The site team has identified a section of soft, unstable ground where the new phase three bridge over the East Coast Main Line and River Witham is being built.
Cllr Richard Davies, executive member for highways, said: “During construction of the relief road’s new bridge, the on-site team found that the ground on the eastern embankment had moved.
"We’re working with the bridge's designer WSP to determine the cause of this issue and why it wasn't accounted for when WSP designed the foundations for the bridge.
“The co-ordination of the ground investigation information and the subsequent design of the bridge was and is the responsibility of the bridge designer, WSP.
“However, because the ground here is too soft to build on, we’re now in the early stages of preparing a planning application to extend the length of the bridge to avoid the unstable ground.
“Unfortunately, this will have an impact on the project’s completion date, which will now stretch beyond our original timeframe of late 2023.
"And with the price of construction materials increasing, it could cost several million more to get the road built – a cost we still believe is far outweighed by the undeniable benefits it will bring to Grantham, and Lincolnshire as a whole.
“Once the new bridge design is complete and we’re ready to submit our planning application, we’ll have a clearer picture of what the full impact of this issue will be in terms of timescales and costs.
“In the meantime, the site team is still hard at work getting the new Somerby Hill Roundabout built, as well as the new mid-route development roundabout.
“We’re also close to opening Phase Two of the relief road, with signage installation on the A1 the only piece of work left.
"We hope to have this work complete and the new junction open in the next 3-6 months, once National Highways allows us onto the A1 to carry the work out.”
The Grantham Southern Relief Road project is being led by Lincolnshire County Council and supported by South Kesteven District Council, Greater Lincolnshire LEP, Highways England, Department for Transport, Network Rail, Homes England and local businesses.
For the latest news on the Grantham Southern Relief Road, visit www.lincolnshire.gov.uk/majorprojects.