ABSTRACT
This chapter deals with the selection of the constitutive models that must represent, as accurately as possible, the stress–strain behavior of an excavation. The main objective of a constitutive model, also referred to as failure criteria, is to define all possible limit stress–strain states of the ground. The chapter presents the stability mechanisms of the ground during tunnel construction and the available constitutive models; whose clear understanding is essential for the selection of the appropriate constitutive model. Constitutive models formulated on stresses are the most numerous, and among them, Mohr–Coulomb, Hoek–Brown and Cam-Clay are mostly used for the design of underground excavations, whose most important characteristics are explained. The strain softening constitutive model allows reproduction of the post-failure behavior, but before doing so it is necessary to perform uniaxial compression tests until failure.
