Risk Assessment of Shield Tunnelling in Mixed Ground Using a Coupled Subjective–Objective Weighting and Fuzzy Set Pair Analysis Model

Shield tunnelling in upper-soft and lower-hard mixed ground is characterised by strong geological heterogeneity and coupled construction risks, including uneven cutterhead loading, face instability, cutter wear and abnormal fluctuations in tunnelling parameters. This study assessed risk of shield tunnelling under such conditions by using a coupled subjective–objective weighting and fuzzy set pair analysis model. Seven key tunnelling parameters, namely total thrust, advance rate, cutterhead rotational speed, cutterhead torque, chamber pressure, grouting pressure and shield attitude, are selected as evaluation indicators. The Analytic Hierarchy Process (AHP) and coefficient of variation method are combined to determine indicator weights, thereby integrating expert judgement with the statistical characteristics of field data. A five-element connection number is then introduced to characterise the identity–discrepancy–contrary relationships between measured indicator values and risk grade standards, while segmented triangular fuzzy numbers are used to determine discrepancy coefficients and address the fuzziness of grade boundaries. The results show that the risk grades for Rings 1–500 fluctuate considerably and are dominated by Grade II–III risks, with 11% of the evaluated segments reaching Grade IV. For Rings 501–1000, the predicted risk level is generally lower, with Grade II and Grade III accounting for 29% and 67% of the evaluated segments, respectively. The assessment results are consistent with the geological conditions and construction observations, indicating that the model can effectively quantify shield tunnelling risk and identify key high-risk parameters in mixed ground. The proposed hybrid framework provides a new theoretical reference for real-time dynamic risk assessment of shield tunnelling under geological transitional conditions.