Rock salt units exhibit different behavioural characteristics according to the ratios and positions of their ingredient mineral components, foreign material contents (such as clay, anhydrite) and environmental conditions (such as stress, temperature and humidity) [1]. In addition, rock salt exhibits a nonlinear behaviour that depends on the in situ stresses formed under different environmental conditions, mineral components and time. The time-related stress-strain behaviour is rather complex [1–5]. Although a considerable number of studies have been performed to understand this behaviour, this complexity brings about important problems in design studies. The mathematical solutions used in the analysis of hard rock salt mechanics are very difficult and time consuming. However, with the aid of in situ observations and experimental data, some numerical analysis methods are utilised to obtain the best model of material behaviour, which will be assumed to be very similar to the actual behaviour [6]. Creep tests are applied to ductile materials, such as clay and rock salt. In the engineering applications of rock salt, the authors suggest that using the optimum strength determined using the creep tests that are performed under dead load (DL) will produce better results in comparison to the UCS values representing failure strength. The creep behaviour based on time consists of three stages, the first of which is formed within the elastic limits [7]. However, damage occurs at the third stage, in which the rock salt material failures continue for a long period of time due to DL. The creep behaviour of salt rock has been investigated by some researchers using laboratory or field tests. Many mathematical models defining creep behaviour have been presented in the literature. The first creep model was developed by Philips [8], and similar studies were performed by other researchers [9–31]. Different mathematical models, which are generally based on time (t), activation energy (Q), gas constant (R), temperature (T), stress (s) and some other statistical coefficients, were developed in these studies. Among them, some models were defined using a logarithmic function, some used a power function, some revealed exponential characteristics, and a few of them were developed using differential equations. Also, it was seen that experimental investigations of the mechanical properties of rock salt under triaxial loading are presented [32–37]. The creep behaviour on rock salt can be well understood from the schematic graph developed by Jeremic [7], which consists of the three stages of creep. If a DL is applied to the material, there will first be an instantaneous elastic strain. A primary strain, which is also called transient strain, follows this elastic strain. In the second creep stage, steady-state creep behaviour is observed, with an almost constant slope. The last stage in the schematic diagram includes a tertiary or accelerating creep leading to sudden failure. According to Jeremic [7], if the applied DL in the transient strain stage is removed, the strain rapidly reduces in the beginning and then approaches zero asymptotically. In this phenomenon, no