Hull online despite tunnel collapse

Sewage is now being treated at Saltend STW in Hull, despite the collapse of the tunnel which should be delivering waste from the west of the city. Peter Minting reports

Yorkshire Water is determined to make a success of its largest ever project, the HumberCare sewage treatment scheme. The scheme consists of a huge flow transfer tunnel beneath the north bank of the Humber and a new STW at Saltend. The STW has now begun treating sewage from the eastern end of Hull, but the western end of the tunnel is yet to be finished.

Spokesman Norman Hurst is by no means ashamed of Yorkshire’s progress: “This puts into perspective the efforts of other water companies, some of which are nowhere near the 31 December deadline for compliance with the EC’s urban wastewater treatment directive.”

Southern Water, for example, is still awaiting an inquiry into proposals for a new STW to treat sewage from Brighton. Planning permission for the Brighton plant was refused after protests from local residents. According to Mr Hurst, residents near the Hull STW also protested, but a satisfactory position was eventually achieved. “Among their concerns, residents were apparently suspicious that Yorkshire Water was saving land in order to build an incinerator.” But Mr Hurst says that this is not true: “A contract has already been agreed with an agricultural supplier to use the dried sludge from the plant.”

Tunnel collapse

The transfer tunnel which should deliver waste to the plant stretches from the western end of the city to the eastern end at Saltend. Work on the western end has been delayed, thanks to a collapse just west of the city centre.

Tunnelling experts Miller are racing to repair the collapsed section, where fluid ground conditions resulted in subsidence around an entry shaft and filling of a large section with sediment last November. The site is only 50m from a main road, so Miller had to act quickly in order to stabilise the ground. To prevent any further collapse, a 2.4km section to the east was pumped full of compressed air and later with water in order to balance the external water pressure. One of the Lovat tunnelling machines has also been trapped by the collapse.

After a long discussion the engineers decided on ground freezing and re-tunnelling along the existing route. Miller should be able to recover the stranded tunnelling machine which is at least 20m under water. Steve Tindall, project manager for Yorkshire said: “We are planning to use external power packs to power the machine once we get to it, as we have to assume that the electrics will be shot.” By sticking with the same route, Yorkshire will not need to apply for extra planning permission or need to compensate any additional businesses. At least one warehouse owner has been compensated for relocation due to the subsidence hazard.

Miller has now frozen the ground for several metres to either side of the entry shaft, the plan being to tunnel through the frozen ground and create a stable concrete tunnel of a greater diameter than the original tunnel (3.6m). A larger tunnel will help to account for the distortion caused by the collapse and allow Miller to link the two ends of segmental tunnel. Tunnelling engineer Steve Round said: “In theory, once the repair is complete the only difference will be that the tunnel either side of the shaft will be smooth, not segmented.”

Mr Tindall said: “We did consider a number of options including splitting the tunnel into two or three smaller pipejacked tunnels.” Pipejacking, where concrete pipes are forced forwards behind a medium-sized tunnelling machine by hydraulic jacks, can be done for pipes up to 2m in diameter. But Miller decided ground freezing would be the most straightforward and practical solution in the circumstances, although quite expensive.

To freeze the ground, supercooled liquid nitrogen at -196°C is injected via copper pipes. Copper pipes are used because at such extreme temperatures most other metals become brittle. Once injected, the liquid nitrogen expands and vaporises at around -180°C, freezing the ground without causing the sediment to significantly expand or contract. At the moment the ground is being excavated with jack hammers, as even the best drills are ineffective in ground at temperatures of -30°C to -50°C. If a nitrogen pipe burst the tunnel could rapidly fill with gas, so all personnel underground are required to carry emergency breathing gear. Although not toxic, the nitrogen could cause suffocation, so there are alarms which will sound if the oxygen level drops below 19% (normal 21%).

Miller is now tunnelling through the frozen ground and continuously freezing 2-3m ahead of the tunnel face. Liquid concrete is being sprayed onto the new tunnel wall at around 40°C to ensure that the concrete has sufficient opportunity to set before it is cooled by the ground behind. The concrete contains steel fibres for additional strength and exhaustive tests on the condition of the wall are being carried out to ensure there is no risk of collapse once the frozen ground has thawed.

Until the western end is completed, the STW will not be able to operate at its maximum capacity. But as Mr Tindall explained: “There are provisions for a derogation when there is a technical problem. And if we haven’t had a technical problem then I don’t know who has!” Expert tunnelling engineers from as far afield as Austria have now been drafted in to help with the project, because Yorkshire is keen to ensure the work takes no longer than necessary as the contract was awarded to Miller on a shared-cost basis.

Biggest project ever

At over £200M, the HumberCare scheme is by far the largest project ever undertaken by Yorkshire Water. The contract for the STW was awarded to Kvaerner Construction on a fixed-cost basis, with project management contracted out to Construction Dynamics Group International (CDGI). The STW is at the eastern end of the tunnel, where there is a complex inlet and outfall construction to deal with both the input to the works and the effluent.

Huge Flygt pumps lift the sewage to the surface from a shaft 25m deep. There are four pumps dedicated to supplying the treatment process and 12 to deal with storm flows.

On entering the works sewage is first screened for any large objects. It is then pumped through a series of fine screens (Brackett Green) and a bank of spiral flow grit and grease removers supplied by Brackett Geiger. Screened material is collected in covered skips for disposal.

The remaining wastewater is then settled in a series of lamella clarifiers, before being pumped to the sequencing batch reactors (SBRs). The SBR tanks are so large that additional steel tendons have been used to reinforce their structure. The SBRs are ‘seeded’ with a special microbiological mixture supplied by InterBio. US Filter’s JetTex aerators pump air through the tank contents to keep it moving and encourage aerobic breakdown of organic matter.

After a few hours the process results in relatively clean water and sludge at the bottom of the tank. The clean water is decanted and transferred to the outfall, while the sludge is thickened with polymers and pumped to a series of anaerobic digesters.

Methane from the digesters is then used to help heat the next stage in the process, sludge drying in Kvaerner Eureka dryers. At the moment only one is needed, as the peak flow (PE 1.1M) is in summer when Birds Eye’s local factory adds a large amount of vegetable waste.

Treated effluent leaves the STW and enters another 1.8km of tunnel, before being released from the north bank of the Humber Estuary.

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