Patchway consists of three tunnels. On the down line there are two single bore tunnels with a short gap of a few hundred yards between them. On the up line there is a single tunnel. The drain running through the down tunnel runs approx 1m below the four foot and had suffered damage during a recent accident. RDL had been invited by Alfred McAlpine Civil Engineering to install a temporary carrier drain through the down tunnel as part of the Great Western Zone earthworks & structures partnering arrangement (GWESPA) with Network Rail.

In December 2002 three teams of safety critical personnel each utilising a 360° road rail excavator and trailers under the RDL railway safety case (RSC) were used to undertake the works. A team worked outside the English end portal installing a new catch pit and under track crossing in order to divert water across to the new down line carrier drain. Modifications were made to a further chamber in the down cess to divert water during peak flow.

The main works consisted of the excavation of material and the installation of 1400m of 300mm carrier and perforated pipe through both down tunnels, down an embankment and in to the up ten foot drain. Much of the drain was surface mounted being secured at ground level by brackets to the tunnel wall.

The works were successfully carried out to timescale and budget for Alfred McAlpine Civil Engineering.