The first new reservoir to be built in London in a century is currently under construction, and it involves some very 21st century thinking, as Margo Cole discovers.
It’s not every day that a UK city gets a new reservoir; in fact, it’s over 100 years since the last one was built in London. But in the leafy and affluent area of St Johns Wood, nestled between Primrose Hill and Regent’s Park, a 28.4Ml capacity reservoir is currently under construction, ready to be filled with potable water and in operation early next year.
The new structure is being built on the site of a brick-built reservoir that was constructed in 1826 and used continuously until owner Thames Water took it out of commission in 2002 after ground movement caused damage to the roof.
Since then, the company has relied on pumping water from other areas of London through its ring main to supply the clean water needs of the area’s residents, but it always planned to replace the old reservoir when the funds were available.
“We have a strategic and water need for it,” explains Thames Water senior contracts manager Neil Franks. “This is a growing area in terms of population and water use, and it is putting a demand on our water infrastructure, so we think that having this asset here best serves the residents.”
It’s a fairly sensitive site in beautiful parkland, so we had a very comprehensive planning process
As one of 26 similarly sized storage tanks around London, the Barrow Hill reservoir, as it is known, also plays an important role in ensuring the resilience of supply, and its capacity will provide the company with the ability to maximise the efficiency of its pumping regime for moving water around the ring main.
Design and construction of the new £11M structure is the responsibility of GBM, a joint venture of Galliford Try, Biwater Treatment and Mott MacDonald, which has a framework contract with Thames Water for work in north and south London during the current asset management plan period, AMP5. Work started on site in May last year, with the dismantling of the roof, columns and floor of the old structure, and construction of the new reservoir began three months later.
But before construction could start, the project had to go through a lengthy planning process. It is in an extremely sensitive area, bordered on two sides by royal parks, surrounded by influential residents, and spanning the boundary of two London boroughs. Primrose Hill is also a well known spot for people to enjoy views over London.
“It’s a fairly sensitive site in beautiful parkland, so we had a very comprehensive planning process,” says Franks.
Although there had previously been a reservoir on the site, filling an almost identical footprint, the project was treated from a planning point of view as a completely new proposal. However, on the whole the new proposal found favour, as its design is far less visually intrusive: it will be topped by a grassed roof that will look far more appealing that the aluminium barrel arched roof that was added to the 19th century reservoir in the 1950s to replace the original brick roof.
“We are moving towards a greener option, so the feedback was very positive,” says Franks. “We are improving the view from the top of the hill.”
We did spend a lot of time in the early days coming up with really, really efficient design and construction techniques
Throughout the two year planning process, GBM and Thames were aware that time was ticking, as the regulatory constraints on the water company mean the reservoir must be in operation by the end of the AMP5 period in March 2015.
As a result, the team was keen to find ways to make the design both structurally efficient and as efficient as possible when it came to construction. Specialist designer GHA Livigunn was brought in to help with the detailed design, and the result is a structure that “pushes the efficiencies as far as possible”, according to GBM senior site manager Allan King.
“We did spend a lot of time in the early days coming up with really, really efficient design and construction techniques,” says GBM area delivery manager Dave Easson. “Everything was thought of.”
In essence, the reservoir consists of two separate cells formed with in situ concrete floors and walls and precast columns, beams and roof panels. The columns are on a 5m grid, and the walls are 6.5m high, although much of the construction is effectively below ground. A valve chamber sits between the two cells.
Since Thames Water decommissioned the old reservoir in 2002, the land in front of the structure has been sold, and a large apartment development is currently under construction, leaving GBM with very little space outside the reservoir’s footprint on which to build.
Originally the JV thought this space requirement would mean having to build the entire structure in its two halves, completing one before starting on the second. However, the quality of the London clay in the area enabled GBM to excavate within a 1:1 batter and to dig down to formation level across the entire site during one big muckshift operation.
The base of the new reservoir is just 1m lower than the old structure, but GTM excavated lower than this and filled and compacted with stone in order to construct the new ground bearing slab. In weaker spots an additional 1.5m was excavated and filled. Exposing the underlying ground enabled the contractor to demonstrate the inherent strength of the clay, leading to approval for a slab that was just 225mm thick, rather than 250mm that is Thames Water’s usual standard.
“We had been deliberating on two or three different designs, but taking all the results, we saw we could have a very efficient slab that was 225mm thick with deepenings for the walls and columns,” explains King.
One of the big risks is propping the reinforcing steel. Generally, wall steel is propped, and then you take the props out when you put the shutter in, and that’s when you have the risk of collapse
The team did look at an all precast option for the structure itself, but with residential streets and tight access it would have been very difficult to get large wall panels onto the site. So, instead, the 500mm thick walls are being cast in situ, while the columns, beams and roof panels are precast.
With so little space around the perimeter of the structure, GBM decided to lay blinding concrete on top of the base slab formation that is strong enough to cope with the loads from the 90t mobile crane being used to lift in the precast units.
For the in situ concrete walls, GBM has been working closely with its subcontractor Ward & Burke to make the process safer and more efficient.
“One of the big risks is propping the reinforcing steel,” explains King. “Generally, wall steel is propped, and then you take the props out when you put the shutter in, and that’s when you have the risk of collapse.”
Ward & Burke has developed a system that incorporates a vertical truss girder made of reinforcing steel into the design of the reinforcement, doing away with the need for propping at all.
Once the wall starter bars have been cast into the deepening at the edge of the base slab, these girders are welded to the starter bars at intervals around the perimeter, acting as supporting columns. The wall steel, prefabricated in panels, is then lifted in and fixed to the girders, and is completely stable without the need for propping. The girders then become part of the permanent reinforcement.
‘It is all done without scaffolding - just using mewps [mobile elevating work platforms],” says King, who explains that during the construction of the reservoir walls the contractor built a 100m length of 6.5m high wall steel using this system.
“It’s been very efficient and very safe,” he adds, emphasising that this construction method means the steel fixers can be working on a completely different section of wall to where the shutters are being erected and concrete is being poured.
“It doesn’t involved any extra steel, so it can be incorporated as part of the constructability,” adds Easson.
GBM and Ward & Burke are pushing for the girders to be given standard reinforcement bar marks, so that they can be incorporated in all wall designs.
The concrete mix for the walls has been designed so that the shutters can be struck the day after they have been poured, to maximise efficiency during construction.
The result of the focus on efficiency is that the first of the cells is almost complete - structural water testing is taking place later this month - with the second cell following shortly after.
Already the drainage layer is going on top of the roof of the first cell, ready for the topsoil and grass seed. GBM is using a seed with shallow roots for the roof itself, but the sloping sides of the mound that will cover the concrete structure will be seeded in the grass mix that is found in the adjacent park.