The 5.5 m wide 740 m long Namroud project diversion tunnel located in the Tehran province of Iran was driven through sedimentary rocks comprising limestone, marlstone and limy shale. The initial design of primary support for the tunnel was based on the data collected from a limited pro-gram of site investigation and the application of rock mass classification approach. During excavation the rock mass conditions encountered were different to those predicted at the project design stage, and as a result the excavation methods and the tunnel support systems had to be changed. Despite the changes the tunnel was constructed on time and within budget. This paper presents (a) the predictions made at the design stage, (b) the results of the classification of rock mass intersected during construction, and (c) the actual excavation and the support methods used.

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