New method for track reconditioning and drainage improvement in tunnel track on heavy haul line

![CDATA[The Ulan Line is part of Australian Rail Track Corporation’s Hunter Valley Corridor. It extends approximately 170km from Muswellbrook to the coal mines at Ulan and on to Gulgong. The line is mainly used by Coal trains with a heavy 30t axle load. An unusual capacity constraint is posed by tunnels on the Ulan line, in particular the Bylong Tunnel 1, which was built in 1915. Within the tunnel, a 200 mm thick subgrade blinding slab sits over the rock subgrade. In recent years, the track condition in the tunnel has deteriorated to the point where routine maintenance activities have struggled to manage the asset. This deterioration has included: ballast fouling with the pumping of fines from the rock subgrade through a suspected fractured blinding slab, which has led to difficulties in keeping track geometry and requiring high-frequency maintenance. Upon initial exposure of the slab, the slab was in very poor condition, broken into slabs of varying dimensions, typically less than 1m. The slab was also found to be sunken below the original founding level up to 500mm. A geotechnical assessment confirmed the presence of high cyclic loads, interbedded and fractured siltstone and claystone with the presence of groundwater in these erodible strata which is considered to have resulted in the observed pumping. Works inside the tunnel consisted of excavating the fouled ballast, removal of the existing slab, removal of unsuitable material beneath the slab to a depth of 1400mm below the top side of the “nib walls” of the tunnel, subgrade strengthening with geofabric and geogrid, drainage trench excavation and installation of a 225mm diameter slotted HDPE pipe backfilled with 20mm aggregate with a no-fines concrete overlay, installation of two strip drains at the top of the trench all encased in geofarbic. One of the key assessed risks associated with the reconditioning work was the potential to induce movement in the tunnel walls (rock and lining) due to stress relief in the floor during and post the excavation. In order to estimate the stability of the structure, a simplified parametric analysis was undertaken using FEA software to assess the potential elastic tunnel movements.]]