Structural response of former tunnel in the construction of closely-spaced cross-river twin tunnels

The interaction between closely-spaced twin tunnels has emerged as a prominent topic in tunnelling engineering, especially under complicated geological conditions like high water pressure and deep burial depth. This paper proposes a novel monitoring technology that utilizes MEMS multi-parameter fusion sensors in conjunction with wireless signal transmission technology to track the structural response of the existing tunnel during the construction of closely-spaced cross-river twin tunnels. The experiment enabled prompt deployment of monitoring devices without disrupting the construction process, and the investigation meticulously monitored the additional stresses, tilt angles, and relative displacement on the former tunnel lining. The results indicate that: (1) The progression of the additional stresses in former tunnel is classified into five stages, corresponding to the advancement of later tunnel's construction and dependent on the position of its heading face. (2) The intensity and spatial distribution of additional axial and circumferential stresses are non-uniform and depend on the proximity to the later tunnel. (3) The degree of change in the tilt angle is directly proportional to the distance from the later tunnel. (4) The deformation of the circumferential joint exhibits oblique uphill deformation with upward deflection at the lower-middle segment and downward deflection at the top, elliptical deformation with radial convergence and vertical expansion, and nearly no rotational deformation in the circumferential direction. The deformation of the longitudinal joint is manifested as an upward displacement of the upper segment blocks in the excavation direction, with radial convergence and leftward horizontal displacement, resulting in circumferential compression. (5) There is a remarkable correlation between the additional stress, tilt angle, and deformation characteristics, with the three factors changing in concert. (6) Secondary grouting exerts a significant influence on the stress and deformation of segments, and despite being localized, it may cause additional damage to the tunnel structure. This paper provides a dependable and efficient approach to monitoring the performance of twin tunnels in complex geological situations. Furthermore, the results highlight the interaction mechanism between twin tunnels and the combined effect of structural response, providing guidance for similar projects.