Queensferry Crossing: the client team's keys to success

Jon Masters
18 September 2017

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The world's longest three-span cable-stayed bridge, the Queensferry Crossing over the Firth of Forth, was opened earlier this month, costing £1.35bn – substantially less than originally forecast. Here, Mike Glover, Paul Baralos and Iain Murray of the delivery team of Transport Scotland and Jacobs Arup Joint Venture, describe how the key technical challenges were overcome.

1. Getting the scope and budget right

When Transport Scotland commissioned the Forth Replacement Crossing Study in 2007, few foresaw the global financial crisis waiting in the wings.  By 2008, funding models feasible just months earlier were off the table and Jacobs Arup JV’s first task on appointment was to reduce the original project estimate by more than half.

The initial brief assumed that the existing Forth Road Bridge (completed in 1964 and unable to take the strain of 21^st century traffic volumes and lorry weights), could not be used in future.  But with support from Transport Scotland, the Jacobs Arup JV showed that the bridge could be adapted for public transport, pedestrians and cyclists, with potential for light rail to be added as part of an integrated ‘managed crossing scheme’.

This meant these provisions would not need to be accommodated in the new structure, allowing it to be substantially narrower. An innovative crossed cable design was introduced to  reduce the size of the deck, foundations and towers, while increasing deck stiffness and controlling deflection. In addition, Intelligent Transport Systems (ITS) could make better use of the existing highways, reducing the requirement for new road construction. Combining these amendments, got the estimated cost into the range of £1.7bn to £2.3bn, which was within the self-funding capability of the Scottish Government.  

Forth Crossing Bridge Constructors (FCBC), the contract-winning consortium, ultimately helped deliver the bridge for even less. This was thanks in part to the competitive dialogue procurement, which enabled open discussion of risks and mitigation during the tender process, and the intensive programme of risk reduction initiatives and targeted construction trials orchestrated by the Jacobs Arup JV in the preparatory phase of the project. The ultimate out-turn cost was just under £1.35bn.

Transport Scotland’s decision to have a lean governance structure was a key factor in the success of the project. It enabled a hands-on management style without duplicating the actions of the Jacobs Arup JV. It significantly shortened communication and reporting lines, and led to timely and durable decision making, which was the hallmark of this project. This was further supported by the Jacobs Arup JV being co-located with Transport Scotland.

Winning the site

The setting of the new bridge is uniquely interesting: the adjacent crossings – the famous cantilever rail bridge, completed in 1890, and the Forth Road Crossing suspension bridge –are both listed structures. The new structure had to be designed to complement the existing bridges and not visually dominate them. The proposed alignment took the route across the estuary where Beamer Rock separates the navigable Forth Deepwater Channel and Rosyth Channel. The rock was to provide the central support location for a three-tower cable-stayed bridge, with two main spans, each of 650m. 

The greatest challenge for bridges that cross estuaries is constructing foundations in deep seawater. All resources have to be provided by marine vessels, often in challenging weather conditions, which makes the process much more difficult and costly than similar construction on land.

Following extensive land and marine site investigations, the chosen crossing location required 15 foundations with 10 in the marine environment for the bridge’s three towers and seven piers. The central support was formed by blasting a rock pocket in Beamer Rock, then forming an in-situ concrete base inside a bespoke sheet-pile and precast concrete cofferdam.  The north and south flanking towers were built within caisson foundations. These were designed to be founded on bedrock (-40mOD), within water depths up to 20m. The steel caissons were twin-hulled, typically 30m internal diameter, up to 41m in height and weighing in excess of 1,200 tonnes.

Excavation within the flooded caissons enabled them to be sunk to the required level, followed by underwater jet-grout base-sealing between the caisson and rock bed prior to the large underwater concrete pours. The south tower holds the world record for the largest continuous underwater concrete pour by marine supply, at 16,869m³ over 15 days supplied by a flotilla of barges. Final dewatering of the completed caissons allowed construction of the required foundations for the superstructure above. 

Designing for durability 

Ease of maintenance and the long-term durability of the bridge were high priorities for Transport Scotland. To combat the unpredictable and often severe weather conditions that affect the Firth of Forth, a combination of design features was specified. This included the use of a parallel strand stay cable system that permits the replacement of individual strands in future as part of routine maintenance. The road surface was laid by plant in echelon formation to remove longitudinal joints and reduce maintenance. A dehumidification system inside the deck prevents condensation, so eliminating corrosion.

The concrete mix designs and testing included determination of chloride diffusion rates and  the modelling of thermal behaviour. GGBS (ground granulated blast-furnace slag) mixes were specified to provide protection against both sulphate and chloride attack. Stainless steel reinforcement was used extensively and the deck steelwork was protected with a five coat paint system in the splash zones of the towers and marine piers.

A critical issue for any stay-cable design is the corrosion-protection system. The system specified for the Queensferry Crossing consists of a multi-layer barrier of high-density polyethylene (HDPE) on the outer stay pipe and HDPE coated strands consisting of seven galvanized and waxed wires. The anchorage, the most vulnerable part of a modern stay-cable in terms of durability, was put through rigorous water tightness testing. Corrosion protection within the anchorage is additionally maintained by an injected flexible gel filler.

Keeping the bridge open, whatever the weather

Traffic restrictions and occasional closures due to strong winds have frequently affected the Forth Road Bridge over the years. To avoid similar disruption on the new bridge, wind shielding was developed for the edges of the deck to protect vehicles without unreasonably increasing the forces the structure must carry.  

The windshields were initially developed using mathematical models and small wind tunnel tests in London, then through an intensive programme of testing using some of the largest wind tunnels in the world in Italy and Denmark. The windshields were made from slats, with acrylic louvres, which act as a grille to reduce the speed and turbulence of the air passing through it, and angled to encourage the wind to be substantially ‘scooped’ over the roadway. The use of transparent materials for the slats provides views down the Forth. By comparison a solid wind shield would have greatly increased the loading on the bridge and initiated undesirable aerodynamic effects in the bridge deck.

Ship impact studies

The Firth of Forth is a busy seaway. The towers and piers of the new bridge lie just outside the shipping channels that serve the refinery and petrochemical plant at Grangemouth, and the docks and shipyards at Rosyth. Tankers, ferries, container vessels and aircraft carriers of up to 70,000 tonnes pass under the bridge. Several of the piers are in sufficient depth of water to mean the largest vessels could feasibly collide with the structure.

A probabilistic approach was used to derive ship impact loads. This was based on the existing vessel transit data with an assessments of future changes in usage and vessel type. For every segment of every transit the probability of aberrance and its consequence was assessed in terms of the frequency of collision with a bridge support and the resulting load: the data set was analysed to discount vessels that would ground before reaching the support. The probability profile of the ship impact design load for each support was established this way. Acceptable level of risk to the client was determined using the widely used principles of “As Low as Reasonably Practical” (ALARP) risk.

Structural health monitoring system 

A system for monitoring structural health has been designed for the Queensferry Crossing, incorporating decision support tools and approximately 1,000 sensors monitoring wind, temperature, corrosion, motion and any strains on the bridge. In significant weather events, the system will raise the alarm and calculate where the likely impact will have been and so what needs to be checked. 

Reports are generated automatically – providing immediate support to the inspectors, as well as long term data that can be analysed to ensure better investment decisions are made. All the information is stored in the Cloud which allows the operator to easily maintain and update records and provides Transport Scotland with detailed oversight of the condition of the bridge and its operational and maintenance budgets.

Dr Mike Glover, Dr Paul Baralos and Iain Murray are members of Transport Scotland’s Employer’s Delivery Team, an integrated team of Transport Scotland and the Jacobs Arup Joint Venture.