On-site Testing of Strain of Feet-lock Pipe and Its Restraining Effect on Tunnel Feet in Soft Rock

Considering the limitations of the traditional resistance strain gauge method for on-site testing the strain of feet-lock pipe, a φ50 Fiber Bragg Grating (FBG) feet-lock pipe was designed and manufactured based on FBG sensing technology. A field test of the FBG method for testing the strain of feet-lock pipe was carried out. The stress features and supporting function of the feet-lock pipe in soft rock tunnel were analyzed. Then, a mechanical analysis model of feet-lock pipe in soft rock tunnel was established, and the formula for calculating the support stiffness of feet-lock pipe on the feet of primary support was derived. The influence law and sensitivity of each parameter of feet-lock pipe on the support stiffness were quantitatively analyzed. The results of this analysis show that the strain variation law at each measuring point of the feet-lock pipe was very complicated owing to the construction disturbance and connection method of the feet-lock pipe. From the overall strain distribution of the pipe, the strain of the feet-lock pipe near the steel rib significantly exceeds those of other parts of the pipe, and the change amplitude of the strain at the end of the pipe near the steel rib obviously exceeds that near the surrounding rock. The feet-lock pipe is primarily subjected to lateral bending deformation in the upper and lower directions, and is subjected to compressive load transmitted by the tunnel feet in the axial direction. As the angle of the feet-lock pipe increases, its axial compression characteristics become increasingly significant. The axial anchoring effect of the feet-lock pipe is very small, primarily exerting lateral bending and shear resistance to constrain the settlement of the tunnel feet, and the constraint effect on the horizontal convergence deformation of tunnel feet is limited. Increasing the diameter of the feet-lock pipe is the most effective way of enhancing the vertical support stiffness of tunnel feet. When the axial support condition of feet-lock pipe is poor, increasing its angle significantly reduces the vertical support stiffness of tunnel feet. At this time, the steel rib should be closely attached to the surrounding rock. As the length of the feet-lock pipe increases, the vertical support stiffness of the tunnel feet provided exhibit rapid growth initially, followed by gradual growth. Considering the stress characteristics of the feet-lock pipe and the engineering economy, a length of 2.5 m is recommended for the φ50 feet-lock pipe. © 2023 Xi'an Highway University. All rights reserved.