Off the road and on the right track
A road project in Scotland needed the rerouting of a water main and power cables. Underground Moling Services director Jim Morrison explains how cooperation between stakeholders led to the desired result
There is, of course, still significant heavy-goods traffic associated with the interchange facility as freight is brought to the site to be placed onto the rail system. One of the road bridges on the approach to the site was more than 100 years old and could no longer carry the HGV traffic loads.
It was decided to renew the deck of the bridge. As the road passed over the rail track, the ultimate client for the work was Network Rail. The Roads Authority, Scottish Water and Scottish Power, however, also needed to be closely involved with operations. This was because, beneath the road surface, there was both a water main and power cabling that needed to be redirected as part of the bridge works. During the planning process it was decided the re-routing 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.
To achieve this, the installation of two new 250mm-diameter MDPE water mains and two new 125mm-diameter MDPE cable ducts would be needed, each installation being about 65m long.
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, especially at the target/exit end of the proposed bores. This was close to the rail boundary and comprised a steep approach to a raised exit pit within a small land area. This meant 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 landownerships in the industrial area. This meant that the target area chosen was the only small parcel of ground that the client had permission to use.
The only window of opportunity in which to complete the essential bridge works was one week, over the Christmas holidays. So, both Network Rail and the utility companies wanted to be sure water and electric cable diversion works would be successfully carried out in the restrictive two-week period leading up to Christmas break.
As well as this, the installation of the new cables and commissioning of the water mains would also 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.
This would allow the new service commissioning works to be finished, while allowing enough time to decommission the old bridge services.
While no particular consulting engineer was employed by Network Rail, Scottish Water employed consultant Donald Stocks, of Water & 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 utilities contractor with experience in trenchless installation.
Ultimately, these discussions led to an acceptance by all parties to utilise the expertise of UMS to install the pipes/ducts beneath the rail track, using HDD techniques for the project. 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.
In the event, UMS used a Ditch Witch JT4020 MK1 horizontal directional drilling rig and a JT2020 MK1 rig. Both systems used DigiTrac Eclipse location and guidance systems to ensure the accuracy of the bores.
The rail track operated continuously, so the Eclipse system was employed 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 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 60-70m 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.
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 of the track to ensure the works did not cause any change in the track levels.
Commenting for UMS client Scottish Water, project manager Willie Aitken said: "UMS has worked with us for many years on a range of projects and the company's professionalism and can-do approach is why we regard them as one of our contractors of choice. All work was completed on time and budget."