In rock mechanics, several researches have been contributed to the creep behavior, however, a limited number of studies have been carried out on the relaxation behavior of rocks. Impression test is a novel technique to predict time-dependent behavior of materials. In relaxation impression, a cylindrical flat ended indenter penetrates into a small region of sample and after holding the indenter at a constant position, the decrease in the stress level is recorded. Impression relaxation tests in comparison with compression relaxation tests are much more economical and less time-consuming.
In this article, 25 impression relaxation tests using indenter diameters of 2, 2.5, 3, 3.5, 4mm and 4 compression relaxation tests are done on rock salt. Then, the parameters of viscoelastic behavior are calculated. The results of both techniques of impression and compression tests illustrate that the value of µ in Maxwell model decreases as the stress increases.
for constructing deep underground spaces in many fields of rock engineering such as mining activities, petroleum production and deep waste material disposals. These constructions are led to a great collapse while contacting salt layers. Consequently, understanding characteristics of time-dependent behavior, deformations and failure around major underground structures and spaces is incredibly important, and stability and safety of the structures must be investigated according to their specifications. 
Creep is the time-dependent behavior of materials under constant stress and relaxation is the timedependent behavior under constant strain. In natural geological systems, the differentiation between pure creep and relaxation is hard and the combination of creep and relaxation is commonly observed. 
When a constant strain is applied on a viscoelastic material, the necessary force to maintain the strain is not constant and changes with time. This behavior is called stress relaxation. Conversely, when a viscoelastic material is subjected to a constant stress, the deformation takes place during the time. This behavior is called creep. Figure 1 shows typical schematic of relaxation and creep behavior of material. [2,3]