A method for selection of optimum distance between twin tunnels under static and pseudo-static conditions, case study: Pooneh tunnel

Abstract

Determination of optimum distance between twin tunnels is very important to reduce the construction’s costs in road or railway projects. When two tunnels are located close together, stress concentration in their space increases and it is possible that plastic zones would be merged. In mining engineering and especially underground mining applications, this zone (the distance between twin tunnels) is called pillar and there are some empirical models to estimate its width. However, these models are not completely suitable for all rock types and different tunnel geometries. In this study, an attempt was made to present a new criterion for the design of twin tunnels. It is based on ground reaction curve and takes width to height ratio (w/h) and shear strain of the pillar (obtained by numerical modeling) into consideration. Strain magnitude observed when normal stress (σ1) in the pillar reaches the value of compressive strength of the rock mass (σcm), denotes the allowable ratio of width to height (w/h) which shows the optimum distance between twin tunnels. Pooneh twin tunnels which located in Iran were selected as a case study. These are road twin tunnels excavated in layers of nine different zones (Arak–Khoramabad Expressway). Results of the suggested method show a good agreement between the pillar dimensions seen in the case study and the present method. Taking all aspects of geomechanical and geometrical characteristics of the tunnels into consideration, it is one of the main advantages of this method.