Opening measurement of rubber gaskets in tunnel segmental joints based on impedance analysis using piezoelectric sensors

Abstract The segmental joint with rubber gaskets is the weakest part of the shield tunnel structure. Poor assembly quality or prolonged loading conditions can compromise the effectiveness of these gaskets, leading to issues such as leakage, seepage, and reduced load-bearing capacity. Currently, there is a lack of effective methods for directly monitoring the state of rubber sealing gaskets. This paper proposes a novel monitoring method for tunnel joint rubber sealing gaskets by integrating flexible piezoelectric sensors (PVDF) with rubber sealing gaskets, based on the principle of structural impedance analysis. First, the mechanical performance equation of the rubber sealing gasket is derived, and a piezoelectric material-rubber sealing gasket impedance model is proposed based on existing research. Subsequently, experiments are conducted on single-set rubber sealing gaskets using piezoelectric impedance analysis, as well as on dual-set rubber sealing gaskets in both series and parallel configurations. The impedance magnitude, phase angle, real part, and imaginary part of the impedance are analyzed to evaluate the performance of the proposed method. The results demonstrate that the impedance magnitude, phase angle, and real part of the impedance of the piezoelectric sensor-rubber sealing gasket structure can effectively reflect the opening state of the rubber sealing gaskets. For the rubber sealing gaskets selected in this study, for every 1 mm of opening, when the applied AC frequency is near the resonant frequency, the impedance magnitude increases by 0.23 Ω, the real part of the impedance increases by 0.3 Ω, and the phase angle decreases by 0.42° at an AC frequency of 300 MHz. In multi-set rubber sealing gasket simultaneous monitoring, the monitoring performance with series-connected internal sensors is found to be superior to that with parallel connections. When both sets of rubber sealing gaskets open simultaneously, compared to a single set of gaskets opening, the impedance magnitude curve generates an additional extreme point near the resonant frequency. This additional extreme point can serve as a signal to determine the opening of multiple sets of rubber sealing gaskets.