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Tower Bridge | Landmark upgrade

One of London’s best known landmarks is getting a much-needed upgrade.

In an age in which the word “iconic” is used to describe even the most mediocre of structures, it is refreshing to report on a project that really does deserve that status: repairs to London’s Tower Bridge. The 122-year-old bridge is one of London’s best known landmarks, and is also a vital river crossing, used by more than 40,000 people and 21,000 vehicles every day.

Despite an 18t weight limit, all that traffic has taken its toll, and the bridge is currently closed to traffic to enable the existing deck to be stripped out, repaired and resurfaced for the first time in almost 40 years. The work has been in the pipeline for more than five years as part of the City of London Corporation’s 50-year plan to maintain the bridges in its control, but this autumn was the first opportunity that the work could be done with minimum disruption.

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pjob 20161111 3

Lifting the surfacing

“We started planning before the Olympics, but we couldn’t do anything until that was over,” explains the Corporation’s assistant director engineering Paul Monaghan. “It has taken me that long to negotiate the closure at the right time, because it has significant impact on traffic in the City.”

The Corporation identified the period from October to December as the most suitable window. “The best time to do roadworks is the middle of the summer, but that’s the busiest time for the bridge, both in terms of tourists and river traffic,” explains Monaghan. “There is less river traffic in the middle of winter, so we opted for these three months, when you usually get the best weather.”

The issue of river traffic is significant: under an Act of Parliament, the Corporation is duty bound to lift the bridge for any vessel that gives 24 hours’ notice. Currently there are about 900 lifts every year, with a full lift taking 10 to 12 minutes. Carrying out the work in the autumn and winter means there are fewer lifts, but tourist boats, cruise ships and Thames barges still regularly go through, meaning multiple lifts every day, even while the deck repairs are underway.

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pjob 20161111 5

Laying the waterproofing

For main contractor Bam Nuttall this is just one of the aspects of the job that make it as much of a logistics challenge as a technical one. The company was appointed at the end of 2015 under an early contractor involvement (ECI) arrangement for the design and delivery of the repairs.

The Corporation undertook a public procurement process for the project, with 18 firms registering expressions of interest. But when it came to the actual tender, only two contractors sent in bids. “I think people are a bit scared of Tower Bridge,” says Monaghan. “We’ve had these issues before.”

But the high profile nature of the project, the complex logistics and rigid deadline did not put Bam Nuttall off. “I see it as a once in a lifetime opportunity to work on Tower Bridge as an engineer,” says contracts manager Janus Moorhouse. “When the tender came in, we looked at the amount of work that had to be done in three months, and thought that it’s a lot of work but it is achievable. The important things were to get the sequencing right, and to make sure we keep the pedestrians moving.

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pjob 20161111 1

The work had to be accomodate road and river traffic

The ECI contract has enabled Bam Nuttall to carry out two phases of investigations into the condition of the bascule deck and the underlying steel structure, as well as giving the contractor and its designer Flint & Neill the chance to look at different materials and technologies. “The nature of the contractor certainly helps with developing the methodology and buildability, and it meant we could try out new techniques,” says Moorhouse.

Tower Bridge is 244m long, with a 61m central section made up of two equal bascules, each weighing over 1,000t. The two 82m long side spans are suspension bridges, with the suspension rods anchored at the abutments and through rods contained within the bridge’s upper walkways.

Steel girders

The bascules consist essentially of steel girders with curved buckle plates and a timber deck. When it was built, the gaps between the buckle plates were filled with creosoted Memel pine, and the road surface was formed in dowelled Memel pine blocks.

From the outset water ingress leaked through this surfacing, causing the timber to swell and rot, and the buckle plates to corrode. By the 1960s a major refurbishment was needed, and the whole deck was removed and exposed. Corroded buckle plates were replaced or repaired; the infill timber was replaced with preformed rigid polyurethane foam blocks; and the Memel pine blocks were relaid.

After further problems with water ingress and swelling of the blocks a decade later, the Memel pine was eventually removed in 1978, and replaced with Acme timber ply panels on top of the polyurethane blocks. Now the bascule surfacing and some of the timber panels are reaching the end of their maintainable life, once again risking corrosion of the bascule girders and buckle plates from water ingress. Bam Nuttall’s contract involves stripping off the surfacing, repairing any damaged steelwork and putting on deck panels and surfacing.

Deck materials options

“We did a lot of investigations into different options for the deck materials, but ended up with like for like,” says Bam Nuttall senior agent Agne Smakovaite. “The overall weight won’t change.”

This is crucial as the 1,000t bascules are balanced so that they require very little energy to be lifted. Kentledge is moved around the deck to ensure the weight does not vary, even when sections of the deck are removed.

During the investigations, the construction team discovered that much of the timber in the carriageway is in very good condition. “We found a specification from the previous work that said the plywood panels are made from veneers of hardwood,” explains City of London senior principal engineer Mark Bailey. Bam Nuttall and timber research specialist Trada investigated further, and concluded that it may be Makore – also known as African Cherry. “The investigations found that it was in very good condition,” adds Bailey. “We still need to strip off the surfacing, but we could retain the timber with confidence that it would last.

Waterproofing

“The rigid polyurethane was also in very good condition – it was still very stiff, and providing a very good seal to the steelwork underneath. We could have done more damage by removing the panels,” he adds.

The footway panels were a 1990s addition, installed when the kerb line on the bridge was moved. Here the Acme panels were formed of a thinner soft wood, and they had deteriorated, allowing water to get through and cause corrosion to the buckle plates. Bam Nuttall is taking up all the footway timber, exposing the steelwork, grit blasting, repairing where necessary and painting it.

On the carriageway around 20mm of surfacing is being planed off, together with the first couple of plys of hardwood to ensure any deterioration is removed. A new surface is then being laid, consisting of an epoxy-based material containing a very hard stone.

The new footway construction consists of two layers of different Acme panels with a skid resistant surfacing containing aluminium oxide built into the top layer.

In addition to the bascule works, Bam Nuttall’s £5M contract also includes installing a new rubberised waterproofing membrane and resurfacing the approaches. The work is being carried out in sections, with pedestrian access being maintained across the bridge – and to the Tower Bridge exhibition – at all times.

Work has progressed well, and the team is on track to finish by 30 December – ready for Tower Bridge to take pride of place as images of the capital’s New Year’s Eve fireworks display are beamed around the world.

 

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