Model test study on the performance of hexagonal segment joints in TBM tunnels

Hexagonal segments for TBM tunnels have attracted extensive attention due to bolt-free connection and efficient assembly technology, and the mechanical behavior of joints acts as the key factor determining the performance of lining structures. Half-arcs of large-scale precast hexagonal segment models with a geometric similarity ratio of 1:10 were assembled to form joint specimens. A self-designed loading system was adopted to apply positive and negative bending moments separately under constant eccentricity, so as to systematically investigate the force-bearing mechanism of joints and the evolution laws of joint opening and dislocation under different bending modes. The results show that under positive bending moments, the force and deformation of joints present staged characteristics, which successively go through three stages: elastic deformation, development of joint opening and dislocation caused by relative slip of contact surfaces, and local crushing of joints at the ultimate state. Upon failure, the strain on the upper surface of joints rises sharply to the ultimate material strength, showing typical material strength failure. Under negative bending moments, the mechanical response of joints is mainly governed by geometric nonlinearity. The arching effect of hexagonal segments gradually weakens and even disappears after linear deformation. Under the coupling effect of axial force and negative bending moment, the effective compression height of joints keeps decreasing, and the segments tend to extrude outward. At the moment of failure, the peak strain of joints is only 72.78% of that under the positive bending moment working condition, presenting an integral instability failure mode of "disintegration", which indicates that the bearing capacity is controlled by structural stability rather than material strength.