Keywords: railway track, tunnel lining, drag force, pore-pressure, liquefaction.

Railway tracks of common types for high-speed trains consist of a well compacted sub-ballast bed of crushed granitic rock and dense overlying ballast of somewhat finer gravel or stone material. Both are self-draining and let infiltrated rain and meltwater through for lateral discharge. For tracks in tunnels it is required that the ballasts are effectively drained since the dynamic impact of heavy fast trains on water saturated ballast can create porewater overpressure leading to liquefaction and loss of bearing capacity. In practice this is avoided by minimizing inflow of groundwater into the tunnels by effective grouting of the rock in combination with lining the tunnels with concrete cast on site or by shotcreting. Tunnel excavation and stabilization in weak and strongly water-bearing rock at great depth can be very difficult and new techniques for fast construction are being tried. For bored tunnels construction of liners by installation of tightly fitting concrete block elements and backfilling of the space between rock and block liner with cast concrete have been applied. Modelling of the performance of such block liners indicates that intense train traffic and a series of emergency brakes can cause mutual shearing and separation of adjacent blocks causing high water inflow and water saturation of the track ballast after a few years if the tunnel inclination is very low. The dynamic impact of the train traffic can then, for long trains and intense traffic, build up a porewater overpressure in the ballast that can generate liquefaction and loss of stability. The rear wagons will leave the rails and the progressive winding motion of a long set of wagons will quickly reach the locomotive and bring it off the rails causing great havoc.

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