Phase 2 strengthening of the Hammersmith Flyover (HFO2), was a phenomenally complex £100m programme including innovative engineering solutions to install a full new prestress without removing the original. With 70,000 users every day on a key strategic route into London, the structure, which had been deteriorating due to significant corrosion, presented many technical, logistical, programme and political challenges.
- Background: 1961 to 2012
- The Structures and Tunnels Investment Programme (STIP)
- Phase 2 – strengthening Hammersmith Flyover (HFO2)
- Above deck
- The new prestressing system
- Anchorage design
- Cutting constraints
- Ultra-high performance fibre reinforced concrete (UHPFRC)
- Replacement of the bearings and central expansion joint
- Precision Engineering & Collaboration
- HFO2 project legacies
- Ramboll Team Leaders
- Image Gallery
An innovative structure in its time, the 16-span, 622m long Hammersmith flyover, is generally considered to be the first major segmental precast post-tensioned highway structure in the UK. It has formed a key part of London’s major A4/M4 link to Heathrow Airport and the west of England since it opened in 1961.
In its design and construction, grout was intended to protect the prestress tendons (which are partly internal and partly external) from corrosion. The Hammersmith Flyover was not originally designed to be subjected to de-icing salt. Instead its designers, G Maunsell & Partners provided electric deck heating, but this was discontinued when electricity prices rose in mid-1960’s.
The Highways Agency revealed in 1999 and 2000 that the post-tensioning tendons holding the precast concrete structure together were corroding and work began by the new bridge owner, Transport for London (TfL) to slow down the corrosion process. However in 2009 further inspections revealed significant deterioration in the tendons which resulted in TfL establishing probably the largest structural monitoring programme in Europe, with 400 acoustic sensors on the eastern section. This proved vitally important at highlighting the urgency of works required. For example, the system picked up about one break a month on the Huntingdon flyover; Hammersmith flyover had one wire break a day. There was no doubt that the situation was critical.
When inspections in December 2011 discovered extensive voids in the grout and active corrosion of the tendons, TfL took the unprecedented step of closing the Hammersmith Flyover immediately. This led to emergency (Phase 1) strengthening to ensure the bridge, which was a key part of the 109-mile Olympic Road Network, could be re-opened ahead of the 2012 Olympic and Paralympic Games.
Following The 2012 Games, TfL identified significant capital expenditure was required on infrastructure and assets to ensure they are fit for the future, which led to TfL’s Structures and Tunnels Investment Programme (STIP).
The objective of the STIP is to replace, strengthen and refurbish key bridges, tunnels and other structures on the Transport for London Road Network (TLRN) to ensure network safety and reliability, while considering the needs of other transport modes. The STIP schedule began by delivering three discrete packages of work within an Early Contractor Involvement (ECI) Framework. Work Package 3 (WP3) consists of the second phase of improvement works on the Hammersmith Flyover.
TfL appointed Ramboll and Parsons Brinckerhoff in a joint venture to begin design work across the portfolio,costing around £200m to deliver. The three tier 1 contractors appointed later by Tfl were Hockteif (WP1), BAMNuttall (WP2), and Costain (WP3).
TFL’s key objective for WP3 was a programme of works which would restore the capacity of Hammersmith Flyover and see it through at least the next 60 years without requiring major maintenance. Ramboll and the rest of the team began the remedial works in October 2013, which included simultaneously:
- Changing drainage and improving road restraint systems, finishing with waterproofing and resurfacing the deck
- Making the structure independent of the original prestress with the installation of a new prestressing system that could be replaceable in the future
- Replacing the bearings upon which the entire structure sits and replacing the massive central expansion joint with a new comb joint
- Work was undertaken 24/7 and any activities requiring road closures above or below had to be scheduled outside the working week and after hours to minimise disruption to the traffic flowing along the A4/M4 link to Heathrow and the West of England.
This second phase of strengthening works, was completed in 2015 and will ensure the flyover serves London with no requirement for major maintenance well into the future.
The surfacing, waterproofing and central reserve have all been replaced. Work above deck focussed on revamping the surface water drainage system, keeping it above the new deck waterproof membrane. This involved the installation of central reserve drainage, kerb drains, and bespoke drainage hoppers running below deck that were shaped to accommodate new post- tensioning.
The design of the new prestressing system began with planning the location of the new tendons; a critical issue because the optimum location was already utilised by the original tendons which it was not practical to remove, and because of the form of the structure with a narrow 3 cell spine beam and multiple diaphragms.
3D modelling was used extensively both in the design and construction methodology for clash detection between the new elements and the existing structure. Developed from a combination of drawings and advanced laser scanning, the 3D model helped inform, verify and challenge design assumptions, test construction scenarios and understand the many interdependent elements at work. Forward-thinking Building Information Modelling workflows and technologies led to targeted physical surveys and identification of key risk areas, and took second prize at Autodesk Excellence in Infrastructure 2015.
A number of options were considered, but the complex nature of the project resulted in an innovative solution, partly based on bridge cable stay technology. A combination of long deviated tendons inside the box and short straight tendons largely outside were used. The straight tendons were hot-dip galvanised and protected by polyethylene sheaths, and anchored with innovative ultra-high performance fibre reinforced concrete anchors (UHPFRC).
”The biggest challenge for the prestress design was finding space. I lost count of the number of times we said ‘we did not ask how much space you wanted: we asked how much space you needed’!”. Paul Jackson Ramboll.
For the tendons, 192 innovative anchors or “blisters”, up to 1500mm x 900mm, were designed to fix to the curved deck box. Those outside the box were precast and installed on the upper vertical web and at the mid-span using special handling rigs. The anchors were carefully designed with the help of BIM and Laser scanning to minimise the stresses on the structure but strengthening backing slabs were needed to avoid overstressing the existing concrete. These and the internal anchors were also cast in UHPFRC but this was cast in situ using a novel syringe.
The long deviated tendons, up to 300m in length, are anchored in larger more conventional anchors at the end of the bridge and in the soffit. These were constructed using self-compacting concrete.
“This is a highly stressed structure so there were several constraints on where we could cut the 12 core holes each blister needs,” Matthew Collings project director.
Each coring operation was fraught with risk, not least because the location of the box reinforcement turned out to be variable. The design team was concerned that as the cantilevers are particularly high stressed, there would be real problems if any of the original reinforcement was not in the right location. For example, for the mid-span blisters the coring had to be through the curved surface from outside.
The restricted space inside the deck boxes made the job incredibly challenging for the engineers on-site, working in confined spaces to install the blisters.
Detailed laser scanning and BIM modelling of the entire structure helped the team to get the orientation right on each blister. In some cases hydrodemolition was successfully used as an alternative to coring.
The use of UHPFRC (up to 170MPa strength) on this project has raised the bar in the concrete repair industry. UHPFRC has not been used in this application before in UK and design is not covered by the Eurocodes. Successful testing by Freyssinet validated the performance under service loads, thus confirming the ability of the blisters to transfer the prestressing anchorage forces.
“We believe HFO2 is unique in its concept and particularly the size of the retro-fitted tendons. Replacing, fully, all the old post-tensioning without first removing it on such a significant structure is truly impressive”. Paul Bottomley, Freyssinet UK managing director.
UHPFRC benefitted the new prestressing system with 50% reduction in size, only minimal conventional reinforcement required and lower weight of the concrete anchorage which enabled the minimum headroom clearance to be maintained for road users passing beneath the flyover.
In addition, the use of UHPFRC also made a significant impact on the aesthetics of the structure because purpose made moulds could be used for the blister construction. The use of efficient off site precast manufacturing and testing techniques ensured consistency of product and full quality compliance.
With the majority of the short tendons attached to the outside of the deck box, and the difficultly of access, it would have been particularly challenging to achieve the level of quality required with in-situ construction using conventional concrete.
The original design of the flyover accommodated movements in each section with roller bearings below each pier and a large central expansion joint between the two structurally independent sections that make up the flyover. As a consequence of the increase in prestress, the two sections would be shortened by up to 150mm, and the original bearings and expansion joint had insufficient capacity to accommodate this in addition to contraction during cold weather.
One part of the solution was to replace the central expansion joint with a new combed design. As for the bearings, replacement was the only realistic option but like for like replacement of the original bearings was not possible as suitable roller bearings were no longer available.
Installation of sliding spherical bearings was the only option, despite them being heavier and larger than the original ones. This was an exceptionally challenging prospect given their size (up to 1.6m length and 2.3 tonne weight), the location of the bearings at the bottom of each pier in pits below ground level, and the need to also accommodate the jacks necessary to temporarily support the bridge.
Access into the pits was limited to a 600mm gap around the piers and the working space below the piers was no more than 800mm. In addition, strengthening of the foundation slab below the bearings and the pit walls was necessary to accommodate the jacking loads. After an initial check of geometrical constraints using BIM, Costain made a full size mock-up of a bearing pit and a pier constructed in timber and steel, with timber replicas of the proposed bearings, to trial the proposed solution.
Successful collaboration, a first principles approach to design and appropriate deployment of technology all played a key role to deliver technical innovation and an accelerated delivery programme. The project’s success is a testament to the ingenuity of the engineering and contractor team, at not only overcoming the known engineering challenges, but also the later discovered issues. At every major juncture additional challenges were faced where unexpected degradation of the structure was found, such as corrosion on the shims (the failsafe stools used to support the piers should the piers ever fail), and honeycombing on the pit foundation slab.
Ramboll, Parsons Brinckerhoff, TfL, Costain, Freyssinet, Structural Systems/Hevilift and Flint and Neill, collaborated with remarkable precision engineering. For example, the 35mm thick steel carrier plates installed underneath each pier had to be positioned within 0.5mm to ensure the correct orientation and loading of the new bearings.
Costain commercial manager Andrew Morse said: "One of the benefits of the absolute trust developed between the parties was that the project team could progress the significantly changing services and works without always relying on fully implemented change. This trust allowed for the smooth progress of the project."
The team used New Engineering Contracts (NEC3) to facilitate project delivery and define legal relationships. NEC3 complies fully with the Achieving Excellence in Construction (AEC) principles, and is recommended by the Efficiency & Reform Group of The UK Cabinet Office for use on public sector construction projects.
Matthew Collings, Ramboll project director said: "The NEC principles of mutual trust and cooperation together with co-location of the design team helped facilitate a dynamic decision-making process. This resulted in a number of non-standard design approaches being adopted to address the unique technical challenges on this project within the tight time frame".
"It really was the best working environment I have ever experienced and should be a model that all projects would do well to follow" said Ken Duguid, Technical Approvals Manager (Structures and Geotechnics) for the Structures Management Group, TfL – Asset Management Directorate.
- NEC3 Contracts Large Project of the Year ‘Highly Commended’ certificate acknowledged the high levels of collaboration within the HFO2 delivery team. 27 April 2016.
- Post Tensioning Association 'Project of Special Recognition' award celebrated the innovative engineering solutions applied to such a significant structure to replace all the old post-tensioning without first removing it. 21 April 2016.
- Chartered Institution of Highways & Transportation, CIHT/Ringway Innovation Award - The judges were impressed by not only the number of innovations and bespoke elements of this scheme that could be easily transferred to other structures, but also the engineering principles and thinking behind the design and construction ingenuity. The judges felt that the concept of post-tensioning a structure from the outside, allowing the original pre-stress to become redundant, is a relatively simple economic solution. They also acknowledged the fact that the work took place with minimal impact on the travelling public over a relatively short period of time. June 2015.
- ICE London Civil Engineering Award ‘Re-Engineering London 2015’ highlighted the technical complexity of the project and all the hard work and the skills and experience that exist in the fully integrated team.
- In Las Vegas, USA, HFO2 took second prize at 'Autodesk Excellence in Infrastructure' for forward-thinking Building Information Modelling (BIM) workflows and technologies. December 2015.
- Freyssinet won the Construct Project Award 2014 for the innovative application of UHPFRC on HFO2, with judges commenting that Freyssinet is driving new techniques like the development of a unique concrete placement system.
This unique project has challenged the application of design codes and has the potential to change the way we think about future refurbishment of similar concrete structures:
- The innovative use of UHPFRC as a key element in the post tensioning strengthening system has set a precedent for extending the life of key structures using advanced concrete with new techniques.
- Intelligent use of 3D scanning technology is driving design efficiencies, eliminating programme and safety risks and speeding up repairs to minimise disruption to the public.
- Believed to be the first time full new prestress has been installed in a bridge where it was not possible to remove the original.
There were many people involved in this project from Ramboll and Parsons Brinckerhoff. The Ramboll team leaders were: