In order to investigate the stress

deformation

and stratigraphic effects of surrounding rock between the supported and unsupported TBM tunnels in deep mixed strata

an experimental investigation was conducted by a self-developed biaxial plane strain similarity model test system. The stress-strain measurement devices and the self-developed refinement digital photogrammetric measurement system named PhotoInfor were applied in this study. The results show that the stress distribution and failure mode are distinctly different between the upper-soft and lower-hard strata

showing an obvious non-uniformity stratigraphic effect. The stress in hard rock is larger than that in soft rock. As the confining pressure increases

the internal stress of surrounding rock firstly increases and then decreases along the radial direction for both supported and unsupported tunnels; under the unsupported condition

the evolution of the deformation and failure process and the broken rock zone of the surrounding rock have been fully investigated. The upper-soft rock is dominated by collapse

while the lower-hard rock is dominated by fracture. The development stages of radial displacement for surrounding rock is also summarized; the supporting structures effectively improve the stress redistribution and the integrity of surrounding rock

slow down the unloading speed and enhance the self-bearing capacity of surrounding rock.