Applicability evaluation of piezoelectric sensing for measuring shield tunnel segment joints openings during operation

In shield tunnel structures, the segment joint is the most vulnerable part. Opening at this location can lead to inadequate stress on the internal water sealing gaskets, resulting in water leakage in the tunnel. Presently, there exists a notable absence of effective direct sensing techniques for monitoring segment joints during tunnel operations. PVDF piezoelectric material presents a promising solution: its flexibility allows it to conform to the mechanical properties of rubber sealing gaskets within segmental joints. Moreover, its ability to convert mechanical force into electrical signals enables it to function as a sensing element within the segment joint. This study conducted unloading tests, vibration tests, shock load tests, and durability tests to verify the possibility of using piezoelectric materials to sense the opening of water sealing gaskets in shield tunnel segment joints. The results of the unloading test indicate that when the opening rate of the segment joint is relatively high, exceeding 3 mm/min, the embedded piezoelectric material can effectively sense the opening of the segment joint. However, when the opening rate is low or static, it is necessary to actively excite the piezoelectric material to obtain sensing data and thereby determine the status of the segment joint. The results of the vibration tests indicate that the piezoelectric material can effectively sense vibration signals. The voltage peak-to-peak difference sensed by the piezoelectric material can to some extent reflect the opening of the segment joint, with the difference increasing as the joint opening increases. However, the results of the impact load tests show that due to environmental disturbances, the regularity between the opening of the segment joint and the voltage peak-to-peak difference sensed by the piezoelectric material is not significant. The durability results indicate that under prolonged exposure to vibration loads, the sensing capability of the piezoelectric material exhibits good stability. Approximately 80 % of the data deviations are below 5 %, with the maximum deviation not exceeding 10 %. While further refinement of sensing methodologies is necessary to establish a quantitative relationship between segment joint opening magnitude and voltage peak-to-peak differences sensed by PVDF piezoelectric material, the potential applications of flexible piezoelectric materials for segment joint monitoring are promising and warrant continued exploration.