One of the UK’s largest retaining structures installed on Sunderland Strategic Transport Corridor
15th December 2020
Partnership between client, contractor and specialist sees a high quality and value engineered geotechnical solution delivered on SSTC3.
Esh Construction, Aarsleff Ground Engineering, and Sunderland City Council have worked in collaboration to deliver 500 metres of retaining structures on the multimillion-pound Sunderland Strategic Transport Corridor, making it one of the largest retaining structures in the UK.
In just over 12 months, one of the UK’s largest soil nail walls, a large king post retaining wall and two further reinforced concrete retaining walls, have been installed in a major engineering feat on Phase 3 of the Sunderland Strategic Transport Corridor (SSTC3).
The £40m SSTC3 project – which is being delivered by Esh Construction’s civil engineering division, Esh Civils, on behalf of Sunderland City Council – will create a new 2.5km uninterrupted dual carriageway link to the Northern Spire bridge and the city centre.
SSTC3 begins at the southern end of the Northern Spire and continues through the former Pallion Shipyard, underneath the Queen Alexandra Bridge, into Deptford Terrace and onto the city centre. The retaining structures were necessary to create space for the highway between the former Pallion Shipyard and the upper existing highway and Tyne & Wear Metro line, given a height difference of 20 metres separated the bordering land.
The largest of the new structures is the soil nail wall which was introduced as a result of a value engineering activity, providing time and cost savings for all parties.
Soil nailing is a construction technique that stabilises steep new or existing slopes by inserting reinforced bars into pre-drilled holes. The bars are then grouted into position and the wall is finished off with a flexible mesh and erosion fabric, while the nail head is held in place by soil nail head plates. A soil nail wall is a much simpler way of using existing ground to retain a structure and is an alternative to a traditional building technique of constructing a large concrete wall.
Installed between the new highway alignment and the metro line, the soil nail wall extends across 1,995m2 of wall face and rises up to 12m high and 250m long. A total of 1,800 soil nails were used in its construction which in total provide a combined length of just over 14 miles of soil nails. The wall is faced with gabion stone and mesh, providing a much more aesthetically pleasing and iconic structure which can be seen from as far as the Queen Alexandra Bridge.
The second major structure comes in the form of a king post retaining wall which is sandwiched between the north side of the new highway and the former Pallion shipyard. Over 100 high-capacity ground anchors, some made of up to 16 strands of steel, were installed by Aarsleff. The ground anchors were drilled into the earth, passing through 20m of granular material, the interface of the old quarry and founded into underlying limestone to retain loads of over 200 tonnes.
A total of 60,000 tonnes of earth was excavated when constructing the retaining structures, the majority of which has been screened and re-engineered for reuse on the project.
Steve Garrigan, Project Director at Esh Civils, said: “The size of the scheme and logistics needed to accommodate construction through extensive temporary works, coupled with maintaining access and assets for numerous stakeholders was a tremendous achievement for all involved. Challenging schemes of this nature and magnitude don’t come along often. The collaboration and efforts by all involved is testimony to its successful completion safely, on time and within budget.”
Kevin Hague, Managing Director, Aarsleff Ground Engineering, said: “This iconic project is becoming the benchmark in soil nail design and execution standards. Working closely with our partners Esh and Sunderland City Council, we are changing the landscape forever and demonstrating along the way how total partnership between client, contractor and specialist can truly work to deliver highly engineered and quality structures.”
Chris Ferry, Project Manager at Sunderland City Council, concluded: “Creating a dual carriageway from the Northern Spire to the City centre would always be challenging to construct. Regular design workshops between all parties have enabled these solutions to be developed and it is a great testament to the individuals involved, who have embraced the collaborative approach needed for a scheme such as this.”
The soil nail wall is due for completion in March 2021 which will mark the project reaching 80% completion.
The size of the scheme and logistics needed to accommodate construction through extensive temporary works, coupled with maintaining access and assets for numerous stakeholders was a tremendous achievement for all involved. Challenging schemes of this nature and magnitude don’t come along often.”