An innovative solution from iron technology leader Saint-Gobain PAM UK has ensured the effective replacement of a failed water main in North Yorkshire while minimising disruption to the surrounding habitat of a rare beetle.
The Tansy is an attractive bright green leaf beetle, with a coppery sheen. The wing cases were so admired by Victorians that they were used as sequins. It received its name because its favoured habitat is around tansy planets on riverbanks. They were once quite widespread throughout Britain, but environmental factors (possibly due to introduced plants that have replaced the Tansy plant) have contributed to a sharp decline in population. Tansy beetles are now very rare and are only found along 16 kilometres of the banks of the River Ouse around York, centred on Fulford Ings near Fulford, leading to them being referred to as ‘Jewels of York’.
A section of an existing 24” cast iron rising main under the A64 dual carriageway at Fulford in Yorkshire had ruptures and required immediate repair. The section lies on the edge of Fulford Ings, between the road and the River Ouse. The issue was how to replace the damaged section while minimising the impact on the Tansy beetles.
The 24” pumped sewer flows from the north to the Naburn Sewage Treatment Plant on the south side of the A64. The project, undertaken by Yorkshire Water’s framework contractor Costain Mouchel, was to install a replacement section, by-passing the failing section, under the A64, and to connect it into the original section at both ends.
In order to minimise disruption to the surrounding habitat as well as to traffic travelling along the busy A64, open cut installation was ruled out, meaning the only viable solution was a trenchless one.
However, with the main situated at 4 metres below ground level on the plain, which in turn was nearly 10m below the (raised) A64, the pipeline material chosen had to be capable of withstanding the surface loading without deformation.
Due to the sandy soils in the flood plain of the River Ouse, the underlying ground conditions were also problematic. The project team established that horizontal directional drilling under the A64 was not viable because of the potential problem of stabilising the bore, along with potential environmental issues of bentonite seepage.
Although there was available space and access to the east of the A64, the access to the west, which is limited to a small track, is part of the Tansy beetle habitat and is tightly squeezed between the A64 and the River Ouse.
Following detailed technical discussions between Saint-Gobain PAM UK
, Costain Mouchel and sub-contractor Perco, the decision was taken to auger bore an 80-metre long 900mm diameter steel casing from south to north under the road. The reception shaft on the western side was to be located away from Tansy plants with fencing erected around them to avoid accidental damage to the beetle’s habitat.
The available installation footprint, and lack of access, on the northern side meant that preassembly of the 74-metre pipe string to be pulled into the casing was not feasible. The decision was therefore taken to use ductile iron Universal anchored pipes as they could be lowered individually down the 4m deep shaft and jointed as they were pulled into the casing.
Since the Universal joint is a push-fit joint, this solution negated the potential health and safety implications of welding within a confined space. The pull-in of the ductile iron pipe could be easily carried out by attaching the lead pipe to the auger bore rods as they were pulled back to the southern side of the installation.
Under normal circumstances, the pipes would have been supported during installation in the tunnel on ‘spiders’ to reduce the risk of dragging and scoring on the pipe wall. However, Saint-Gobain PAM UK
proposed the use of directional pipes. These use Universal self-anchored flexible joints but have a fibre reinforced mortar external coating, specifically designed to withstand the demands of trenchless technology, meaning no need for ‘spiders’, thereby reducing the diameter required for the casing and also the CO2 emissions for its installation. Since there was a possibility that the new pipeline could be grouted in the casing, Saint-Gobain PAM UK
recommended the use of a rubber muff with a steel retaining cone over the joint area. This would protect against grout ingress and reduce friction between the pipe and the casing.
Directional pipes are supplied in 6-metre lengths so the shaft on the northern side of the project only needed to be 8 metres in length: this small footprint reduced the impact on the habitat and also the power requirements for the excavation itself. Given the soft sandy nature of the local soil, a trench box was required.
With the directional pipes and assembly equipment stored on the western side of the A64, Land & Marine undertook the south-north auger bore under the A64 according to schedule, with no problems.
As the auger rods were retracted, the lead pipe was manoeuvred into position in the shaft, attached to the trailing rod and pulled into the casing. As this was the first time that this particular technique had been used in the UK, Saint-Gobain PAM UK
engineers worked with the Land & Marine team to demonstrate the pipe jointing technique. The pipeline was externally supported in the shaft while being pulled into the casing. The next pipe was lifted into the shaft and jointed into the lead pipe’s socket as the socket reached the outer edge of the casing. This process was repeated until all 74m of pipe had been pulled into the casing.
The use of bentonite was specifically discounted in the original project planning, due to possible environmental impact. The calculated, and actual, frictional forces on the directional pipes were so low that the pull back into the casing was made without the need for bentonite, further reducing the project cost and environmental impact.
The speed of jointing was consistent with the pulling speed, with this part of the job completed in one day, minimising the overall installation time. This was of ultimate importance as the Tansy beetle briefly appears during May, June and July, mates and then disappears again for the rest of the year.
With the new section in place, a short shutdown of the pumped main allowed the connections to be carried out, leaving the failing section abandoned.
Thanks to all parties working together to find a viable trenchless solution, there was no damage to the Tansy plants, and thus to the beetles’ habitat. The project was completed ahead of schedule with only minimum interruption to the flow of sewage. It was ‘highly commended’ in the United Kingdom Society for Trenchless Technology 2010 Awards held in Birmingham during April.