1.Introduction 1.3 Design Principle Principle 1 The ground constitutes the main structural element of a tunnel Principle 1.1 Minimise disturbance to the ground during excavation or even increase its strength Principle 1.2 Allow displacements to mobilise its support capacity,leading to the ring effect Principle 1.3 Prevent excessive displacements to avoid loosening and consequently strength loss 百年同濟5 TONGII UNIVERSITY5 1.3 Design Principle Principle 1 The ground constitutes the main structural element of a tunnel Principle 1.1 Minimise disturbance to the ground during excavation or even increase its strength Principle 1.2 Allow displacements to mobilise its support capacity, leading to the ring effect Principle 1.3 Prevent excessive displacements to avoid loosening and consequently strength loss 1. Introduction