Design of sequential excavation tunneling in weak rocks through findings obtained from displacements based back analysis

Design of Sequential Excavation Method (SEM) and its support system in weathered and incompetent rocks is a primary challenge in tunneling. The Shibli tunnels that are being constructed within Zanjan–Tabriz freeway are located 25 km away from Tabriz with total length of 4533 m (north tunnel: 2244 m, south tunnel: 2289 m), 14 m width, and 11 m height. Three collapses that occurred at initial 800 m length of southern tunnel necessitated modification of either or both of the support system or excavation sequences. In this study, modification of the excavation sequences was merely taken into consideration for the high costs required to change the support system. Initially, the method of top heading and benching was proposed based on size of tunnels span and the ratio of Uniaxial Compressive Strength (UCS) to vertical in situ stress. Subsequently the excavation sequences were examined and designed precisely. Application of back analysis technique on three aforementioned collapsed zones led to identification of the most probable rock mass shear strength parameters. Results obtained from this analysis showed that in crown part of collapsed zones the displacement values had laid in an interval between 70 and 75 mm. Therefore, based on the weakest strength parameters obtained from the back analysis, three different sequences of excavation were proposed and sent to a finite difference numerical modeling which followed by an efficient SEM design with safety factor of 2 that reduced the displacements after excavation of top heading and whole tunnel section in the collapsed zones to less than 45 mm and 70 mm respectively. Thereafter, the modified SEM design has been applied successfully without occurrence of further collapses throughout excavation of the remained length of Shibli tunnels.

► We modified the excavation sequences based on learned lessons from several collapses during construction.
► Three collapses at initial 800 m length of southern Shibli tunnel dictated to modify the excavation sequences.
► We calculated the most probable rock mass strength properties using back analysis of collapsed.
► We suggested three different SEM and a safe and efficient SEM were selected.
► The modified SEM design successfully applied without collapses at the remained length of Shibli tunnels.