Large Up Size Pipe Replacement Beneath The West Highland Railway

Running at around 5m depth, the 450mm existing culvert needed to be not only replaced but also up-sized to 630mm with the replacement pipe being HDPE. Whilst the project at first appeared to be a straight replacement job, with the need to utilise a 730mm diameter expander behind the bursting head, an equivalent up-size of over 250mm diameter, the installation of the new pipe was not going to be easy.

Although the contractor had used the Hammerhead® HB125 on an earlier section of works on the West Highland rail route, given the massive up-size required for the new installation, operator training of Carillion personnel was completed by U Mole engineers prior to the works on site at project site at Taynuilt.

Other parts of the challenge on the Taynuilt site over and above the up-size required, was access and site preparation. To achieve the replacement installation site works were undertaken for 14 days before the bursting work could commence. This was because ground conditions at the site proved to be soft which meant the footings for the pipe bursting system had to be correctly constructed. The site also required the removal of a line of trees in the rail embankment. These site preparations of the launch and burst site had to be completed with imported stone etc. Also required was the environmental protection of local waterways in case of any hydraulic leaks etc.

Once the site prep was completed the job could be started. Carillion’s workforce was ultimately delighted at the ease of operation of the bursting system once set up. The bursting works, including machine set up were completed over the course of just two days. The actual simultaneous burst run and new pipe installation were actually completed in less than half a working with day no disturbance to the railway line on the embankment above. Looking ahead to a series of future bursts beneath the railway line members of the crew commented that they were looking forward to once again using the HB125 system.

In terms of the site set-up, another complication which was faced by the contractor was ensuring that launch pit face was able to resist the pulling forces of the bursting rig. Carillion’s Section Engineer for the works, Glen Henderson, had to develop a temporary jacking face system which was capable of distributing the pulling force of the rig against the relatively soft railway embankment. Glen said: “Due to the added resistance caused by the expansion process when bursting the existing pipe, the pit face had to be very robust. The set-up ultimately consisted of driven sheet piles with 13 mm (½ in) thick steel road plates.”

Works were carried under the normal working times of the trains. RETB tokens of the OBN2 line between Dalmally station and Connel station were taken and pipe pulling operations to install the new pipe took place between trains. The contractor also continuously monitored the cant and level of the track so that the works could be stopped at the first sign of any movement. The depth of the fill between the pipe installation and the track allowed sufficient consolidation through the embankment such that during the operation no track movements occurred.

Glen went on to say: “Due to the depth of this culvert below the track level, the traditional deep excavation and replacement would not have been feasible. With guidance and training from U Mole we were able to exploit the benefits of trenchless technology. We were able to cut down on our material usage in terms of not requiring substantial amounts of backfill and we minimised our wastage by having no excavated material to dispose of. Any opportunity where we are able to keep the railway open to traffic whilst undertaking this type of work puts us ahead of our competition and allows us to achieve a more economical and sustainable end result for both Carillion and ultimately Network Rail and the service’s customers.”

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