Today, the construction of structures with a long life, such as underground spaces, is necessary considering the population growth. Given the nature of these structures, they function for a long time. Hence, analyzing the time-dependent behavior of underground structures is substantially important. Creep is one of the most important time-dependent natural phenomena. This phenomenon generally occurs in soft and loose rocks. Time-dependent behavior is also one of the properties of rock material which can help explain and study many phenomena such as the predicted stability of rock structures. Despite the importance and necessity of experimental tests in geomechanical projects, the results of these experiments do not present a precise assessment of the creep parameters due to the small rock sample size. In in-situ dilatometry tests, the precise assessment of time-dependent behavior is possible through the identification of the creep parameters. In this thesis, first, the creep parameters of the tests carried out by Goodman were identified to correctly assess the creep parameters. The results were, thereafter, compared with the results reported by Goodman. Afterward, the creep parameters of several rock types were identified by studying the rock mass deformation behavior in in-situ dilatometry tests. Finally, the rock mass long-term creep was calculated by obtaining Burger’s time-dependent curve for different rocks. It was also indicated how the behavior of rocks varies over time. One of the remarkable results of this study was the calculation of the rock mass time-dependent behavior curve using in-situ tests, which could be used in the numerical modeling of rock masses and calculation of long-term creep, especially in rocks exhibiting time-dependent behavior