Verification of geotechnical data of Qazvin _ Rasht tunnel using back analysis of instrumentation data

Various types of numerical analyses such as the Finite Element Method, the Boundary Element Method and the Distinct Element Method, are used in rock mechanics and in engineering practices for designing rock structures such as tunnels, under ground caverns, slopes, dam foundations and so on. In this paper, the results of back analysis of Koohin tunnel which is located in the first section of Qazvin – Rasht railway have been presented. The main purpose of this paper is to perform the back analysis of the mentioned tunnel with the use of numerical models. For modeling the tunnel, two different sections are analyzed with Flac 2-D software, which are 30+150 and 30+900. To perform back analysis the suitable interval of geomechanical parameters according to the tests which performed on the core drillings has been determined. With the use of Direct Method in back analysis, the errors of models have been corrected in several steps and finally the geomechanical parameters of control in 30+150 station (Elastic Modules = 0.3 GPa, Cohesion = 0.21 MPa & Internal Angle of Friction = 34°) and in 30+900 station (Elastic Modules = 0.3 GPa, Cohesion = 0.21 MPa & Internal Angle of Friction = 35°) have been achieved. The geomechanical parameters which achieved from back analysis are completely in the chosen interval and compliance with the results of tests which performed on core drillings. On this basis and according to the match of the geomechanical parameters which obtained from back analysis with the parameters which used in designing of the tunnel, the tunnel design and the structure method were confirmed.