Water under the bridge

Underground Moling Services has completed a complex operation involving the rerouting of a water main and power cables during the renovation of a 100-year-old bridge. Space was tight, and time even tighter.

As part of the national drive towards reducing road congestion and carbon emissions from road traffic, the multimillion-pound Central Scotland Rail/Road Freight Interchange facility has been built at Grangemouth, Stirlingshire.

This is intended to take freight traffic off the roads and place it on to the rail network. But there is still significant heavy goods traffic associated with the interchange facility – freight is brought to the site to be placed on to the rail system.

One of the road bridges on the approach to the site, which was more than 100 years old and passed over the rail tracks serving the interchange facility, could no longer carry the HGV traffic loads. So, it was decided to renew the deck of the bridge.

As the road passed over the railway track, the ultimate client for the work was Network Rail. But the Roads Authority, Scottish Water and Scottish Power were closely involved with the operations. Beneath the existing road surface, there was both a water main and power cabling, which would need to be redirected as part of the bridge works.

Rerouting utilities

During the planning process, it was decided the rerouting of these utilities would be best achieved by removing them from the bridge structure altogether and installing new parallel services beneath the rail tracks to the west of the bridge. The installation of two new 250mm diameter MDPE water mains and two new 125mm diameter MDPE cable ducts would be required, each installation being around 65m long.

The ground conditions at the proposed rail crossing site were good, being predominantly reasonable clay soils. But this was not a simple operation.

While no particular consulting engineer was employed by Network Rail, Scottish Water brought in consultant Donald Stocks, of Water and Pipeline Services, for its part in the crossing operation. The main contractor for the overall works was Carillion.

Several months of discussions between Carillion, Scottish Water, and its consultant ensued to find the best solution among the crossing installation options available.

These discussions included consultations with Underground Moling Services (UMS), a specialist utilities contractor with experience in trenchless installation for this type of work.

Ultimately, these discussions led to an acceptance by all parties to use UMS’s expertise to install the pipes/ducts beneath the rail track using HDD techniques for the project.

One of the main obstacles to be overcome in planning the trenchless installations was that the ground levels on either side of the rail tracks differed considerably. This was especially the case at the target/exit end of the proposed bores, which was close to the rail boundary and comprised a steep approach to a raised exit pit within a small land area.

This meant that the bores would need to be installed with great accuracy so as to hit the target area and yet maintain cover under the rail tracks required by Network Rail.

The reason that the target area was so small was the number of different landownership in the industrial area. This meant that target area chosen was the only small piece of ground that the client had permission to use to get the water mains and electricity ducting into from the rail to the public roadway.

Topography and space were not the only site limitations as time was also of the essence. The only window of opportunity in which to complete the essential bridge works was one week over the Christmas holidays. So, both the Network Rail and the utility companies wanted to be sure that water and electric cable diversion works would be successfully carried out in the restricted two-week period leading up to the Christmas break.

Also, the installation of the new cables and commissioning of the water mains would need to be completed. So, it was decided the new pipeline and ducting installation works had to be completed over just one weekend at the beginning of the two-week run up to Christmas to allow the new service commissioning works to be finished while allowing time to decommission the old bridge services.

Having placed this urgency on UMS to complete its part of the works, it was decided by UMS engineers that two units from the company’s fleet of HDD rigs would be used in order to meet the schedule.

UMS used a Ditch Witch JT4020 MK1 horizontal directional drilling rig and a new JT2020 MK1 rig. Both systems used DigiTrac Eclipse location and guidance systems to ensure the accuracy of the bores. Given the continuing operation of the rail track, the Eclipse system was used in its remote monitoring mode while the bore passed beneath the track section. This eliminated any need to have personnel accessing the track during the pilot bores.

After careful planning and with good co-operation between all the interested parties UMS completed the total of four bores in the one weekend as required. Four individual pilot shots were carried out with at least 1m separation between each of the parallel bores.

The JT4020 rig was used to install the two 250mm diameter water pipes and the JT2020 was used to complete the two 125mm diameter electrical duct installations. Using the two machines in parallel allowed the works programme to be completed in half the time it would normally take. All bores were between 60m and 70m long.

On the 250mm diameter installations, the pipe string was laid out along a private access road, by arrangement with the owner, on a Sunday, so allowing the pull to be completed despite limited access. The pipes for the 125mm installations were provided on pipe coils so the space limitations did not affect these operations in the same way.

Bore path

In terms of reaming and pipe pull-in, the 250mm diameter installations required one careful pre-ream to bring the bore to 200mm diameter prior to the pipe pull-in run itself due to the nature of the bore path. The conditions on the electrical duct bore however and their small diameter meant these could be pulled in directly without a pre-ream operation.

Throughout the pilot bore and pipe pull-in operations, Network Rail undertook continuous monitoring to ensure the works did not cause any change in the track levels.

Commenting for UMS’s client Scot Water, Willie Aitken, project manager, says: “UMS has worked with us for many years on a range of projects. And the company’s professionalism and can-do approach has reinforced why we regard them as one of our contractors of choice. All work was completed on time and budget.”

For UMS, Jim Morrison, director, says: “I am delighted with the way the job has gone and that the very tight timescale was met with a comfortable margin of safety.”


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