Face Stability Analysis for the Earth Pressure Balance Method in Nonhomogeneous Inclined Soil Layers: Case Study

Earth pressure balance tunnel boring machines (EPB TBMs) were successfully used for tunnel excavation in loose grounds. Determining the minimal support pressure in the tunnel face is one of the most critical steps in tunneling by the earth pressure balance method. This study aimed to investigate the tunnel face stability of the Dez-Ghomrood water transfer tunnel analytically and numerically. During the water transfer tunnel project from Dez to Ghomrood, a very weak rock mass similar to an alluvial zone was encountered. Considering the depth of the tunnel and also the layering of the ground, calculating the proper support pressure of the tunnel face was significantly complex. The analytical method was based on the slope of layers relative to the tunnel face and the limit equilibrium of forces. Numerical analyses were conducted based on three-dimensional finite-element simulations using the PLAXIS 3D tunnel. The results demonstrate that the formation of soil arching yields to the curvilinear distribution of the weight of upper levels, therefore, pressure reduction at the tunnel face. The geomechanics parameters of the ground, shield geometry, and ground convergence in the excavated section are the determining factors in calculating the support pressure of the tunnel face. The slope of ground layers should also be considered in the tunnel face failure mechanism and in support pressure. The operational support pressure is compared to the analysis results, and the good accuracy of the method is confirmed.