BENEATH London's streets construction is continuing on the latest addition to the maze of railway tunnels that serve the British capital.

At £14.8bn Crossrail is currently Europe's largest infrastructure project and London's most important transport undertaking in a generation. The 110km east-west route from Shenfield and Abbey Wood to Reading and Heathrow Airport covers 40 stations, including eight in a new 21km twin-bore tunnel right through the heart of London.

The first passengers will use services through the new tunnel in late 2018, and once fully operational the following year, Crossrail will help to reduce journey times, ease congestion and improve connections for an estimated 200 million annual passengers. It will also increase rail capacity by 10%, which is essential as London's population continues to rise.

Crossrail prides itself on its sustainability record and a sustainable approach is evident across the project in three key areas: economic, environmental and social, in activities ranging from procurement to disposal of tunnel spoil and regard for the livelihood of employees. In fact, according to Crossrail's 2014 Sustainability Report, the project is on target to achieve an 8% carbon dioxide reduction which would save approximately 57,000 tonnes of emissions. Crossrail has also employed nearly 2800 people from London boroughs, many of whom were previously unemployed, while 97% of contracts have been awarded to British companies.

These achievements are no accident. To incorporate sustainability into business planning from the start, Crossrail produced a sustainability strategy which highlighted seven sustainability themes critical to the delivery of the project. Within this there are 15 Key Sustainability Initiatives (KSI) designed to deliver the requirements of each theme, with responsibility for their realisation assigned to senior management. This structure is intended to take a balanced view of the project's environmental, social and economic performance, with each theme and KSI reflecting Transport for London's (TfL) sustainability framework, government priorities, and Department for Transport (DfT) policy.

Senior management, under the overall direction of the chief executive, is responsible for delivering the KSIs, with each Crossrail directorate given or responsible for identifying specific sustainability objectives and targets. Corporate KPIs are established each year against which top management and partner organisations are offered monetary incentives to achieve these objectives, many of which seek a high level of sustainability performance.

Bechtel was appointed project delivery partner for Crossrail in 2009 and developed the sustainability strategy in partnership with Crossrail. Dr Mike de Silva, Bechtel's Crossrail sustainability manager, says the strategy is reviewed annually so it remains fit for purpose, and given the vast scale and importance of the project, this sustainable approach offers valuable lessons to similar schemes that are planned or currently underway around the world.

De Silva says it was understood from the beginning of the project that contractual requirements were insufficient to support the level of performance needed to meet the project's sustainability goals and the "world class performance" desired for the project.

As a result a critical factor in achieving these goals has been the establishment of a common vision for the project through developing a philosophy of collaborative working and information sharing. Regular forums between the contractors' respective environmental managers, procurement specialists and sustainability managers were subsequently established which have helped to mobilise resources.

These include the Carbon Working Group which emphasises reducing carbon emmissions, Ethical Supply Chains in Construction (ESCIC) which looks to promote ethical sourcing, a Contractor Environmental Managers forum, and an innovation forum, which evaluates ideas through a competition and provides funding for development.

These groups are enabling contractors to share best practice and lessons learned, which is aiding the deliveryof Crossrail's sustainable objectives.

In addition, a supplier performance framework is now in use where contractors are assessed against contract requirements with performance going beyond compliance deemed as "added value" and "world class." Achievements are also recognised in the annual sustainability awards, while contractors can secure "Green Line" status, which recognises and increases awareness of good practice and environmental behaviour on construction sites.

"The intention was to create an atmosphere where contractors were not only achieving their minimal targets, but are given the flexibility to think innovatively and to go above and beyond these goals," de Silva says.

"We developed an interactive platform in which all parties within the supply chain were encouraged to come forward with ideas that would potentially benefit the project. Many of these companies are used to working in other industries so their way of thinking may be quite new to those used to working solely within the railway industry."

Crossrail's annual sustainability report documents the achievements of the project, with 2013-14's edition the fourth since construction began. 2015 is a critical year for Crossrail, which is now at its peak rate of construction with 10,000 people working across the project. The eight tunnel boring machines are set to conclude their work in the spring as attention shifts to fitting out the tunnels and installing railway systems before the expected completion date of 2017. In addition the section between Liverpool Street and Shenfield is also set to join the TfL network on May 31, the first step in introducing the Crossrail service, which MTR will operate under a contract awarded by Transport for London in 2014.

To achieve its environmental sustainability goals, de Silva says that the emphasis of the project is to improve efficiency and reduce energy consumption in both the construction and operation of the railway. An array of environmentally-friendly technologies and working practices are consequently in use across the project.

Crossrail-sustainabilityFor example, LED and hydrogen fuel cell lights used during construction in stations and tunnels are reducing energy consumption by 38% compared with conventional lighting sources, while LED lights will be used in stations and tunnels when the railway is operational. Variable speed escalators, which offer a 23% improvement in energy consumption compared with current legacy escalators, are also being used and will be incorporated into future upgrades to the London Underground network.

There is also a significant emphasis on reducing rolling stock energy consumption. Bombardier's Aventra trains were the lightest design offered in the bidding process and will all use regenerative braking and sleep modes for heating and ventilation systems. In addition, the use of a hump back profile at stations will reduce the energy required for braking and acceleration.

In addition, the depot at Old Oak Common, which Bombardier is delivering as part of the rolling stock contract, is set to incorporate rainwater harvesting technology and roof mounted photovoltaic panels, which combined with an under-floor heating solution, are estimated to reduce heat loss and CO2 emissions by 35%, well above the target of 20%. Custom House, one of Crossrail's new above-ground stations, will also utilise rain harvesting technology, as will Whitechapel where a green roof is being installed.

Crossrail estimates that CO2 emissions from the construction process will amount to 1.7 million tonnes, with the project set to offer a payback period of nine to 13 years. This figure is relatively low for such a large project and de Silva says there is significant attention on minimising carbon and particulate emissions. By mid-2014, 73% of construction equipment in use on the project was using emission control technology such as diesel particulate filters, or European stage 3b-compliant diesel engines.

In addition trials are underway with the David Ball Group to use Cemfree concrete. This low-carbon alternative eliminates the need for cement in concrete and could have a substantial impact on reducing emissions during cement production, which produces 5-8% of all the world's estimated CO2. Similarly Crossrail is exploring the use of used aggregate in concrete through a research scheme with Nustone and the University of Wolverhampton.

"A big feature of the project is to try and innovate where possible," de Silva says. "If these trials are successful they will form a major part of Crossrail's legacy for future infrastructure projects."

Utilising the latest enviromentally-friendly working practices is a key element of the sustainability strategy. For example, all tier one contractors and their subcontractors have committed to a Civil Engineering Environmental Quality (Ceequal) assessment scheme as an environmental assessment methodology and benchmarking tool for all tunnels, portals and shafts. The Building Research Establishment Environmental Assessment Methodology (Breeam), is playing a similar role for all stations in the central section. Indeed Crossrail is pioneering the use of Breeam for a railway construction project and its legacy is already evident in London Underground's Northern Line extension project and Bank station upgrade.

In practice these initiatives are resulting in the reuse or recycling of 99% of the 4 million tonnes of waste materials generated from the project so far and sourcing construction materials which contain 32% of recycled content, well above the 15% project target. However, this figure is set to fall during the fitting out period when attention shifts away from concrete and steel to materials for finishing, cladding and flooring, which are less sustainable, although steps are being taken to select lower impact options.

Projects that are benefiting from the reuse and recycling of Crossrail's waste include the Wallasea Island Nature Reserve through an initiative with the Royal Society for Protection of Birds (RSPB), where tunnel spoil is used to build new wildilife areas, and Pitsea, also in Essex, where 187,179 tonnes of chalk slurry from the tunnels is being used to convert a former landfill site into calcareous grassland.

Water conservation has also been considered and the Hochtief Murphy JV is saving an estimated 2 million litres of potable water per week by using surplus groundwater from the dewatering scheme from Benotite mixing, slurry dilution and grout mixing to treat chalk slurry spoil from tunnels, which is required before this material is reused.

Crossrail is attempting to reduce the impact of noise and vibration from construction activities, particularly around Whitechapel where it has offered to temporarily rehouse affected residents. In addition, an extensive archaeology programme was conducted during excavation, and the recorded outputs are proving invaluable in understanding London's past.

Economic sustainability

Crossrail is estimated to be worth £42bn to the British economy, or £1.97 for every £1 of the project's capital cost, and it will bring an additional 1.5 million people to within 45 minutes of the centre of London. Indeed 41% of planning applications in the vicinity of the project in 2013-14 cited the railway as the reason for their development.

As Britain's largest railway project for a generation, all of which is publicly funded, Crossrail has endeavoured to support British industry throughout its development.

Consequently 97% of contracts awarded are to British-based companies, with three-fifths located outside of London and each region of the country contributing something to the project. Crossrail is expected to create 55,000 jobs during construction and 75,000 business opportunities, with 62% of contracts awarded to small and medium enterprises (SMEs).

To guarantee that British companies, particularly SMEs, have sufficient opportunity to participate in the project, Crossrail held several meet the buyer and meet the contractor events from which several attendees have subsequently won contracts.

Similarly it held a drop-in event with members of parliament so they could promote the project in their constituencies, and regional road shows and presentations with 250 potential suppliers at a CompeteFor supply chain summit. Indeed the CompeteFor initiative, a supply chain brokerage service first developed for the London Olympics, is playing a critical role in recruiting the supply chain. All tier one contractors are required to advertise their opportunities on CompeteFor, with 80% of the contracts awarded available through the platform.

"We are very mindful of the holistic approach to sustainability and the importance of working with SMEs to guarantee the project's economic sustainability," de Silva says. "We have been very conscious of offering opportunities to the domestic supply chain and that opportunities are realised across Britain, not just in London and the South East."

Crossrail's emphasis on sustaining British industry is extended to the project's workers. Maintaining high levels of health and safety has been a priority throughout, while employee well-being has been taken into consideration through a range of initiatives. Contractors are also obliged to pay at least the London Living Wage, the minimum amount deemed sufficient to live in Britain's capital where prices are higher than the rest of the country.

Contractors are similarly obliged to develop local skills, with a particular emphasis on reaching out to groups currently under-represented in the engineering sector, as well as offer employee training programmes, employ apprentices, and contribute to community engagement.

Skills are consequently being harnessed through community development programmes and various training initiatives, including the Tunnelling and Underground Construction Academy (Tuca). So far more than 8000 people have participated in Tuca courses learning skills that will benefit their work on Crossrail and set them up for future opportunities. Tuca has also secured £4m in funding from the Skills Funding Agency for programmes for skills development for 3000 local learners, which has translated into 900 people securing jobs, and £1.1m from the UK Commission for Employment and Skills to support skills development, particularly at SMEs.

Young people are a major target for these engagement programmes. By the middle of 2014 more than 350 people had participated in work experience programmes on site and at head office, and 241 graduates had been recruited through schemes led by tier one contractors. Other programmes such as the Young Crossrail scheme, which is led by volunteer ambassadors, and the Budding Brunels initiative, are promoting the benefits of a Science, Technology, Engineering and Mathematics (Stem) based education to young people, and careers in engineering.

"400 apprentices was the target for the project, and we have already exceeded this," de Silva says. "We are very proud of the legacy of this as it is providing the pipeline of engineering talent from London and the South East for us and for other projects for many years to come. We are also providing opportunities for the unemployed and offering a more stable footing for many people to establish careers when the project has concluded. The Tunelling Academy is the only soft rock tunnelling academy and is one of the key parts of the legacy of the project to train the British workforce and there is nothing preventing people from further afield using it as well."

The legacy of Crossrail will be felt long after construction has concluded and trains start carrying passengers. Indeed the Thames Tideway, Northern Line extension, Crossrail 2 and High-Speed 2 are all set to benefit from the project's approach to sustainability.

While construction on HS2, the London - Midlands - North high-speed line is not set to start until 2019, the lessons learned during Crossrail could prove essential to selling the economic, environmental and social benefits of an expensive and controversial project to a sceptical public. A generation of British engineers trained during Crossrail are then set to shape the future of railway infrastructure projects at home and abroad.