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Engineers battle to repair Preston sewer collapse

Engineers in Preston were this week tackling the third sewer emergency in 13 months, after a crucial section of Victorian infrastructure was discovered to have collapsed.

Emergency excavation

An emergency project to repair the collapse of one of Preston’s main sewers began on Monday with a 5m deep excavation to remove the damaged clay pipe.

The sewer runs underneath a major road into the city and very close to on-going work to revamp the city’s sewer system.

The incident follows a similar but unrelated sewer collapse in Preston in April 2010, and the breach of a tunnel lining in November 2010 that left a TBM immobilised on the £114M Preston Environmental Improvement Scheme (NCE 19 January).

Engineers from KMI Water − a joint venture comprising Kier Construction, J Murphy & Sons and Interserve Project Services, which is also constructing the adjacent improvements scheme − will excavate the damaged sewer in two phases.

It will then replace it with 20m of 600mm diameter concrete pipe.

Mobile cameras in use

United Utilities senior project manager Brian Edwards told NCE that engineers believe that the 20m length will slightly exceed the collapsed pipe section. This conclusion was reached after investigators inspected the collapse using mobile cameras lowered into the sewer via manholes.

The collapse is close to a major junction on Fishergate Hill, a major traffic route which had to be partially closed while repairs took place. Two lanes of the three lane carriageway were closed from Sunday for the beginning of phase 1 of the repairs.

A 2m section of the road was being excavated to 5m depth as NCE went to press. This was to enable removal and replacement of the damaged pipe, before work could switch over to the third lane for phase 2.

Repairs are expected to take six weeks. The cause of the collapse is not yet known. The age of the 161 year old clay sewer could be a factor, as could the proximity of a nearby shaft construction site for the ongoing improvements by KMI Water (see map).

‘Not identified causes’

But United Utilities and KMI Water said they had not identified possible causes of the collapse as NCE went to press.

“Once we get down into [the excavation] we will probably have a better idea,” said a United Utilities spokesman. The collapsed pipe is believed to be one of the city’s last Victorian clay pipes. The company said the earlier sewer collapse in April 2010 was not connected to this incident, because it was in a separate part of the city and was made from modern materials.

“Once we get down into [the excavation] we will probably have a better idea”

United Untilities spokesperson

United Utilities said KMI Water’s work on the £114M Combined Sewer Overflow (CSO) improvement scheme was unaffected by the emergency, despite the shafts’ proximity.

The CSO project comprises a 3.5km long, 3.05m diameter storm water storage tunnel to prevent overflows discharging to the River Ribble plus associated shafts; a 5km long, 2.85m diameter rising main tunnel; 445m of 1.5m diameter microtunnels; a new pumping station and a river crossing to a wastewater treatment works. Four of five microtunnels have been completed to date.

United Utilities faced construction problems in November when a Lovat tunnel boring machine (TBM) became immobilised while boring the river crossing, when silt breached the tunnel lining. United Utilities said this week that the TBM had still not been recovered.

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