The brief
Affinity Water called us in to a pair of leaks on two 9 inch water mains running side by side on a bridge crossing the M4 at Southall. Both mains were leaking. Both were live. Taking either of them out of service would have meant pressure and supply consequences downstream, and working on either of them meant working directly over a live motorway.
The challenge
Bridge-mounted pipework is its own discipline. The mains sit on bridge supports rather than in the ground, so the usual assumptions about excavation, shoring, and bedding don’t apply. What does apply: working at height, working over live traffic, working within the engineering envelope of someone else’s structural asset, and coordinating any access arrangements with the bridge owner and Highways England.
Digging up and replacing a bridge-mounted main in the traditional sense wasn’t an option. The question was how to restore the integrity of two failing mains without disrupting the motorway underneath and without an extended supply shutdown.
Our approach
Slip lining, twice. A 180mm polyethylene liner was installed inside each of the two 9 inch originals, turning both damaged host mains into conduits for new PE mains. Polyethylene was the right choice for a bridge crossing: light enough to handle at height, flexible enough to thread through the existing pipework, and durable enough to sit as the long-term asset.
With both liners in, each new main was pressure tested and chlorinated. Pressure testing confirms integrity before commissioning; chlorination brings the new main up to drinking water hygiene standards. Both are regulatory requirements before a water main returns to potable service.
- Original mains
- Dual 9 inch (both leaking)
- Slip liner
- 180mm PE, both mains
- Location
- Bridge crossing M4 motorway
- Technique
- Slip lining + pressure test + chlorination
- Duration
- Back in service within 1 week
- Client framework
- Affinity Water R&M
Outcome
Both mains back in service within a week, no M4 closure required. Dual slip lining on a motorway-crossing bridge is not an everyday job. The technique, the setting, and the doubled-up scope together make this one of our more distinctive R&M pieces.
For technique context see our no-dig and trenchless methods and chlorination and pressure testing work. The job sits under water mains repair and maintenance for water companies.
