Queensferry Crossing

As part of the overall network connection design, Sweco designed and managed the implementation of ground improvement beneath the Queensferry Crossing’s northern approach. Sweco employed innovative deep ground improvement techniques and designed an alternative to a multi-span viaduct, saving millions of pounds in capital costs with improved programme delivery.

KEY FACTS

Location: Edinburgh
Contractor: Forth Crossing Bridge Constructors
Client: Transport Scotland
Construction value: £790m
• Opened in 2017

PROJECT

The Forth Crossing Design Joint Venture (comprising Sweco, Ramboll and Leonhardt, Andrä und Partners) was appointed under a design-and-build contract to provide multi-disciplinary design services for a new 2.7 km cable-stayed bridge (the ‘Queensferry Crossing’) spanning the Firth of Forth in Scotland. This includes design of all road network connections either side of the crossing.

Sweco worked extensively on the design of the network connections, including 6.4 km of new, improved motorway standard dual carriageway and significant modifications to the local road network. We provided design services for two new major grade separated junctions, 26 overhead sign gantries and 10 road bridges, including the 100m long Ferrytoll Viaduct, and several kilometres of new embankments, retaining walls and environmental bunds. Sweco also provided environmental and landscape design services.

Services provided:

• Ground Investigation
• Environmental Surveys
• Spatial Modelling
• Transport Planning
• Optioneering
• Earthworks and Foundation Design
• Highways, Pavements and Drainage Design
• Landscape Design
• Project Management
• Value Engineering
• Risk Management
• Construction Supervision
• Structural and Bridge Engineering Design

BACKGROUND

The Forth Replacement Crossing, a project to provide a second road crossing across the Fife of Forth, was initiated after an options study in 2007. This was the biggest infrastructure project in Scotland for a generation and was initiated in response to defects discovered on the 40-year old Forth Road Bridge in 2004.

This project comprised the Queensferry Crossing, a new 2.7km cable stayed motorway bridge, with significant upgrades to the connecting road network on either side. The project included the upgrading of 19.7 km of roads connecting to the new crossing and construction of 4km of completely new connecting roads.

In 2010, the Forth Crossing Bridge Constructors were appointed to design and construct the new crossing, with the Forth Crossing Design Joint Venture (including Sweco) appointed as scheme designer. The bridge opened to traffic in August 2017.

SOLUTION

Sweco led the design of the northern and southern road network connections and devised several innovative design measures to overcome challenging and complex ground conditions.

We identified the need for deep ground improvement measures beneath the following two areas of the northern road network to control embankment stability and long-term settlement:

• Road B981 – 8m-high road embankment located in marshland, overlying 15m of reclaimed estuarine deposits historically dredged from the Firth of Forth
• FT19 Reinforced Earth Wall – 9m-high retaining wall supporting the western edge of the M90 motorway overlying 12m peat and alluvium

To provide a cost-effective and sustainable solution that achieved capital cost and programme savings, we implemented innovative ground improvement techniques to support road embankments on the northern network connections.

To achieve cost savings of tens of millions of pounds, we challenged the client’s original seven-span design for Ferrytoll viaduct to the north of the crossing by proposing an alternative twin-span solution.

The complexity of the ground improvement solution provided many design challenges. As the Principal Designer, we worked closely with the Principal Contractor and ground improvement specialist contractor to specify a suitable ground treatment solution. To verify our solution, we used advance 2D and 3D finite element analysis techniques.

To enable design conceptualisation and detailed design analysis in accordance with BS8006 and Eurocode 7, we worked closely with specialist contractors to define ground models, engineering parameters and loading conditions.

To facilitate the procurement of specialist ground treatment measures we produced a performance specification and to improve performance we implemented hybrid ground treatment solutions where required.

We also specified a rigorous monitoring regime across the ground improvement areas to monitor and verify the performance of our solutions.

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