A Simplified Algorithm for Designing Tunnel Composite Support System in Discontinuous Rock Media

Abstract

Since the introduction of the primary support systems in tunnels, many researchers tried to enhance their composite-like behavior by improving the behavior of structural elements. In this study, the behavior of composite lining with three elements (mesh + frame + shotcrete) was investigated based on the share of each structural element from section’s internal forces using equations derived from shell theory. Then, the result was compared to results of composite section consisting of two elements (frame + shotcrete) in order to determine the role of mesh in bending and axial capacities of composite section. The results showed that mesh element helps the section’s composite-like behavior and increases the bending and axial capacities. Based on this comparison, an algorithm was presented for determining frame spacing as a critical parameter with technical and economical points of view. In this algorithm, an iterative process was introduced to determine lining equivalent section’s properties, inputs for universal distinct element numerical model, based on structural elements, frame spacing and structural analysis methods. The outputs of the numerical method (bending moment distribution, axial force and shear force in the equivalent section) were returned to theoretical model and were compared to structural elements’ capacity diagrams. This process continued until the internal forces reached the range within that of the capacity diagrams. Results indicate that the addition of mesh element reduces the shotcrete’s and steel frame’s share of section’s internal forces. Thus, the steel frame spacing could be increased and the costs of primary support system per meter could be reduced.