1. Experiments reveal not only a quantitative difference but also a qualitative discrepancy between the model of a continuous medium on which the mechanics of deformed bodies is based and the properties of a real solid. Thus for salt rocks it is found that the volume increases owing to gradual opening-up and development of defects and microcracks. The effect of compaction, found in Cambrian clay, is influenced by drying-out of the material during an experiment, by closing of pores and gaps, and by recrystallization processes which occur during longterm tests. The time factor involves serious changes in our ideas on the behavior of a solid under load. Bulk deformations may exert a serious influence on the formation of the state of stress in the solid rock; in particular, in the abutment pressure zone, the expansion effect may create an additional force which tends to eject the rock into the worked-out area this is especially important in the problem of shock bumps and rock bursts. A decrease of volume reduces the repulsive force directed towards the worked-out area. 2. As a result of laboratory tests on creep and long-term strength in specimens of potash salt and Cambrian clay, it is found that the creep rate\(\dot \varepsilon _1\) is related, within the investigated range of conditions, to the stress as follows: $$\dot \varepsilon _1 = \dot \varepsilon _0 e^{\alpha \sigma }$$ (1) . The relation between the endurance t and the stress is $$t = t_0 e^{ - \alpha \sigma }$$ (2) . 3. Experiments reveal that the coefficient \ga in Eq. (1) is nearly equal to that in Eq. (2). 4. Since the absolute values of \ga in Eqs. (1) and (2) are nearly equal, it is suggested that the acting stresses and times to fracture of salt pillars should be determined from the known creep rates.