Articles & Case Studies

Lanes sees through blind sewer

Posted: Wednesday 24th July 2013

Lanes pulled off a tricky drain reinforcement to keep Manchester’s multi-million pound tram network extension scheme on track. The no-dig specialist lined 54-metres of sewer with only a single point of access, instead of the usual two manholes, using a technique known as a ‘blind shot’.

Wherever new tramlines are laid, the utility pipes running beneath the new lines are reinforced to withstand the 39-tonne weight of the trams. As sub-contractor, Lanes has been using trenchless technology to line (and so reinforce) around eight kilometres of drains during the scheme which will see the transport system almost double in size and capacity.

To date, most had been straightforward, but a 600mm diameter sewer in Smith Street, Rochdale, presented a problem. Lee Bow, Lanes’ Operations Manager, explained: “Usually we install a liner from manhole to manhole, but in this case there was only an upstream manhole, no downstream exit point. Introducing the liner into the pipe would not have been a problem, but we would not have been able to trim off the liner’s sealed end afterwards to re-open the sewer.

“The site was unusual. It is one of the widest bridges in the world, which was created when the River Roch was culverted in the early 1900s and seven bridges were joined together. Four attempts by the contractor to find an exit manhole failed because the bridge struts prevented excavation. We had to find a way of reinforcing the sewer via no-dig methods or excavate the full 54-metres to access the pipe, and last time the bridge was excavated for repairs back in the 1990s, the bill was £3.5million.

“A blind shot liner installation is difficult but not impossible, as long as it is planned meticulously, though it is a technique usually only attempted on pipes up to 12” diameter. There were additional considerations too.

“Firstly, we had to negotiate the liner through two blind chambers. That can be difficult because the size and space of the chamber may encourage the liner to go off course. Secondly, if anything went wrong, styrene-contaminated water (0.008 parts per million) from the curing process would have been released into the river via the drain outfall. And, thirdly, at the end of the 54-metre run, the sewer changed from 600mm to 400mm diameter, so if the liner was even slightly too long it could bunch up in the pipe, potentially impeding future flow.

“Most worryingly though, once a liner has gone into the sewer past the point of no return (usually half way), there would be no way of retrieving it, short of full scale excavation. There would be no second chance. We had to get it right first time.”

Lanes came up with a plan. To manage the single point of entry, the team used a special open-ended liner instead of one that had to be cut open manually at the downstream manhole once cured. The liner was inverted with water to the midway point; the inversion tower filled to the maximum level with enough water to control the rate of inversion once the liner was released and four jet vac tankers used to supply water at 80-gallons per minute from each unit. The trick was to ensure sufficient head of water so that as the liner inverted through the sewer, there was enough water in the liner to invert it the full length and push open the blind end. Without an adequate supply of water, the liner would have run out of steam and stalled — a disaster, since it would have passed that crucial point of no return.

To ensure the liner had a safe passage through the drain and the blind chambers, a pre-liner hose was used as a carrier. This plastic, triple-stitched, heat-welded tube was inverted into the drain first, so that the liner had a guide to inflate to, and was helped through any areas where the drain veered or changed shape. Pre-liner hose is cheaper than liner should it spoil; it also provides a good indicator as to how the wet-out liner will travel through the sewer.

A second calibration hose used inside the liner forced it to inflate against the outer, pre-liner hose carrier. Lanes doubled up this second hose as a precaution against it rupturing and spilling process water into the river.

Once the liner had completely cured, the calibration hose was pulled back to the inversion manhole bringing all process water with it, which the vacuum tankers removed for disposal at a registered waste plant.

Finally the sewer was CCTV-surveyed to ensure that the lining operation was a complete success. And, as with all liners on this contract, a sample was sent for independent testing to ensure that it met or exceeded specifications.

Lanes’ expertise and careful planning paid off. The 54-metre sewer — thought to be one of the largest diameter ‘blind shots’ attempted — was a complete success. And the tram extension scheme was back on track.

A spokesperson for the main contractor said: “There’s no doubt that it was a challenging project. Not being able to locate an exit chamber meant we were facing a hugely expensive undertaking. Fortunately the Lanes reline specialists came up with this alternative solution, which allowed us not only to complete this section but avoid any knock-on delay to the wider project too.”

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