A Galliford Try and Black & Veatch JV has used a clever solution to ensure a flood defence scheme gains full marks for sustainability.
Client: Environment Agency
Contract value: £6m
Main contractor: Black & Veatch
Main contractor: Galliford Try
Earthworks subcontractor: D Morgan
Playing fields subcontractor: White Horse
Start date: April 2015
Completion date: May 2016
“The secret of the job was to try and do it while the sun was shining,” says Galliford Try project manager Costas Pantelidakis at the contractor’s flood alleviation scheme in Salford.
Ironically, the rain is so bad on the day Construction News pays a visit to the site that works on the project have had to be stopped, and the approach to the main works has turned into a mud bath.
Galliford Try and Black & Veatch are working as part of a joint venture to make sure that nearly 2,000 homes along the river Irwell will be protected from flooding.
Fortunately, a spell of good weather in the summer and an innovative approach to materials helped the team to get a headstart on what has proved to be a challenging project.
The site itself is situated on a loop of the Irwell, some 4 km from the centre of Manchester.
Galliford Try and Black & Veatch are constructing a detention basin with a capacity of around 450,000 cu m to reduce the flooding risk to surrounding properties.
Desperately seeking approval
The scheme has been a long time in the planning, with an initial business case put forward following the completion in 2005 of a neighbouring flood alleviation scheme at Littleton Road, only a stone’s throw away from the site.
However, under the funding rules at the time, the Environment Agency was unable to justify the construction of a second scheme.
Thanks to revised funding guidelines, the business case for the new flood alleviation scheme was approved in June 2014 to the tune of £11.7m.
The agency is currently forecasting a total project cost of approximately £8m, of which around £6m is construction, meaning the scheme is on course to come in under budget.
“Currently the standard of protection offered with the existing flood basin upstream and the linear defences going through down towards Manchester is one-in-75 years,” says Environment Agency project manager Paul Robertshaw.
“With this flood basin, that standard of protection for these 2,000 properties is raised to one-in-100 years.”
This has the added bonus of protecting brownfield land from flooding, which Mr Robertshaw says was a key element behind the scheme gaining money from a Department for Environment, Food and Rural Affairs growth fund.
“With this flood basin, the standard of protection is raised to one-in-100 years”
Paul Robertshaw, Environment Agency
“Given the enhancement of protection, it means these areas that Salford City Council or the University of Salford are looking to develop suddenly become viable for regeneration through the planning process,” he says.
To help manage any potential flooding, while maximising future land use, the team has had to come up with some innovative solutions while making the scheme as sustainable and environmentally friendly as possible.
There were existing flood defences prior to the project getting under way, but the whole area the site covers was used as a floodplain and was typically flooded once every five years.
The team has reconfigured the land to allow for the installation of new sports pitches at the south end of the site, a new 8 ha wetland habitat and a knoll at the north end, as well as improved public access in the form of public footpaths.
“Effectively what we’re doing is cutting the natural floodplain off with an embankment, but we’ve set a spillway here [midway through the site] and an inlet weir,” Mr Robertshaw explains.
“To build the embankment we’ve lowered the floodplain area to get the material to build the banks, and surplus material has been used to build the knoll to avoid any export from site.”
Once the new flood alleviation scheme is complete, the area will only flood once every 10 years, compared with once every five years previously.
In this event, water would flow over the weir, and then the penstocks on the east side would be shut so nothing can flow back into the river, meaning the flood basin would fill up.
“In a really big flood event, once the river drops, some of the water will go back out over the spillway and the rest will be drained through the penstocks”
Paul Robertshaw, Environment Agency
“In a really big flood event – the one-in-100 year even – once the river drops, some of the water will go back out over the spillway and the rest will be drained through the penstocks, through two 1.5 m pipes,” Mr Robertshaw adds.
The approach the team has taken to the project has aimed to maximise its post-completion efficiency and to minimise the environmental impact while it is under construction.
Mr Robertshaw says the team looked at a range of different options before deciding on the solution being employed, but options including an online flood storage structure and tilting floodgates would have led to significant works being undertaken on the opposite bank.
“This offline option is as passive a design as we can get – there are two penstocks that will operate but we’ve kept future maintenance and operational requirements to a minimum with this design,” he explains.
320,000 cu m all site-won
The scheme is also highly sustainable in that all 320,000 cu m of the material used to construct the embankments, the 2.3 km-long bund and the knoll will be entirely site-won.
Mr Pantelidakis says that using the onsite material proved to be a challenge for the contractor.
“You couldn’t call the material we were using the best you could get from a geotechnical point of view, in terms of constructing the embankments and achieving the criteria that our engineer wanted to achieve,” he says.
Mr Pantelidakis explains that the team had to work with two different types of material: one area contained silt, sand and clay, while other areas were made up of sand and gravel of higher permeability.
The team began to excavate the material in early spring, but found that once they reached the water table, much of the material was too wet, making it difficult to compact.
The nature of the material also made the project very weather-susceptible, though the good conditions over the summer allowed the team to make quick progress.
“You couldn’t call the material we were using the best from a geotechnical point of view”
Costas Pantelidakis, Galliford Try
However, the site-won material still presented a number of issues. Its nature meant the team had to partly lime-stabilise the cohesive sand, silt and clay.
“Once we started on site we tested whether we could use the material without lime, but we found that in some cases it was almost impossible to compact it,” Mr Pantelidakis says.
“The higher percentage of lime you add to the material, the more you’re open to shrinkage within the core.”
The team went through the process of lime-stabilising the material where necessary, using a maximum of 2 per cent lime.
Thanks to the warm summer weather, however, only 0.75 to 1 per cent lime was used, meaning that only around 20,000 cu m of the material needed to build the 150,000 cu m embankment was lime-stabilised, making it much more secure.
“It was a case of hitting it very hard in the dry months and working late into the evening, but, as a result, we’ve managed to achieve 95 per cent of the project by using little lime stabilisation,” he adds.
Battling the elements
Unfortunately, adverse weather conditions in the autumn – very much like the rain storm that greeted our site visit – have meant that a good amount of the works have had to be suspended until the spring.
The knoll area is still under construction, as the team still has to take out around 10,000 cu m of material from what will be the wetlands area to complete it.
Mr Pantelidakis says that work on the sports pitches has now become impossible due to the weather.
“The construction of the playing fields is quite an involved operation,” he explains.
“The construction of the playing fields is quite an involved operation. It requires a number of cross drains as well as a main drain and is a very weather-susceptible operation”
Costas Pantelidakis, Galliford Try
“It requires a number of cross drains as well as a main drain and again is a very weather-susceptible operation, so we have suspended the works on that for the rest of the year.”
The team can still continue with the M&E works on the site, including around some of the ducts and penstocks, until the weather takes a turn for the better.
It is envisaged that the major construction work on the site will complete early next year, although certain criteria – including grass coverage of the knoll – mean that the site cannot be fully handed over until Environment Agency standards are met.
Following the main construction works, the pitches will also be reinstated and the habitat area developed, which could take a further six months.
Once these stages are complete, however, the team will have created a project that will not only provide flood protection for nearly 2,000 homes, but will also aid development in neighbouring areas and provide a new nature area for local residents.
“We’re putting in something that’s a lot better for public access and we’ve done a lot of work with local community groups to try and give them something that they can be proud of,” Mr Robertshaw says.
Building a habitat for nature
Galliford Try and the Environment Agency have worked with the Wildfowl and Wetlands Trust (WWT) to design the 8 ha area of wetlands on the site’s northern end.
With the input of the WWT, the team are attempting to create an area that will be both suitable for wildlife and an integral part of the flood storage area.
In addition, the scheme will see Galliford Try install a sandmartin nest bank, kingfisher tunnels and an otter holt to add to the site’s wildlife-friendly credentials.
“The approach we’ve taken will create a habitat that the local community will see as a really great resource going forward,” says Mr Robertshaw.