Tunnel vision

13th June 2016

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James B Diamond

Paul Suff learns how a new sewer tunnel under London builds on the innovative system constructed by the Victorians

In a typical year, 39 million cu m of sewage is discharged into the tidal River Thames as the existing system, built nearly 150 years ago, struggles to cope with runoff from land development and waste from a rising population. However, when the Thames Tideway tunnel opens in 2023, completing the London Tideway improvement programme, the amount discharged is unlikely to exceed 2.4 million cu m.

‘The sewer system developed and constructed by Sir Joseph Bazalgette was designed for a population of four million. We’re now at eight and it is expected to rise to ten million by 2031,’ says Roger Bailey, asset management director at Tideway, the company responsible for delivering the tunnel project.

Without the planned 25 km tunnel, dubbed the ‘super sewer’, improvements to sewer treatment works (STWs) and the recently opened Lee tunnel, annual combined sewer overflow discharges into the Thames would have been expected to reach 70 million cu m in the 2020s.

Dealing with the problem

Bazalgette’s is primarily a combined system, transporting wastewater from buildings and industry as well as surface water runoff. It has a built-in failsafe mechanism, consisting of 57 combined sewer overflows (CSOs). These allow waste to overflow into the river to prevent sewage backing up and flooding buildings and streets. Storm runoff frequently triggers overflows, with those in the summer, when it is hot and the river level low, causing most ecological damage. Storms in August 2004 and June 2011 both resulted in the deaths of significant numbers of fish.

Bailey says most long-established major cities suffer similar problems and it is only relatively recently that sewage and runoff have been separated in urban environments, with foul sewage transported to treatment works and storm run-off allowed to discharge into watercourses. ‘Newer parts of London are like that,’ he says. ‘But to separate the existing combined system into separate foul and storm systems would cost about £14bn and take about 20 years.’

When Bazalgette designed the system, many parts of London were still market gardens and orchards, and discharges were infrequent. Now there are between 50 and 60 a year and are breaches of the EU urban wastewater directive and UK legislation. Untreated sewage can stay in the river for up to three months before the tide takes it out to sea. The Tideway tunnel, working in conjunction with the Lee tunnel and the improved STWs, will control about 95% of discharges each year from the most polluting CSOs.

The project has encountered opposition and some critics called for sustainable urban drainage schemes (SuDS), such as green roofs to absorb water and reduce runoff, to be used instead. Bailey says SuDS can provide benefits to alleviate flooding and are encouraged for new developments, but to retrofit the developed areas of London would be problematic. Studies have shown that, even if retrofit SuDS converted significant land – for example, almost 40 times the area of Hyde Park – this would not control CSO discharges to an acceptable level. It would also cost at least four times the cost of the Tideway tunnel and be immensely disruptive to property owners.

One complication that the planners have to address is the variance in the volume of discharge along the river. The construction of the 6.9 km Lee tunnel in east London, between Abbey Mills pumping station in West Ham and the Beckton STWs, will help prevent more than 21 million tonnes of sewage mixed with rainwater overflowing into the River Lee each year. The Lee tunnel opened in January and, combined with improvements at sewage treatment works along the Thames, including increasing capacity at Beckton by 60%, was the first step in reducing overflows into the Thames. However, control of CSOs throughout London is only achieved with the completion of the Tideway tunnel.

Designed for life

The Tideway tunnel starts in Acton, west London, and ends at Abbey Mills. It drops 1 m every 790 m across its 25 km, the tunnel depth falling from about 35 m at Acton Storm Tanks to 66 m at Abbey Mills, where it connects to the Lee tunnel. Due to the storm flow operation of the Tideway tunnel, it is predominantly self-cleaning. There are no intermediate pumping stations, with the new one built as part of the Lee tunnel project capturing flow for treatment at the Beckton STWs.

The tunnel will pass through variable ground conditions, such as London clay, Thanet sands and chalk as well as areas of high water pressure. ‘It spans London’s entire geology, but central London is the most challenging,’ says Bailey. Tunnelling is split into three sections (west, central London and east). The geology of the central section, between Carnwath Road in Fulham and Chambers Wharf in Bermondsey, is more variable than in the west or east.

Most of the 1,301 buildings the tunnel passes underneath are in the western section, between Ealing and Hounslow, and the south east, between Greenwich and Bermondsey, although the 24 listed buildings along the route are mainly in the central region. However, unlike Crossrail (the environmentalist, July 2014), which includes 42 km of tunnels, the Tideway tunnel passes under comparatively few buildings. ‘Because it follows the river, more than 90% of the tunnel is under water,’ says Bailey. Even where it breaks off to connect to Abbey Mills it follows the Limehouse Cut, a canal linking the lower reaches of the Lee Navigation to the Thames.

The existing sewers rely on gravity (and pumping stations) to allow sewage to flow eastwards, but Bailey says the large drop for water into the new system would generate a lot of energy and pockets of air. These can cause hydraulic surges and geysering at the surface, so the Tideway tunnel uses a vortex drop pipe and specially designed chambers to dissipate energy and minimise air entrapment. ‘At Blackfriars Bridge there will be about 50 tonnes of water dropping per second. That’s a massive flow and would create a huge amount of energy at the bottom of the shaft,’ says Bailey. ‘Our solution is to create a vortex and spin the flow of wastewater, which is then controlled at the bottom of the shaft before entering the main tunnel. At each [drop] shaft there are facilities to treat the air that will be released from the tunnel during operations.’

Starting work

Bazalgette’s system consists of more than 132 km of main interceptor sewers as well as 1,800 km of street sewers. It took about 16 years to complete, cost £4.2m and comprises around 318 million bricks. The main construction work for the Tideway tunnel started in February, including piling for a new pier near Blackfriars Bridge. Tunnelling is due to begin next year and the system is expected to open in 2023. It will be constructed of pre-cast concrete tunnel segments, similar to those used by Crossrail, and cost £4.2bn.

Some 2.5 million cu m of earth were excavated to construct the Victorian system. Tideway aims to transport around 4.2 million tonnes, or 90% of excavated material, from 11 sites along the Thames by barge or ships.

The sewer system has operated for almost 150 years and has contributed to improving the health of Victorian Londoners and subsequent generations. Completion of the super sewer should help keep the Thames clean, allowing marine species to thrive and enable the capital to cope with its ever-expanding population.

Applying lessons from other projects

Environment and sustainability manager Darren White says three main lessons learned from previous infrastructure projects have been applied on the Thames Tideway Tunnel scheme.

Engage with industry regarding targets – The London Olympics project set some challenging targets on recycled content. However, after talks with members of the Construction Products Association, it became apparent that better results could have been achieved if a different approach had been adopted. For example, in some cases an increase in recycled content might increase the curing periods, or more fixing agents or water may be needed. Also, if the recycled content is shipped around the world to meet the target instead of using material sourced nearer the point of production the carbon footprint might be greater.

Get the contract right first time – Make sure the works information is clear and is supported by comprehensive documents, such as the Code of the Construction Practice. Incentives to meet key targets are also necessary. One thing Tideway is doing differently is that it is working in ‘alliance’ with the main contractors, so all parties ‘share the pain and gain’.

Respect – A principal lesson learned from other projects is the importance of showing respect to all those involved. One of Tideway’s key values is to treat people fairly, whether they are nearby residents and businesses, employees or suppliers. .

The route to consent

Anna Sutherland joined Tideway in 2012 and was a member of the team that put together the information for the planning application. It was submitted in February 2013 and consisted of 50 documents, including a 25,000-page environmental statement (ES). A further 70,000 pages of information was submitted. Consent, under the Planning Act 2008, was granted in September 2014.

Sutherland says the team endeavoured to keep the ES as short as possible, while ensuring it contained crucial information, such as how the project would comply with regulations and evidence to back up the environmental assessments.

Two public consultations were staged, eliciting 9,400 responses. Sutherland says community engagement was a priority, and the team worked closely with resident groups. ‘Some sites are in residential areas and noise impact was a major concern, particularly around the main [tunnel] drive sites, such as at Chambers Wharf in Bermondsey and at Greenwich, where the site is in the town centre,’ she says. ‘We presented our assessment findings and have worked with residents and regulators to develop plans to mitigate noise impacts.’

Sutherland’s role now includes visiting sites to ensure contractors are delivering on the commitments made in the planning application. She is also part of a team providing assurance checks for contractors’ permit applications. ‘We work with officers from the 14 boroughs the project passes through and regulators, such as the Environment Agency, and check everything before a contractor submits an application to a consent-granting body authority,’ she says. ‘Our role is to check but also to facilitate relationships between the contractors and the boroughs.’

The science of behaviour

Tideway has adopted a behavioural science approach to developing the right culture across the project. It is based the concept that people who know why they are carrying out an activity in a particular way are more likely to perform better than if they are just told what to do.

‘The approach begins with our “EPIC” induction, an interactive day based on the principles of: tell me – I forget; show me – I might remember; teach me and I’ll do,’ says environmental sustainability manager Darren White.

Although behavioural science is used to develop a safety culture among the workforce, White says the principles can be transferred to environment or sustainability because they give people the confidence to challenge decisions, whether it is poor practice or traditional design options.

Tideway is also linking behavioural science into its leadership and coaching training. The areas covered include:

  • Working with people – a range of units, including how to deal effectively with stress and conflict, manage remote workers and build excellent customer relations.
  • Managing yourself and personal skills – including units that focus on assessing your own leadership performance and developing critical thinking.
  • Providing direction – such as leading teams to achieve organisational goals and objectives, and making strong and informed management decisions.
  • Facilitating innovation and change – for example, building a culture of continuous improvement and leading people through change.
  • Achieving results – such as managing efficiency and effectiveness, and managing projects that bring results.
  • Using resources – including managing facilities and information.

Sustainability matters

The main objectives of the Thames Tideway tunnel are to improve the environment, ecology, public health, appearance and reputation of London as its population rises. Given its environmental ambitions it is not surprising that sustainability is integral to the delivery of the tunnel. ‘Sustainability was a fundamental component of the planning application and the development consent,’ says Darren White, the project’s environmental sustainability manager. He describes Tideway’s approach as holistic, covering environment, social and economic issues. ‘The aim is to achieve the best balance across the three factors.’

The commitment to sustainability is reflected in the project’s vision, which was set out in a sustainability statement: ‘[To] deliver a world class infrastructure project fit for the low-carbon economy, which benefits the community, supports a healthier, cleaner River Thames and demonstrates best practice performance in sustainability across the project lifecycle.’ The statement, which was published in 2013, also said: ‘Achieving a sustainable outcome for the project requires taking a balanced approach to social, economic and environmental objectives, whereby objectives for all three principal components of sustainable development can be successfully achieved.’

The project has 15 sustainability objectives, including minimising the carbon footprint. Early on, the footprint of the tunnel over its 120-year design life was assessed to identify interventions that could be incorporated into the design to reduce greenhouse-gas emissions. The footprint was calculated to be 840,000 tonnes of CO2 equivalent emissions, most of which were during the construction phase. Embodied carbon in materials accounted for 84% of total emissions, while 10% were from construction plant and machinery, including from tunnel boring. By contrast, emissions over the operational life of the tunnel represented just 2.5% of the overall total. Measures adopted to reduce GHG emissions include minimising the length of the tunnel and using low-carbon materials where possible.

There are 20 members of the environment and sustainability team, although the number and make-up have fluctuated since work started in 2010. White says during the environmental impact assessment (EIA) phase the team consisted largely of EIA managers led by Thames Water, who looked after specific sections of the route and particular topics of the assessment.

‘They were supported by some of the best consultants in the country, some of whom were used as expert witnesses during the examination,’ he says.

Now that the project has moved into the delivery phase, the environment and sustainability team operates as a matrix function. This comprises three environment and sustainability specialists in each area delivery team, mostly from consultancy CH2M, Tideway’s programme management organisation, supported by nine subject matter experts in the core team from various consultancies, such as AECOM and Arup.

The in-house team boasts several environment practitioners who have returned to work after a career break as part of Tideway’s returners programme. One in particular, Ines Faden, who works in the corporate finance function, was part of the team that developed the Equator Principles, the framework used by banks to manage environmental and social issues in project financing. White and asset management director Roger Bailey are part of the senior leadership team at Tideway and have responsibility for environmental sustainability. As well as the in-house function, each of the three contractor joint-ventures has at least three full-time environment and sustainability specialists, supported by consultants and consent advisers.

White says: ‘All the teams are supported by a raft of monitoring and testing consultancies that are carrying out baseline assessments and real-time environmental monitoring to ensure compliance to the various thresholds that are set in the development consent order or specific consents. Aside from the tried and tested systems, we’re also trialling new and innovative technology to see what improvements can be made.’


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