Field-validated wireless vertical graphene sensors for autonomous crack and leakage monitoring in metro tunnels

Simultaneous and autonomous monitoring of crack evolution and water leakage in operational tunnels remains challenging, particularly under non-intrusive installation constraints. This study presents a physics-guided vertical graphene (VG) sensing framework for the continuous and quantitative monitoring of both deterioration processes. A VG strain sensor was developed and characterised using a physically interpretable three-stage resistance–strain constitutive model governed by microcrack evolution within an island–bridge conductive network. The same VG-based transduction principle was extended to leakage monitoring through the incorporation of a superabsorbent polymer (SAP) layer, which converts water ingress into deformation-induced resistance changes. Laboratory tests demonstrated stable sensing responses under monotonic, cyclic, and sustained loading conditions, with bounded hysteresis and limited baseline drift. The sensing components were integrated with a low-power Sub-GHz wireless platform for continuous, cable-free data acquisition in confined tunnel environments. A three-month field deployment in an operational metro tunnel demonstrated the field applicability of the framework by capturing quasi-static crack evolution, including an opening of approximately 0.53 mm, and quantifying low-rate leakage at approximately 1 mL/day. By converting resistance measurements into physically interpretable engineering quantities, including crack-opening displacement and leakage volume, the proposed framework provides continuous condition information for quantitative tunnel assessment and may support future condition-informed maintenance planning. These results demonstrate the feasibility of non-intrusive wireless VG sensing for operational tunnel monitoring, although longer-term, multi-site validation is required to assess durability and broader applicability.