## 1 Introduction
On the other hand, the KST model provides a description of both volumetric and deviatoric behaviours of geomaterials (rocks and concrete) under confining pressure, where the deviatoric response corresponds to the increase of the maximum principal stress difference as function of the hydrostatic pressure (defined as minus the mean of principle stresses) and the volumetric response is defined as the change of volumetric strain (compaction) as a function of the hydrostatic pressure. In the deviatoric part, the locus of the yield surface is a quadratic function of the mean pressure in the principal stress space (Equation (4)), where \(\sigma_{q}\) is the equivalent (von Mises) stress, \(P\) is the hydrostatic stress, and \(a_{0}\), \(a_{1}\), and \(a_{2}\) are constant coefficients related to the considered material (Figure 1b). A piecewise linear equation of state linking the volumetric strain to the hydrostatic stress (Figure 1b) is used to describe the collapse of pores that occurs in the case of porous rocks under high confining pressure. When all pores are closed, the material exhibits a higher bulk modulus (\(K_{final}\)) which corresponds to the compacted material. As illustrated in the present paper, both responses can be determined based on a single quasistatic, quasioedometric compression (QQC) test. At this stage, it is necessary to mention that there exist other constitutive models, for example, the Drucker-Prager law [10], which can be utilized in particular cases when the plastic behaviour of the material deviates from the associative flow. This feature is most often described by the so-called dilation angle of the material. A typical rock material that has been constitutively characterised pertinent to the KST-DFH model is granite, see [11; 12; 13; 14]. In particular, this constitutive model was employed with the application of percussive drilling in mind. Apart from the explicit determination of material constants and functions, these investigations also showed that pre-existing (or structural) cracks have a substantial effect on the mechanical response and on the fracture pattern at, for example, impact loading. Obviously, as these investigations rely on the KST-DFH model, the material behaviour at both high- and low-strain-rate values were determined. In a recent study, the effect on the mechanical behaviour of granite from nonassociative plastic flow was also investigated [15; 16]. In particular, in [16], it was shown that the direction of the plastic flow has a significant influence on the mechanical response at spherical indentation. Indeed, not only is it important to account for nonassociativity, but also to determine change of the plastic flow direction at different values on the hydrostatic pressure. In the constitutive modelling, a Drucker-Prager law was employed together with a variable dilation angle. The influence from changing nonassociativity is, of course, a feature that can be of great interest also for other types of rock materials [17]. In the present work, an important experimental campaign was conducted using a sandstone rock provided by the international company Epiroc Rock Drills to identify the mechanical behaviour of this rock material. Three types of static experiments were used to obtain first the Weibull parameters associated to the DFH model and second the parameters related to the KST plasticity model describing the response under high confinement. In particular, a comparison between the results from the direct compression tests and quasioedometric tests highlights the influence from structural defects at different loading conditions. It should be emphasized that, although many of the experiments  
Figure 1: Description of DFH–KST coupled plasticity-damage model. (**a**) DFH (Denoual–Forquin–Hil) anisotropic damage model. (**b**) KST (Krieg–Swenson–Taylor) plasticity model.