New index to identify karst caves based on operational parameters during shield tunnelling

Real-time karst cave identification is crucial to ensuring safety during shield tunnelling. However, owing to a sparsity of drilling data, karst caves often remain undetected before shield tunnelling begins. An approach is proposed for identifying karst caves in real time during shield operations. A new index, called the karst cave identification index (KCII), is proposed based on a force equilibrium analysis on the excavation face of a shield. The force balance conditions necessary to maintain the stability of the excavation face are derived according to soil pressure balance principles. Operational parameters related to the pressure balance process are then used to calculate the KCII, which leverages the operational parameters recorded by the sensory system of the shield machine. The effectiveness of the KCII was validated through a field case of shield tunnelling in Xuzhou, China, and it was compared with four other indices: specific energy, face penetration index, torque penetration index, and geological feather identification index. The comparison shows the difficulty of identifying karst caves using the other indices, but the KCII can be used to distinguish geological types and differentiate various types of karst cave filling. The KCII enables engineers to identify types of karst cave in real time, thus making necessary adjustments to shield operations possible and enhancing safety and efficiency in tunnelling.