No-dig solution saves the day

The mission: fixing a large hole in a 1,200mm-diameter sewage pipe - with 36 pumping stations feeding into it - on a main road. Andrew Bellamy, site manager of UCS Civils, recounts the successful no-dig rescue operation

Lincolnshire civil-engineering firm UCS Civils recently completed a technically challenging localised pipe repair, using no-dig trenchless technology, to a concrete sewer. At 1,200mm in diameter, this was the largest local liner repair successfully completed in the UK to date.

When Anglian Water received a call from the Highways Authority informing it of a hole in St Thomas's Road in Spalding, Lincolnshire, civil asset engineer Mick White called in UCS Civils' emergency crew.

By the time they arrived on site, the hole revealed a void under the road surface and it became clear the problem was more serious than initially thought. The team was initially called out to a suspected sewer pipe collapse. But, following further excavations, the team realised the midriff of the concrete pipe, from the half-barrel upwards, had been corroded away by hydrogen sulphide gas emitted by sewage.

Pumping stations
White said: "As it was one of the main sewers in Spalding, to install a new pipe would have meant tankering at up to 36 pumping stations. This would have affected three-quarters of the homes and businesses in the town, and cost a colossal amount of money. The pipe also carried some surface water, and in the event of any heavy rainfall the flows would have increased immensely."

To overcome these problems, UCS Civils and Anglian Water jointly decided the best option would be to employ a patch repair technique, normally reserved for much smaller pipes, to prevent further collapses. Due to the problems of constant high flows, it was necessary to install over-pumping into the nearby pumping station while the patch repair was carried out.

Although the pipe had been eroded to an enlarged 1,350mm in diameter by the effect of the hydrogen sulphide, the largest inflatable packer available was only 1,200mm. According to John Kelly, managing director of CJ Kelly Associates - UK consultants for MC Building Chemicals who manufacture the trenchless technology system - there was some scepticism as to whether the technique would actually work.

Kelly said: "This patch repair system is widely used in the UK, but had not been tried on pipes of this diameter before. And, because the packer was smaller than the pipe, we weren't sure how effective it would be. But, by using this method, UCS Civils managed to avoid having to excavate and replace a vital piece of pipe in the Spalding sewerage system, which would have been a lengthy and costly undertaking."

The patch repair technique involved mixing together an epoxy resin with a hardener that was pasted onto the patch. The inflatable pipe packer was covered with a polythene tube to prevent the patch from sticking to it.

The patch lining was then rolled around the packer and positioned in place in the sewer. The packer was filled with air to press the patch against the existing sewer wall. It was left for two to three hours while the resin cured. The packer was deflated and removed, leaving the fibreglass patch lining in place, which the team then inspected, using CCTV, to verify its integrity.

All the packers had to be lined up from the wet well to the excavation site. Initially, Anglian Water considered creating a manhole, but decided this was not possible because of the cost and safety implications. Instead, UCS Civils formed by a new pipe by double-lining what was left of the existing main with the fibreglass patch, laying a reinforced concrete slab over the top, and backfilling it to highway specifications.

Using the patch repair system - distributed by the Sewer Centre in the UK and Ireland - UCS Civils managed to line about 15m of pipework in a densely populated residential area - without having to dig any trenches. The only place the crew had to dig was where the pipe had collapsed. They also managed to avoid creating any new manholes, and considerably extended the lifespan of the existing pipe. This saved Anglian Water thousands of pounds. Although UCS Civils had done a number of patch repair jobs for Anglian Water using this technique, none had been on a pipe of this size.

Tanker access
Anglian Water's White said: "The collapsed pipe was adjacent to a manhole, which had a total of 36 pumping stations flowing into it - so the flow was very high and constant. The alternative to packing the pipe was to switch the incoming pumping stations off. But, as time was of the essence, and a number of the stations were either located in people's back gardens or where tanker access proved extremely difficult, this was not a viable option.

"It was also unfortunate timing as the work was happening at the same time as the world- famous Spalding Flower Parade. But the job took a lot less time than initially feared, and we only had to close one road, which caused very little disruption to the parade and other road users.

"As far as we can tell it's been a very successful repair. And, if circumstances allow, we will definitely consider using this kind of technique again in the future."

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