Crossrail: Thames Tunnel

Crossrail: Thames Tunnel - TBM “Mary” breaks through at Woolwich station
Crossrail: Thames Tunnel | Image: © Crossrail Ltd.

CEEQUAL Excellent (89.1%) – Whole Project Award
Version 4, July 2015

London, England, UK

Winner – CEEQUAL Outstanding Achievement Awards 2016 – Water Resources

Project team

Client: Crossrail Ltd;
Designers: Arup / Capita Symonds;
Constructor: HOCHTIEF Solutions AG / J. Murphy & Sons Ltd JV (HMJV);

Assessors: Stephen Marr (J. Murphy & Sons) and Mike de Silva (Crossrail)
Verifier: Karl Pitman (Pitman Associates)

Summary

The £14.8bn Crossrail project has created a total of 42km of new twin bore tunnels, which have been constructed using 1000 tonne boring machines. One of these was Thames Tunnel C310, a project delivered by a joint venture between Hochtief and J. Murphy & Sons Limited, known as HMJV, which saw a 2.6km drive from Plumstead in south east London emerging in north Woolwich – the only Crossrail tunnel to run under the Thames. Two portals (400m long, 20m deep ramp structures) were constructed at either end of the tunnel to bring trains up to ground level.

The £260m tunnel project was highly complex with a distinct set of challenges such as a large head of water being present above the tunnel’s deepest points and tunnelling through chalk ground. This was unlike the London clay of the other Crossrail tunnels, meaning a mix shield tunnel boring machine (Slurry TBM) had to be used. The portals were constructed within extreme space constraints adjacent to live roads, listed buildings, a live railway and residential properties.

The team achieved a tunnelling rate of up to 130m per week utilising two mixed shield Tunnel Boring Machines. HMJV constructed a total of 5,457m of 6.2m internal diameter tunnels in a depth of up to 15m beneath the Thames, excavating more than 700 000 tonnes of spoil and building 3411 precast concrete tunnel rings.

Extensive dewatering was required for the construction of the tunnel portal as the tunnelling work was wholly within the chalk layer. At the same time tunnelling support processes were projected to use approximately 300 000 litres of potable water per-day. As it was not possible to recharge all the water being abstracted back in to the ground, the site team implemented a scheme to use the surplus groundwater from the dewatering. This resulted in saving over 2 million litres of potable water per week during tunnelling.

The project’s high levels of sustainable performance were also recognised when it became the first Crossrail Contract to win the Commendation Level, Green Line Award as part of Crossrail’s vision for a sustainable railway. The HMJV team received the award for excellent engagement with the workforce to drive sustainability forward. To celebrate this success Environmental Manager, Cathy Myatt, and Technical Director, Chris Sexton awarded the C310 team their commendation during a special presentation at their North Woolwich site. Cathy said: “As well as improving performance on site it’s also about improving everyone’s awareness about environmental behaviour. This is exactly what Green Line is all about.” Key innovations such as water savings, ultra-efficient and hybrid plant, diesel particulate filters retrofitted on all machines, as well as design changes led to an overall reduction in carbon emissions of 11% across the life of the project. Speaking at the awards ceremony, Chris said: “If everyone else follows your example we will end up in a very good place on environmental and sustainability areas on Crossrail.”

Gallery

This slideshow requires JavaScript.

Material Use

With a contract value of approximately £255 million this major project required significant quantities of concrete (>250 000 tonnes), steel (>7 700 tonnes), and granular fill (>82 500 tonnes). By using high GGBS and PFA replacement concrete, 100% recycled steel, and recycled granular fill the project achieved an overall recycled content in the materials used of 35%.

From the initial stages through to completion of this four year project, the C310 construction team actively sought solutions to reduce the environmental impact of the materials used. During the pre-construction optimised contractor involvement (OCI) stage innovative design changes were suggested by HMJV for client and designer approval. All of the following examples resulted in significant savings of resources, waste, embodied carbon, cost and time.

Reusable TBM Launch Rig

HMJV employed an unusual technique for launching the two Tunnel Boring Machines (TBMs). The traditional method is to cast concrete blocks and embed large steel frames that cannot easily be re-used, to act as launch blocks for the TBM to push off from at the start of tunnelling. Once it has built several rings it is within the tunnel and can push off the last ring built. At this point the launch blocks and first “sacrificial rings” outside the tunnel become waste.

Instead of this approach, the C310 team installed a re-usable steel frame around the eye of the tunnel for the TBM to “pull” itself forward during launch. This frame was then dismantled and moved to facilitate the launch of the second TBM. This took place at Plumstead and Woolwich Box and meant that the frame was used four times in total. This approach removed the need for the large concrete blocks (200-240 tonnes) and steel frames (300-320 tonnes) that could not be re-used. It also reduced the noise associated with breaking them out, and the lorry movements associated with pouring the concrete and then disposing of the broken out material. It also removed the need for a total of 20 sacrificial tunnel rings (500 tonnes of concrete and steel fibres) during the four launches.

Slurry Wall Panels

The original design accounted for a jet grout block behind each headwall for launching and receiving the TBMs at Plumstead Portal, Woolwich Box, and North Woolwich Portal (four headwalls in total). To achieve the required ground stability and control groundwater pressure behind each headwall, a total of approximately 8000m3 of grout was initially proposed to be injected into the ground. Due to the nature of jet grouting an equal volume of waste grout/soil slurry would have to be disposed of, in addition to the energy and water required to mix the grout. This would have been removed from site by truck to appropriate waste sites. C310 replaced the grout blocks with a 1 metre thick slurry wall panel constructed by mixing inert clay bentonite with the existing soil using a diaphragm wall grab. This achieved the necessary ground stability and water tight seal for the launch and reception of the TBMs at all four headwalls and removed the requirement for 8000m3 of jet grouting and the associated 8000m3 of grout soil waste slurry.

Lengthened Universal Tunnel Ring

HMJV proposed an innovative change in tunnel ring design. The traditional left and right rings have to be cast differently and installed in the correct sequence to allow for changes in tunnel direction. HMJV used a universal ring with a tapered wedge shape, which allows for changes in direction by rotating the ring. This meant only one mould was required in the casting process and there were never any delays or waste caused by the wrong ring being sent to the tunnel face – a common occurrence with the left/right ring types. In addition to the universal ring design, HMJV increased the length of the ring by 100mm. This meant the same tunnel length was constructed with 7% less rings. Due to the size and weight of the tunnel rings only one could be transported to site per lorry. With 7% (>250) less rings required, HMJV took over 250 lorry movements off the roads.

Relations with the Local Community

The four year project was constructed in close proximity to many residential properties in a densely populated area of south east London. The team was initially met with local opposition to the project and this made community relations a significant aspect of the project’s success.

The heavy civil works required to construct the portals included secant piling, diaphragm walling, bulk excavation and the in-situ pouring of many thousands of metres cubed of concrete. This work was also coupled with 24 hour tunnelling – with tunnel segments going into the tunnel and excavated material being brought out. The majority of the disruptive activity was at either end of the tunnel where the portals were constructed and the tunnelling operations were supported from.

Community Relations

As soon as HMJV arrived on site, open and transparent communication with local residents was a key priority. Open day site visits were well attended by the public and were a useful tool for explaining works methodology, space constraints, plus the need for road closures, night works, and other disruptive operations. HMJV did not “hide behind the hoarding” but actively generated interest in the project and provided job opportunities for the local area by employing direct labour.

Various investment schemes were implemented over the course of the four year project with tens of thousands of pounds donated to local worthy causes. Each initiative was assessed against criteria such as benefit to whole community, sustainability, and value for money. Volunteer hours were provided by the HMJV workforce as well as senior management. Volunteers spent many hours painting, planting and constructing with donated materials that were either surplus from the project or procured for donation.

As a result, HMJV won round much of the opposition to the project and the relationship that developed between the team and the local community was complimented.

Stakeholder Relations

HMJV also developed close working relationships with third party stakeholders and local authorities. These close relationships positively influenced the progress of the project. From consenting and permitting issues to agreeing working methodology and traffic management designs, an open and honest dialogue from planning stages and regular progress meetings kept the project on programme and stakeholders on side. Excellent relationships were developed over the course of the project between HMJV and the Royal Borough of Greenwich, London Borough of Newham, Thames Water, Network Rail and the Environment Agency.

One specific example of the close working relationship is illustrated by the collaboration achieved with Network Rail.

Working in close proximity to a live railway

A derogation was issued to HMJV for constructing the Plumstead portal (an approach structure at the southern end of the Thames Tunnel). Heavy plant including crawler cranes, piling rigs, and diaphragm wall rigs was required to construct the 300m long portal walls adjacent to the railway. Network Rail standards stated that no such equipment was to work facing the line or at an angle such that the jib, load or other part would foul the line should the machine fail or overturn. Due to the location of a large Grade II listed building to the south, the portal could not be moved further from the railway. Mitigations that reduced the risks had to be devised and adopted. Several key changes were agreed in order for Network Rail to sign off this unique deviation from their safety standards and keep the railway running throughout the construction period. These included:

  • the use of heavier machines, down-rated to the required capacity to reduce the risk of failure,
  • the construction of substantial reinforced concrete platforms for the machines to work on, reducing the risk of overturning,
  • the provision of attendant rail safety staff to monitor the works and carry out emergency procedures in the event of an incident,
  • the provision of detailed emergency procedures and communications systems in case of an incident including sensors along the site perimeter that would detect an obstruction and trigger audible and visual alarms to alert the rail safety attendants.

The result was very successful. The works were completed in eight months without the need for disruptive railway possessions and this resulted in significant time and cost savings.

This is considered to be the benchmark for piling and heavy lifting operations adjacent to live railway and resulted from collaboration and close working with a key stakeholder.

Water Resources

The use of groundwater for site activities such as dust control and road sweeping is common best-practice in UK construction sites. However, the use of groundwater rather than potable water for large scale processes such as grout and bentonite mixing is a new, innovative practice. The idea to use groundwater for C310’s Tunnel Boring Machines (TBMs) was an outcome of HMJV ‘thinking outside the box’ and looking into ways to change environmental performance by reducing reliance on a large amount of potable water.

The use of groundwater was proposed for two important tunnel processes:

  1. Mixing of grout to be continuously injected through the tunnel rings during TBM advance.
  2. Mixing with bentonite powder in a Slurry Treatment Plant (STP) to be pumped under pressure to the cutting face of the TBM to help stabilise the tunnel face.

Historically, groundwater has never been used for such highly technical and programme critical processes. Therefore, it was essential that all interested parties were involved from the earliest stage. Initial concerns that the untreated groundwater could affect the chalk or machines were addressed by the environment and technical team working together to carry out water quality and machinery tests to ensure there would be no effect on the tunnelling process or programme. HMJV collaborated with dewatering specialists who were recharging and abstracting the groundwater at Plumstead Portal. It was essential that enough surplus water was consistently available to supply the TBM processes without adversely affecting the groundwater levels beneath the nearby railway line and houses. Too much abstraction could have led to settlement issues and too much recharge can lead to flooding. Adding a third variable into the process needed very careful management.

The Environment Agency was consulted from initial planning stages to obtain consent to use the groundwater, which is protected in England and Wales. A detailed environmental risk assessment and report was compiled to assess the potential impacts on groundwater resources. Groundwater levels and quality was continuously monitored throughout the process to ensure there was no adverse effects and the Environment Agency was frequently updated with the groundwater monitoring results.

By replacing potable water with surplus groundwater the HMJV saved around 68 million litres of potable water, reducing pressure on potable water supplies in London and also reducing the carbon emissions associated with the project. By re-using surplus groundwater the project saved approximately 23 tCO2e emissions associated with the energy required to treat the equivalent volume of drinking water and pump it to the point of use. In addition to the water and carbon reductions, this initiative was economically viable saving the project approximately £100,000 in water supply costs.

The success and environmental/cost benefits from this operation has been an incentive to share this information with other construction projects across the country. This operation has now proven to work for mix shield tunnelling. Therefore, any tunnelling project that uses a Mix Shield TBM could save large amounts of potable water and costs. However, this process is not restricted to Mix Shield TBM use, the reuse of groundwater could potentially be used on any site that is left with surplus groundwater from abstraction.  The success of this process has instilled confidence in the Environment Agency that large scale reuse of groundwater can be safely carried out as long as the correct procedures are in place.

HMJV have proven that this process is not only successful but also achievable. This idea has been shared on Innovate18 for other Crossrail projects to adapt according to their needs and passed on through best practice sharing by the joint venture’s respective head offices. This is a perfect example of combining technical design and engineering to drive sustainability in construction whilst increasing environmental performance to a world class level.