A novel real-time monitoring system for the measurement of the annular grout thickness during simultaneous backfill grouting

Backfill grouting is generally used to limit the ground surface settlement induced by shield tunneling. The control of this settlement constitutes a major challenge in the use of shield machines in urban areas. This control could be monitored using the ground penetrating radar (GPR) for its high precision and efficiency. This paper presents the development and use of a real-time back-fill grouting monitoring system, which is based on the integration of a new GPR monitoring system in a shield tunnel machine. The three-dimensional Finite Difference Time Domain (FDTD) was adopted to analyze the propagation of the electromagnetic waves in the backfill grouting. After verification of the use of a GPR center frequency band of 300 MHz, the system was used in real in the construction of the R3 of Jinan metro. The application showed that only 3-4 min were required for the backfill monitoring, which provided enough time to adapt the grouting process to the settlement requirements. Results also allowed establishing criteria for the evaluation of the grouting thickness. The use of this system allowed engineers to limit the tunneling-induced settlement in urban areas.