Numerical modeling of umbrella arch technique to reduce tunnelling induced ground movements

One of the most important aspects of tunnelling in soft grounds is to reduce any possible adverse environmental impact. That is, pre-supporting techniques are found to boost tunnelling advance speed whilst, at the same time, keeping the environmental consequences due to tunnelling process appreciably low. The umbrella arch method is one of the most common ground pre-supporting techniques used in tunnelling. The interactions between the ground and umbrella arches are very complex. There are also several parameters affecting the performance of the umbrella arch technique. Sophisticated numerical modeling is, therefore, required to gain a proper understanding of the mechanism of ground movement based on the umbrella arch method. In this paper, the role of the pipe roofing and face bolting on the stabilization of the northern portal of Sabzkooh tunnel is assessed using three-dimensional (3D) numerical method. Furthermore, the effects of design parameters such as the pipes installation angle, the pipes diameter and the transverse spacing of pipes on the performance of umbrella arch system are investigated through numerical modeling. The vertical displacements of the tunnel crown and the internal forces induced in the initial support system are extracted from the numerical modeling and are carefully assessed to identify the role of the pre-supporting methods. The outcomes of the numerical modeling demonstrate that when pipe roofing and face bolts are utilized, the tunnel convergence decreases and the induced axial forces in the initial support system tend to increase. These clearly highlight the role of pre-supporting technique on stabilizing the tunnel and transferring ground loads to the support measure. Results also show that reduction in the installation angle and transverse spacing of pipes and increasing the diameter of pipes can all considerably reduce the tunnel crown displacement while they increase the maximum induced axial forces in the initial support system. Stability analysis using bearing capacity diagrams also indicates that the initial support system with different configurations of pipe roofing and face bolts at different cross sections along the tunnel alignment is fairly sturdy and robust against the applied loads.