Triaxial creep tests and back analysis of time-dependent behavior of Siah Bisheh cavern by 3-Dimensional Distinct Element Method

Time dependent effects or creep behavior of rocks has great importance in further development of knowledge in the field of rock mechanics. An increase of pressure on support system due to creep behavior of rock is one of the most important issues in underground structure with weak surrounding rock mass. In this contribution a time-dependent behavior analysis of Siah Bisheh pumped storage powerhouse cavern with complex geometry being under construction on the Chalus River at the north of Iran were investigated. The cavern surrounded rocks containing Shale, Limestone, Sandstone and igneous rock in major parts is located in Alborz Structural Zone. The cavern is being built in a region that is highly prone to sheared and faulted zones. Therefore, it is essential to analyze and design underground structures to prevent any serious long-term damages in this region. The rock mass may exhibit continuous or discontinuous deformations due to excavation of large underground openings; therefore, deformation include shearing of joints and creep deformation of rock material. Because of the fractured and jointed rock mass, the Discrete Element method used to back analysis the time-dependent behavior of the Siah Bisheh cavern. In addition, triaxial creep tests were performed on rock specimens in order to estimate the time-dependent behavior of rock around the cavern. The creep tests and in situ measurements were employed to estimate parameters of power constitutive creep model being able to model the primary and secondary creep regions of rock masses implemented in the 3-Dimensional Distinct Element Code. Simulation results show good agreement with monitoring data. By excavating the lower stages of the cavern, some instantaneous deformation occurs in displacement–time curve of the crown.