Investigation of the effects of load reduction coefficient (deconfinement) with the new Austrian tunnelling method (NATM)

In this thesis, the finite element (FEM) method was applied to investigate the deformations in the tunnel excavation using the New Austrian Tunneling Method (NATM) in the (0+162)-(4+118) kilometers of road work in Hakkari Yüksekova-Yeniköprü area. The soil properties of the tunnel excavated by NATM were modeled in Plaxis 2D program using parameters obtained from borings and laboratory tests and the results of the analysis are presented. In order to obtain more realistic results, calculations based on the staged excavation method were performed; effective stresses, axial force, shear force, bending moment and maximum displacement graphs were obtained and evaluated for each case.In addition, a tunnel excavation using the New Austrian Tunneling Method (NATM) was numerically modeled in two dimensions using the deconfinement coefficient. In the modeling, load reduction coefficients (1-β) were determined at various values such as 10%, 50%, 60%, 70%, 70%, 80% and 90% in order to reduce the stresses during the construction phases of the upper and lower half excavations. As a result of the analyses, it was observed that as the deconfinement (1-β) value increases, the stress and displacement values in the excavation and lining stages increase. However, it was found that the shear and tensile stresses in the tunnel excavation section were mostly concentrated in the wall and floor. Based on this, it was determined that the appropriate selection of excavation stages is the most important factor.