A novel weathering control index (WCI) for performance assessment of EPB TBMs in urban tunnelling

Rock weathering induced mineralogical, lithological variability and sudden water inflows lead to notable difficulties during urban EPB tunneling, affecting excavation performance and face stability control by altering the engineering properties of the rock mass. Under such weathering-controlled regimes, an integrated, quantitative operational metric is needed to support the interpretation of excavation performance and face-pressure response under weathering-affected EPB tunnelling conditions. In this study, a novel Weathering Control Index (WCI), was developed to integrate net thrust and working chamber pressure into a weathering related operational metric. Because the chamber-pressure term is derived from the pressure applied during excavation, WCI is interpreted here as an operational indicator influenced by both ground conditions and operational response. Approximately 11,000 performance data obtained from three different EPB-TBMs excavating along a sedimentary tunnel alignment were reanalyzed. Three weathering zones (W 1 , W 2–3 , W 3–4 ) were evaluated as they represent the most frequently encountered conditions along the tunnel alignment. The $$SE = f(FIR, WCI)$$ exponential nomogram indicated that, WCI is associated with excavation performance and may also help distinguish operational responses under different ground conditions, particularly in clay-rich sections. In the W 3–4 zone, two internally different SE regimes were observed, suggesting that excavation response under highly weathered conditions may vary with changes in fine-grained content, water sensitivity, and chamber flow behavior. Moreover, Response Surface Methodology (RSM) was used as an operational optimization method to identify five operational regions on the FIR–WCI plane for SE values. In this context, WCI may provide an additional operational perspective for tracking excavation-state changes, supporting soil-conditioning adjustment, and screening potentially demanding ground conditions during EPB tunnelling. However, its use as a predictive early-warning indicator requires further validation using time-synchronized control logs and independent excavation datasets. The proposed WCI ranges and SE–FIR–WCI operational regions should be interpreted as project-specific findings, since independent external validation data were not available in the present study.