Interference law and gain method of reinforcement layer on GPR detection signal of cavities behind tunnel linings

Cavities behind tunnel linings can weaken lining performance and are commonly detected using ground-penetrating radar (GPR). In reinforced concrete linings, reinforcement reflections may overlap with cavity responses and reduce image clarity. To examine this effect, controlled model tests were conducted using reinforced and non-reinforced lining specimens. Air-filled and water-filled cavities with different sizes and burial depths were compared. The results show that the reinforcement layer partly disturbs the continuity of hyperbolic features and reduces reflection amplitude, especially when the cavity is close to the reinforcement layer. Under the tested conditions, the depth interpretation error was generally within 10%. The size-related result should be regarded as the apparent cavity thickness along the radar wave propagation direction, rather than the directly measured geometric diameter, with an error within 16%. For air-filled cavities, the amplitude difference decreased with burial depth, and an empirical compensation formula was established with a verification error within 15%. For water-filled cavities, the amplitude difference was more closely related to cavity size, and the compensation error was within 8%. The proposed gain relationship is an empirical correction method applicable to conditions similar to those tested.