North Strathfield Rail Underpass A One Pass Synthetic Fibre Reinforced Shotcrete Lining For A Very Shallow Cover Tunnel

The single track North Strathfield Rail Underpass (NSRU) consists of two dive structures either end of a 148m long driven tunnel. The underpass is for diesel hauled freight trains up to 1.5km in length. The tunnel excavation dimensions are 7 to 8m in height and 9m in width with has a horseshoe shaped profile. The permanent tunnel structural lining consists of synthetic fibre reinforced shotcrete. The ground cover over the crown of the tunnel varies between 2.5m and 3.5m. The ground profile is predominately Ashfield Shale with graded weathering from the surface to fresh shale. Finite Element analysis, calculated the stresses in the shotcrete lining, was also used to assist in predicting surface settlements. Apart for the initial steel canopy tubes no other steel support is installed in the driven tunnel (a first for civil transport tunnel in Australia). A repeated grid pattern of thirty-five grouted 12m long fibreglass dowels ensured tunnel face stability. The tunnel face was mapped daily. The 12m long canopy tube array installations are staggered relative to the 12m long face dowels by 4.5m. The excavation/shotcrete support cycle advanced in increments and the next cycle cannot commence until the initial 150mm thickness of shotcrete has reached an early strength of 6MPa. Early strength measurements of the shotcrete are a vital part of the construction. At the tunnel face, to support the train live loads, there are three levels of redundancy, the canopy tubes, the shear capacity of the ground slot to the surface and the structural/deflection capacity of the rails. The synthetic fibres in the shotcrete provide shrinkage crack control, residual strength if cracking occurs due to deformation and enhanced durability of the tunnel lining compared any alternative using steel such as steel lattice girders(and with no electrical stray current issues). Both the macro and micro synthetic fibres (the latter in the final 100mm fire protection layer placed over a spray-on waterproofing membrane) will reduce potential fire event related shotcrete spalling. Surface settlement minimisation relies on the construction methodology with the shotcrete over the arch always being very close to the tunnel face not allowing the ground to relax. Real time surface settlement monitoring was carried out using robotic scanning theodolites aimed at reflective prisms. In tunnel monitoring included convergence taping and optical targets. Excavation of the driven tunnel commenced in February 2014 was completed in late August.