Experimental study of airbag off-wall tunnels model under strike-slip fault dislocation

Tunnels are highly susceptible to structural damage induced by active fault dislocations. Current engineering practice offers limited effective protection measures against large-magnitude fault displacements. To address this issue, this study proposes an off-wall lining system integrated with airbag technology for tunnels traversing active fault zones. The structural assembly consists of four main components: the outer lining, the airbags, the inner lining, and the abutments. The outer lining acts as the primary support, while the inner lining encloses the operational tunnel space. Circumferentially arranged airbag units occupy the annular gap between these structures. The abutments provide dedicated spaces for airbag maintenance and personnel access, while also enhancing structural stability. To evaluate the performance of this novel system under various configurations, three series of dislocation experiments were conducted using a self-designed biaxial dislocation testing platform. A three-dimensional finite element model was developed and validated. Comparative analysis of three anti-dislocation structural systems was performed. Experimental results indicated that tunnel structures underwent significant bending moments and shear forces under strike-slip fault dislocations. The airbag-off-wall system effectively safeguarded the main tunnel structure during fault events. During minor dislocations, the airbag reinforcement considerably enhanced the compressive stiffness of the system. Under large dislocations, the pressure-regulated airbags mitigated direct damage transfer to primary structural elements through active pressure relief. The inner lining maintained structural integrity even under a model dislocation of 50 mm (equivalent to 2.5 m at prototype scale). However, transient pressure spikes caused by delayed relief led to localized stress concentrations. A rapid airbag pressure release mechanism can alleviate the risk of inner lining cracking.