A royal success

The Queen Elizabeth II (QEII) Reservoir is a raw water reservoir in South West London that is owned and operated by Thames Water. The reservoir holds over 19 million litres of water, equating to 10% of the raw water storage for London. Once treated, this provides clean drinking water for millions of people across Surrey and London. 

Barhale was engaged by Thames Water to complete the £11million tunnel relining contract to guarantee the integrity of the inlet and outlet pipes. The goal of the project was to increase resilience by strengthening the pipes and preventing any leakage from what is a crucial clean water asset. 

The project involved relining 2x2.5m internal diameter tunnels. Before works could begin, the reservoir needed to be drained down and the pipes fully isolated for safety. Ultimately, the water level was reduced by 11m, leaving 6m of water in place to sustain the wildlife in and around the water. 

Relined

The first tunnel to be relined was the 1060m long inlet tunnel, which draws water into the QEII Reservoir from the River Thames, via Walton Raw Water Abstraction Pumping Station. The second tunnel was the 826m long outlet pipe, which takes water from the QEII to Hampton Water Treatment Works for treatment before being fed into trunk distribution mains and the Thames Water Ring Main network. Both were constructed in the early 1960s using an unbolted wedgeblock technique.  

Both tunnels required two access points. For the inlet tunnel, this was at Walton Treatment Works, where a surge tower pipe was removed for access and refurbishment. Additional works here included adding a 600mm butterfly valve to futureproof the surge tower. The secondary access point was a temporary 6m internal diameter access shaft constructed at the North West boundary of the reservoir. 

The access points for the outlet tunnel were at an existing, permanent shaft at Boormans Field site (in a nearby residential area) and at a new 6m diameter temporary access shaft within the boundaries of the reservoir near its north east entrance. 

The temporary access shafts were constructed through a combination of caisson and underpin techniques. Both shafts were sunk to a depth of around 18m, using a 40t excavator with a telescopic pole grab and clamshell bucket. The team then swapped to under-pinning, using a 3t excavator at the base of the shaft to muck out to formation level that encompassed the existing tunnels. 

Reinforcing ribs were installed inside the tunnels to ensure their structural integrity was maintained during the shaft construction. Once the crowns of the tunnels were reached, the existing wedgeblocks were carefully removed to facilitate the lining works. Each shaft progressed to around 2m below the invert of the tunnels, with a final depth of 44m. 

The temporary shaft required for the inlet tunnel was located at the base of the QEII Reservoir’s embankment. This access shaft was between the reservoir embankment and Walton Road, which runs parallel to the northern side of the reservoir and is a well-used route between residential areas. To gain access safely and at the correct location over the tunnel, it was necessary for the shaft to encroach on the road. Initially it was essential that the road was closed, but as soon as it was safe to do so, one side of the road was reopened to minimise disruption to the travelling public as much as possible. 

Collaboration

To protect its workforce while relining the tunnels, Barhale worked collaboratively with Thames Water to complete and implement a robust and thorough isolation procedure. This included making sure that every valve and outfall into the tunnels was double isolated – with primary and secondary physical and electronic isolations. 

In total there were over 30 isolation points, with the primary physical isolation being at the location of the valve or outfall, and the secondary electronic isolation occurring at the remote control room. Additionally, there were a number of large valves within the reservoir that needed to be isolated. Specialist divers worked with the Barhale team to install blanking plates. These were 3.5m in diameter and 2.3 tonnes each, so required careful fitting and placement to guarantee full isolation.

Once shaft construction at the inlet tunnel was complete, the team moved straight onto the outlet tunnel temporary shaft which was within the grounds of the reservoir. At the location of the outlet shaft, an existing disused sampling building was demolished to enable works to progress. 

While relining works were underway on the inlet tunnel, the outlet tunnel was being cleaned to enable its relining to progress. This involved the removal of silt and zebra mussels which had settled along the tunnel walls and invert. This was done manually using scrapers and trollies which were then emptied in the tunnel using a vacuum system.

Steel fixing was completed and then the tunnels were both relined. Shutters were installed in situ by the team, with concrete poured and cast in place. The team were working at a rate of 39m of tunnel concreted per day. 

The concrete was pumped down the shaft and along the tunnel, into the crown of each shutter section. The maximum distance of pour from the pump was 560m, utilising a 4 inch pipeline. 

Challenging 

Communication in tunnels often presents a challenge as it requires maintaining contact over long distances, where there is likely to be minimal signal. Barhale’s team decided on the use of traditional baby monitors as they are simple and effective devices. This was a resounding success, with functionality that enabled clear communication over distances of 500m within the tunnels. 

There were numerous challenges to overcome during the course of the project, not least those presented by the global pandemic. In line with Barhale’s company approach, the site took every effort to keep workers safe. This included social distancing, increased hygiene measures and regular temperature and COVID testing checks.

Two of the four project sites were close to residential developments. While acoustic barriers were used to reduce noise from 24-hour working, relationships with the community were closely managed to understand any end-customer issues. Barhale’s customer manager regularly visited residents throughout the project. Through ongoing, open dialogue, it was possible to listen to concerns and to work with the operational teams to find best solutions to any problems. 

The site at Walton Treatment Works (inlet tunnel access) was a live Thames Water site, so access and movement around site was carefully co-ordinated with Thames Water Operational Teams to avoid any impact on site operations. The collaborative approach between Barhale’s and Thames Water’s teams extended to permitting Barhale full access to the necessary part of the treatment works via a secondary entrance gate, with a full compound set up within the boundary of the treatment works.

Award winning

As a direct result of its hard work and commitment to the project, the Barhale team were awarded Thames Water’s Project Pride Award. This accolade recognises and rewards projects working as part of the Capital Delivery AMP7 Framework that achieve Health and Safety Excellence. 

Barhale put in every effort to beat the outline construction programme and worked in collaboration with Thames Water to finish four months ahead of schedule. This was a significant efficiency saving for Thames Water, allowing the reservoir refilling operations to commence in November 2021 prior to the Winter flows in the River Thames, and reducing the logistical challenge of sourcing water from elsewhere.