Geomechanical evaluations of Karaj tuffs for rock tunneling in Tehran-Shomal Freeway, Iran

This paper attempts to present a general approach to geomechanical evaluations of Karaj tuffs required for design and support of Tehran-Shomal Freeway tunnels. The freeway is to cross the Alborz Mountain Range through twin road tunnels varying from 300 to 6300 m in length with as much as 500 m of cover. Eocene Karaj tuffs crop out in more than 50% of the freeway route. Along this sector of the freeway, the Taloun and Alborz tunnels, each with 12 m width, are to be excavated. Geomechanical evaluations of the tuff masses were carried out using material properties, site investigations, and rock mass classification (RMC). The results of the evaluations show that the tuffs are structurally sensitive to fractures due to tectonic setting of the region. The degree of fracturing that controls the tuff mass characteristics was used to classify the rock masses into three main groups. The rock mass strengths were also determined using the integration of Rock Mass Rating (RMR) and Hoek-Brown failure criterion. Both empirical and numerical analyses indicate that the faulted tuff mass is not stable enough in tunnel openings and thus should be supported. Although the crystalline tuff mass analyses showed no indications of the plastic zone developments in tunnel surroundings, application of support system is recommended due to the jointed nature of the rock mass. Similarly, the required support system for the stability of each rock mass group in openings was also analyzed.