Abstract: Traditional open-cut cast-in-place tunnels face challenges related to construction efficiency and labor costs. The absence of a simplified formwork support system in assembled monolithic structures has led to inefficiencies and poor working conditions in full-scaffold construction. To address these issues, the authors propose a structural system and its construction techniques. Based on the design concept of the support-free composite slab, a novel type of assembled monolithic structure is proposed. This structure is assembled using prefabricated bedplate reinforcement cages, lattice girder-type precast walls, and roof slabs, and then cast in place. Recyclable corner connectors are used to provide mutual support between the bedplate, sidewall, and roof slab. The precast slab and wall-attached enclosure serve as formwork for on-site pouring to realize support- and formwork-free construction. In addition, the force transformation mode during the construction and service stages is analyzed, and the roof slab and sidewall are simplified as simply supported beams and two-dimensional frame structures in the construction and service stages, respectively. Stress analysis of the support-free composite slab and bearing capacity assessment of the connecting nodes demonstrate the feasibility of the proposed scheme. A technical and economic analysis demonstrates that the proposed system significantly improves construction efficiency and reduces labor costs compared with traditional open-cut cast-in-place tunnels. In addition, it outperforms full-scaffold assembled monolithic open-cut tunnels in terms of efficiency and working conditions. It also offers easier construction, lower cost, and better adaptability  in soft soil foundation pit support. Therefore, the proposed structure scheme offers better environmental and economic benefits.

Key words: assembled monolithic structures, open-cut tunnel, support-free composite slab, reinforcement truss, concrete