A better understanding of the failure process makes it possible to evaluate the behavior of underground excavation with regard to instability problems such as rock fallouts. The failure process of brittle rock has been the subject of many studies. Some studies have focused on using crack initiation and crack damage stress to better evaluate the rock behavior around underground excavations. Furthermore, a number of failure criteria have been presented in the literature in order to predict differing rock behavior. However, only a few failure criteria for rock are formulated in terms of deformation parameters. The stress-based failure criteria are often not adapted to predicting fallouts in an underground excavations and the connection between in situ behavior and predicted behavior of the rock is often lacking. Since it is possible to perform in situ deformation measurement, the strain behavior of the excavation more easily could be determined and evaluated. Hence, a failure criterion formulated in terms of deformation parameters could be considered as a criterion to study and evaluate the failure process of hard rock underground excavations. The study presented in this paper was based on uniaxial laboratory tests carried out on rock-specimens typically found in Fennoscandia.The rock types tested include:limestone,quartzite,diorite,granite,norite,gabbro,diabase,and syenite porphyry.The paper evaluates the stages and critical states of deformation of these rock types.The aim of this paper was to identify the deformation parameters that govern the failure of hard rock.Different relations between deformation parameters were evaluated.The parameters studied and relations were:(i) volumetric strain vs.rock types, (ii) axial and lateral strain vs.rock types,(iii) volumetric strain vs.axial and lateral strain,and(iv) axial strain vs. lateral strain. The scatter in the normalized axial stress,and axial and lateral strain relations leads to the conclusion that deformation stages such as crack closure,crack initiation,and crack damage are influenced by the rock characteristics.The normalized relations indicate that rock types such as diabase,granite,syenite porphyry,limestone and quartzite gave values that were consistently outside the limit(i.e.,mean±standard deviation) of these stages.These standard deviation values also indicate that it may be better to consider strain parameters as indicators of failure instead of only using stress parameters.These findings further suggest that failure behavior is dependent on the rock type, stress capacity, and strain behavior.Thus,deformation stages should be defined on the basis of(i) individual rock type,or(ii) groups of certain rock types. The standard deviation values also show that the volumetric crack damage may be an important parameter in the evaluation of failure characteristics. The correlation coefficient showed a linear relation between the volumetric crack damage strain and(i) the axial crack damage strain, and(ii) the lateral strain at crack closure, crack initiation and crack damage. Linear relation between axial strain and lateral strain quantities also exist with exception between lateral crack damage strain and(i) axial crack closure strain,and(ii) axial crack initiation strain,