Conflicting challenges are encountered in the mechanized construction of large-section mountainous tunnels in complex geological settings, particularly the trade-off between "construction progress vs. safety" and "support stiffness vs. coordinated deformation between surrounding rock and support". To address these issues, a novel steel pipe spatial grid "face support" structure is developed. This study employs full-scale laboratory model tests and finite element numerical simulations to compare the mechanical properties of traditional steel-reinforced concrete structures and those of the novel steel pipe spatial grid concrete structure. The mechanical bearing mechanisms, failure modes, and stress-strain evolution patterns of the support structures are investigated. In addition, field tests are conducted to explore real-world differences in support characteristics between reinforced-steel supports and triangular-steel pipe spatial grid supports, particularly under conditions of stress redistribution in the soft and fractured surrounding rock of large-section tunnels. The findings are as follows: (1) Significant differences in failure modes and crack propagation patterns are found between reinforced-steel concrete structures and steel pipe spatial grid concrete structures. The two novel steel pipe spatial grid concrete specimens demonstrate more complex crack distribution patterns and superior capacity for maintaining bearing characteristics during the plastic stage compared to the reinforced-steel concrete specimen. (2) In terms of load-deformation relationships and concrete strain development, the steel pipe spatial grid structures show improved composite action with concrete, exhibiting remarkable flexural capacity and structural ductility. In their ultimate bearing state, the mid-span deflection and bearing capacity of the triangular-steel pipe spatial grid concrete specimen increase by 24.09% and 25.52%, respectively, compared to those of the reinforced-steel concrete specimen. (3) Field measurements reveal that the triangular-steel pipe spatial grid support reduces tunnel crown settlement by 19.62% and horizontal convergence by 32.46%, confirming its efficacy in controlling the convergence deformation of surrounding rock in large-section tunnels. (4) The novel steel pipe spatial grid support allows for convenient installation and a high degree of mechanization, enabling rapid formation of closed-loop "face support" resistance.