A criterion for determination of minimum distance between twin tunnels by pillar reaction curve (PRC)

Determination of optimum distance between twin tunnels is very important. There are many empirical models to estimate width of the pillar, all of which have their own limitations. In this study an attempt was made to present a new criterion for design of the pillar between twin (parallel) tunnels. It is based on ground reaction curve and numerical modelling. 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 located in Arak-Khoramabad Expressway, was selected as a case study. This Tunnel is made of eight zones in layers of marly limestone and one zone in alluvial deposits. Results of the suggested method show a good agreement between the pillar dimensions seen in the case study obtained from instrumentation and the method (new criterion). In zone 3, as an example, the amounts of rock mass compressive strength and strain in the pillar are 3.23 MPa and 0.053, respectively. In this zone, 1.8 was obtained for the w/h ratio, according to the analysis the minimum width for the stable pillar must be 17.1 m. Taking all aspects of geomechanical and geometrical characteristics of the tunnels into consideration, this method is capable of determining the optimum width of the pillar when designing the twin tunnels in static conditions with a high level of precision in all Iranian tunnels.