Analysis of the rock fatigue in tunnels by finite element method

Abstract

This thesis investigates the fatigue effect caused by cyclic loading on the deformation development around the underground openings. Employing PLAXIS 3D FEM package for the numerical modelling of a hypothetical tunnel opened by the NATM in Bayburt Tuff for static and cyclic loading conditions to analyze stresses and deformation developments. The investigation is divided into two stages: 1) physico-mechanical characterization of rock properties and performance of static and cyclic loading tests, and 2) numerical analysis applied to static and cyclic loading cases. PLAXIS 3D simulated four hypothetical NATM tunnels considering the Hoek-Brown failure criterion; being three of them based only on frequency variation (0.2 Hz, 0.6 Hz, and 1.0 Hz) while keeping the number of cycles (500) and load amplitude (30-50% from UCS) constant. Concluding as follow: i) the frequency directly influences the strength and elasticity values of the Bayburt tuff; in other words, the fatigue effects on rock specimen will be significant at lower frequencies. ii) |umax| (0.2 Hz) < |umax| (0.6 Hz) < |umax| (1.0 Hz) < |umax| (static), being |umax| the maximum total displacement. iii) tuff responds with an inverse relationship between principal stresses and frequency. iv) the fatigue effect influence appears through proportionality between frequency and safety factor (SF) values.