Bridge foundations: Spreading the load

A change to the design of the river-based foundations for the 1km Mersey Gateway Bridge allowed development of an optimised solution and helped to win the mega-project for the Merseylink concession consortium appointed to deliver the scheme for Halton Borough Council.

Flint and Neill director Hugo Wood highlights the decision to adopt shallow spread-footing foundations for the three pylon, cable-stayed crossing as one of the key strategic decisions behind the successful bid for the project. Flint and Neill is a Cowi company and was appointed as the lead consultant in the design consortium for the project by Merseylink Civil Contractors JV (MCCJV).

The bold move came when Flint and Neill, working in collaboration with Aecom, developed the tender design for the Mersey Gateway project on behalf of the Merseylink Civil Contractors JV, which consists of FCC, Kier and Samsung.

The main cable-stayed bridge forms the centrepiece of the 9.5km long link road connecting the national motorway network in north Cheshire with Merseyside. The bridge forms part of the 2.25km long estuary crossing and will carry six lanes of traffic between Runcorn and Widnes, relieving the congested and ageing Silver Jubilee Bridge “We proposed an alternative spread footing in place of an embedded foundation for the three pylons forming the cable-stayed bridge,” says Wood.

“Working with the construction consortium we showed that the temporary works and excavation needed for traditional deep foundations could be more extensive than those required for shallow foundations. A spread footing would therefore be quicker to deliver if the challenge of demonstrating their feasibility could be overcome.”

“We wanted to save the contractor time and money and give them the best opportunity of winning the tender,” says Wood.

A further bonus to the design was that it allowed for a more cost effective reworking of the bridge superstructure design.

“The zones where the foundations could be placed within the river were restricted from the planning consent,” explains Wood. “Because we were able to reduce the size of the foundations, we could push them to the boundaries of the approved zones and optimise the span arrangements for the main bridge, resulting in a more efficient design.”

Nonetheless, putting its faith in spread footing foundations required the design team – and hence the construction consortium – to trust the qualities of the rock they would sit on.

“Uncertainty about the quality of the rock, and its strength and stiffness, would have influenced the choice of deep barrette foundations shown on the reference design,” says Wood.

“We had to ensure the foundation was large enough to give sufficient bearing capacity but also that the foundation stiff ness was adequate so that the response under loading was compatible with the bridge design.”

Flint and Neill’s foundation and superstructure teams worked closely together, using a fully integrated model that incorporated the stiff ness of the foundations to predict the stresses that would develop during construction.

In addition to detailed analysis of the ground conditions, using results from previous site investigations, Flint and Neill together with the contractor, planned two further stages of investigation prior to construction and various inspections during construction to ensure that the final solution was successfully delivered.

“One of the requirements of our design was that we validated strength and stiff ness assumptions through detailed further ground investigations,” says Wood.

“We used crosshole seismic tomography to provide a picture of the shear-wave velocity and the compression-wave velocity within the rock to assess the magnitude of the stiff ness of the rock and to see if there was any significant variation within the footprint of the foundation.

“We wanted to identify if there was any variation in the ground stiffness which could potentially lead to differential rotation of the foundations under loading.”

Work on the foundations started on site in the second half of 2014 following financial close of the concession agreement.

The first job was to install sheet piles to create twin wall cofferdams in the river allowing construction work to take place in a dry environment. “The cofferdams had a working platform between the two lines of sheet piles and the construction team then excavated down to the founding surface,” says Wood.

Once exposed, this surface was subjected to a detailed inspection by Flint and Neill.

“Based on this inspection we were able to maintain the founding level that we had assumed in the design. This provided confidence to the contractor, concession company and Halton Borough Council in our solution.”

A blinding layer of concrete was placed on the sandstone before each reinforced concrete footing was cast to form the foundations.

“Because there had been a lot of planning and co-ordination with the contractor, there were no nasty surprises,” says Wood. After about a year of work on site, the foundations were completed in September 2015.

Construction has progressed smoothly thereafter with the main pylons complete and deck construction well progressed by autumn 2016.

“We are now monitoring the foundations to check their response to loading as the bridge is built,” says Wood.

He says Flint and Neill’s experience working on such projects, and its in-house expertise, helps it see where it can make a difference.

“We have a willingness to understand what is of benefit to contractors,” he says.

“We understand where contractors can save time and money and with our experience of working on jobs including similar challenges, we have the confidence to investigate the process, working in partnership with the contractor.”

Formed in 1958, Flint and Neill joined Danish consultancy Cowi 50 years later in 2008. It will rebrand as Cowi from 1 January 2017. Wood says the parent company had a big role to play in the success of the Mersey Gateway scheme.

“Part of the strength of the team on Mersey Gateway was built on bringing in global expertise from other parts of the parent company,” he says. “Given the sheer size of the scheme, it was great to be able to pull in additional engineers from Denmark to meet the demanding programme.”