Evaluating the influence of engineering geological parameters on TBM performance during grinding process in limestone strata

The chipping and grinding processes are the basis for understanding rock cutting during TBM excavation in hard rock conditions. The chips are produced when tensile fractures created by adjacent cutters propagate parallel to the tunnel face and coalesce whereas grinding occurs when fractures do not fully propagate through the rock and only fines are generated. Chipping is the normal and efficient boring process using machines equipped with disc cutters. Consequently, almost all of the models proposed for predicting the TBM penetration have been developed by the assumption that chipping is the dominant mode. It is obvious that these models are unable to predict correctly TBM performance when grinding is dominated in the rock cutting process. In this study, the relationships between engineering geological properties of the rock units (Cretaceous limestones) and operating and performance parameters of a refurbished EPB-TBM, employed for completion of SEL6 (the Southern Extension of Line 6, Tehran Metro) tunnel, were evaluated and analyzed. In this project due to limitations in the load capacity of the employed cutters and insufficient initial penetration in rock, the excavation was mostly carried out on grinding-dominated mode. So, the suitable data for establishing a special database and consequent analyses were provided. The established database composed of actual operating and performance parameters of the machine (i.e., cutterhead penetration and RPM and cutter load) and also engineering geological properties (i.e., UCS, alpha angle, GSI, and RQD) of limestone strata. Finally, by statistical analyses performed on collected and screened data (41 data sets), some simple, site-specific, empirical equations were developed to predict the TBM performance on grinding mode. In addition, according to the multivariate regression method, a significant relationship between the cutterhead penetration (PRev), the cutter load (Fn), intact rock strength (UCS), and geological strength index (GSI) was established, statistically. The obtained results demonstrate that this engineering geological-based model could provide a new applicable equation for accurately predicting TBM performance for similar geological conditions where grinding is dominant.