentation

Mechanized tunneling using tunnel boring machine is mainly a thrust controlled process that can conveniently be modeled by indentation tests. Under confinement, the indentation force required to fragment rocks is significantly larger than that at ambient conditions, which necessitates consideration of lateral confinement in predicting penetration rate. Yet, most of the proposed methods for estimating penetration rate are of phenomenological nature, with only a few based on theoretical analysis for which it is not straightforward to extend to confinement conditions. In this study, we exploited the model recently developed for analyzing data of rock indentation tests with lateral confinement on specimens of variable sizes and extended it to predict penetration rate and thrust required to fragment rocks. The effect of confinement on indentation is accounted for by the increases in compressive strength with lateral confinement. In comparison to CSM model, the model predictions require three extra parameters: confinement, the simplified cavity shape that relates to the cutter shape and the ratio between damage zone size and cutter spacing. The three parameters can be obtained/calibrated by using experimental data obtained from laboratory-controlled linear cutting tests. The predicted operating limits using power of TBM head suggests that under confinement conditions the maximum feasible penetration rate that can be reached is reduced. The proposed method can serve as a start-point for estimating operating limits for TBM under confinement conditions. 1. INTRODUCTION In mechanized tunneling using tunnel boring machine (TBM) the thrust and penetration rate that can be achieved, or operating limits, are of great interest for the planning and construction stages for a project. Tunneling normally takes place around tens to hundreds meter in depth, thus subjected to elevated in-situ stresses at a magnitude of up to tens of megapascals. Rock behavior at confinement conditions differs from that at ambient conditions according to conventional triaxial compression test results (e.g., Paterson & Wong, 2005), which is also reflected in in the significantly larger thrust required to fragment rocks in rock cutting/indentation tests (e.g., Wang et al., 2021; Wang et al., 2022; Yin et al., 2014). Therefore, predictions of the operating limits would need to include the expected strengthening effect of confinement on rock excavation.