Mechanical properties of a ductile composite support structure enhanced with ECC: Primary support–damping layer–secondary lining

Tunnels crossing active faults are subjected to complex mechanical loads, including tension, compression, bending and shear, caused by surrounding rock pressure, fault displacement and seismic ground motion. However, the mechanical behavior of composite tunnel support systems and the interlayer interface response under such complex loading remain insufficiently understood, and methods to further enhance the deformation adaptability of tunnel supports require investigation. In this study, high-toughness cementitious material Engineered Cementitious Composites (ECC) was introduced into a multilayer support system to replace the primary support or the secondary lining, forming a primary support–damping layer–secondary lining ductile composite support. Combining laboratory tests and numerical simulations, the mechanical performance and failure mechanisms of these composite ductile support members under combined compression–bending were systematically investigated. The full loading processes, crack development, interfacial slip and failure modes of three component types were compared: S-P-C (C25 shotcrete–foamed concrete–C30 ordinary concrete), E-P-C (ECC–foamed concrete–C30 ordinary concrete), and S-P-E (C25 shotcrete–foamed concrete–ECC). Results show that the introduction of ECC markedly improves crack resistance, ductility and toughness. The cracking loads of E-P-C and S-P-E were 1.11 and 1.17 times that of S-P-C, respectively; cracking displacements increased to 1.69 and 2.56 times; and toughness at failure increased by 17 % and 41 %, respectively. The load-carrying capacity of S-P-E increased by 20 % relative to S-P-C. Use of ECC produced finer, more distributed cracks, effectively suppressing crack propagation and preventing spalling. Parametric analysis indicates that increasing the elastic modulus and thickness of the primary support enhances the composite member’s bearing capacity and stiffness but reduces its ductility. Thus, the results of this study provide a reference for the design of R/ECC ductile composite support structures as primary supports for tunnels. • Novel ECC-enhanced composite support structure developed. • Compression-bending tests on ductile composite members conducted. • Failure mechanisms of ECC support structures systematically revealed. • ECC improved load capacity, deformation tolerance and crack control.