Crack Width Simulation and Nonlinear Finite Element Analysis of Bursting and Spalling Stresses in Precast FRC Tunnel Segments Under TBM Thrust Jack Forces

Tunnel boring machine (TBM) thrust jack force is one of the governing load cases for the design of precast Fiber-Reinforced Concrete (FRC) tunnel segments. Due to the concentration of the jacking forces and partial loading of the circumferential joints, the bursting stresses are formed deep under the thrust pads while the spalling stresses are generated in the areas between the thrust pads, and between the thrust pads and the end faces of the segments. Under high thrust conditions and in conjunction with eccentricity of the thrust pads, bursting and spalling tensile stresses can exceed the tensile strength of FRC which results in segment cracking. Serviceability Limit State (SLS) design includes limiting the crack width to allowable values. This paper presents results of three-dimensional nonlinear finite element analyses (3D NFEA) of TBM thrust jack forces by modeling the actual parallelogram shape of segments, the actual size and positions of thrust pads, and maximum jacking forces for an ongoing construction project in North America. Tensile response of FRC as a multi-linear stress-strain curve is obtained following an inverse analysis procedure conducted on nominal response of standard beam tests. Presented NFEA simulation is recommended as a reliable and optimized procedure for design of FRC segments loaded by TBM thrust jack forces.