Significant investment in rail, water and energy infrastructure will drive a strong pipeline in tunnelling for the next decade across the UK. The design and engineering of tunnels can mitigate their impact and optimise construction

Fit-out taking place in Crossrail Thames Tunnel

Fit-out taking place in Crossrail Thames Tunnel

01 / Introduction

The UK has a proud tunnelling heritage, including the world’s first subaqueous tunnel, built under the river Thames in 1843. The good tunnelling conditions north of the River Thames (London Clay) have allowed many mass transit tunnels to be built since 1863. However, the more sandy and gravelly ground to the east and south of London has traditionally been more difficult to work in, explaining the wider use of surface rail elsewhere.

The design and construction of tunnels and underground infrastructure will continue to be key to sustainable development, balancing the demands of a growing urbanised population and the desire to preserve the environment. A tunnel is a one-off solution, relying on a variety of tunnelling methods optimised against the constraints imposed by the ground conditions and tunnel’s function, as well as challenges surrounding logistics and land acquisition.

Putting effective contract and procurement strategies in place is essential to effectively manage risks associated with tunnelling, and the long lead in time for the machinery involved.

Infrastructure projects will turn to tunnelling when it presents the most cost-effective way of achieving desired outcomes. However, tunnelling projects impose significant impacts on the local environment, particularly in busy metropolitan areas, which must be mitigated through pre-planning and innovation, making tunnelling safer, and reducing the impacts.

Tunnelling is widely used across a multitude of sectors, including transport, water, and so on. The UK tunnelling industry requires a consistent, predictable and long-term pipeline of work to maintain capacity and investment and with a large number of potential projects, including HS2 and Crossrail 2, prospects are currently very bright.

02 / When is a tunnel the right solution?

Tunnelling is only used when it is the best way of achieving the desired socio-economic benefits from an infrastructure project. A wide range of factors determine the viability of a tunnel, including engineering feasibility, environment and economic impacts, public consultation and land acquisition.

For example, the Northern Line Extension (NLE) delivers benefits of increased mass transit access to the Nine Elms and Battersea regeneration area generating a cost-benefit ratio of nearly 10:1.

There is a rural-urban divide in the drivers behind major tunnelling projects. In urban spaces, tunnelling will often deliver immediate benefit to the local population, be it improved transport or increased power capacity.

By contrast, in rural environments, tunnels are mostly used to minimise the impact of infrastructure that will have a direct benefit elsewhere, such as the National Grid North West Coast Connection Tunnel, across Morecambe Bay, used to re-route transmission cables.

In summary, while tunnels in cities increase benefit through increased accessibility, rural tunnels mitigate the wider impacts of national investment.

Rural and urban spaces therefore have strong, but different drivers for the choice of tunnelling: in rural environments there is plenty of space for necessary infrastructure, but limitations surrounding its environmental impacts, whereas in urban environments, space limitations are often a significant challenge affecting both design and construction. In both cases, a balanced solution delivering social, environmental and economic benefits must be achieved.

Other drivers of the need for a tunnel as opposed to above ground infrastructure include:

  • Keeping infrastructure away from the population (high voltage power, for example)
  • Environmental preservation
  • Topographic reasons (tunnelling below mountainous regions such as the Pennines, or to cross bodies of water – for example, Channel Tunnel).

Due to high initial costs and the extended life of tunnelled infrastructure, benefits do need to be sustained over an extended period of time to pass the viability hurdle.

If demand for the asset is neither predictable nor sustainable over time, then clearly investment will be much more difficult to justify.

03 / Design and engineering

Tunnels are one-off projects – using well established solutions adapted to the geology, the route taken, tunnel function, groundwater conditions, and many other factors.

There are many factors that need to be considered when designing and constructing a tunnel including:

  • The alignment and clearances with other underground structures
  • The geology and its impact on route
  • The construction method
  • Drainage and lining requirements
  • Functional elements of the tunnel, such as safety provisions, service buildings, ventilation, lighting, and so on
  • Operation and maintenance procedures
  • Durability, especially for sewage tunnels or immersed tunnels

Disciplines involved in the design of a tunnel are wide ranging and include geotechnical, examining ground conditions and predicting ground movement, environmental sciences to assess and mitigate wider impacts of tunnelling, and soil and rock mechanics as well as specialist skills in aeraulics, hydraulics and safety.

Tunnelling techniqueWhat is it?When is it used?
Conventional tunnellingConstruction underground of any shape built by cyclic process of excavation with drill and blast or mechanical excavation and placement of lining (e.g. sprayed concrete lining).Smaller tunnel runs and irregular underground spaces including interconnecting tunnels, station platforms. Extended tunnel construction in hard rock
Bored tunnels using Tunnel Boring Machines (TBMs)Different types of machine to suit different geology and ground water conditions – Hard Rock: Main Beam, Shield, etc. Soft Ground: Slurry Shield, Earth Pressure Balance etc.Extended continuous tunnel running through reasonably consistent geology.
Length of tunnel runs may be determined by geological conditions.
Surface tunnelling (cut and cover)Built in a temporary or permanent open trench in surface, then usually covered. eg. Metropolitan lineShallow tunnels
Sub-aquatic tunnellingImmersed tunnel – constructed in a trench using prefabricated sections, then backfilled to provide protectionWhen crossing a narrow body of water or when combined with a bridge

Route design optimisation
Tunnel design requires significant pay-offs between the best engineering solution and other considerations. Route optimisation is concerned with the design of a solution which represents the best balance of utility, cost and programme, risk and wider impacts, which include:

  • Geology which determines the depth and the route of the tunnel, passing through, as far as is practicable, the safest and easiest tunnelling geology. An example could be changing the layout of a station to avoid a geological fault.
  • Hydrological conditions Groundwater pressure is a significant determinant of the selection of the tunnelling method, and the structural design of the tunnel including the support required once the tunnel is constructed.
  • Function which determines the size and configuration of tunnel bores, the route and depth and the complexity of tunnel interfaces. For example the Thames Tideway Tunnel is tunnelled to a fall which means that the east end is 30m deeper than the West, as well as having a wider bore to accommodate larger water volumes. Other aspects of function affecting design include rail alignments and even the performance requirements of linings and so on.
  • Land take Minimising the extent of above ground development during and after construction which may require compulsory purchase and demolition – increasing cost and the wider impact of the development.
  • Space limitations Where existing underground infrastructure, and allowable clearances will determine both the route and the depth of tunnels. The impact of existing underground structures is a significant factor in cities like London. Complex utility diversions could be required, access to favourable geological conditions blocked, and tunnels forced deeper underground. Potential obstacles include existing mass transit tunnels, utilities, building foundations, and so on. Some cities, such as Helsinki, are leading the way in planning the use of underground space through an underground master plan, facilitating route optimisation in an increasingly congested underground environment.
  • Space proofing Ensuring that all required functions can be accommodated within the planned tunnel volume. This is a particular challenge with station design, which involves reconciling the functional space needs of a large number of stakeholders and resolving many complex physical interfaces.

Construction optimisation
Construction optimisation is concerned with making best use of fixed-resources such as tunnel boring machines and conveyors as well as the scarce above-ground worksites available in crowded city locations – delivering the best combination of programme, cost and risk.

  • Geology determines the tunnelling method to be used, which in turn has a significant impact on cost and programme. Different ground conditions may require different tunnelling techniques which will determine the number and length of tunnelling drives.
  • Tunnelling method includes the impact of transitions between different geologies, which may be accommodated using a single type of machine using the shield method. Optimising the tunnelling process involves balancing the costs of the methods against their relative efficiency, given the constraints of the ground and groundwater conditions.
  • Logistics, such as fixed costs associated with conveyors and ventilating plant, together with variable costs associated with muck disposal and delivery of tunnel linings will all be influenced by the number of above-ground access sites, together with access to non-road transportation.

Innovation
Innovation in design and tunnelling techniques has had a key role in managing the risks and impacts associated with tunnelling, as well as increasing the safety of working conditions for operatives.

Key developments include:

  • Use of sensors for monitoring and instrumentation that improves the accuracy of ground movement prediction and monitoring, both during the tunnelling process, and once the tunnel is built. Remote monitoring has also decreased manpower required to monitor ground movements.
  • Improved analysis and design tools including advances in finite element modelling, providing a very detailed analysis of stress factors and how they interact and giving stakeholders additional assurance of safety.
  • Greater use of BIM, used for design development and coordination, planning of construction and logistical challenges and better understanding interfaces and clashes. Wider use of BIM is also increasing safety in tunnelling through better visualisation of the workface and workface access.

A worker with a tunnel boring machine

A worker with a tunnel boring machine

Risk mitigation and management in tunnelling
Tunnelling in congested urban environments has the potential to create massive damage and disruption as the result of an accident, as well as high-levels of nuisance to neighbouring uses both during construction and operation. Environmental impact assessments are naturally a key aspect of the permitting process – driving an early commitment to detailed design so that full impacts are understood and mitigations assured as part of the approvals.

High priority areas of mitigation include:

  • Health and safety risks are present in tunnelling environments which involve the use of complex fast-moving machinery in confined spaces. Health and safety culture is well developed so in order to push towards zero-harm clients are adopting behavioural change programmes to promote a health and safety culture from the bottom-up. The EPIC (Employee Project Induction Centre) safety training used by clients including Thames Tideway Tunnel (TTT) is a good example of this, using scenarios to increase worker awareness of the impacts of poor safety practice.
  • Noise and vibration associated with construction works, ventilation, conveyors road-traffic and so on through to the location of worksites and limitations on working hours.
  • Ground movement resulting from tunnelling and associated settlement, mitigated by detailed modelling and design, precision tunnelling and intensive monitoring.
  • Loss of visual amenity requiring sensitive design of portals and other elements of above ground infrastructure.
  • Impacts on existing occupiers either restricting access or use of sites or requiring compulsory purchase and relocation.
  • Effects on public services and utilities including the timing and impact of services diversions and a wider impact on the operation and resilience of networks, including blockades and closure.
  • Effects on historic and archaeological assets.
  • Long term changes to geology, hydrology and water resources.

04 / Economics and procurement of tunnelling

Cost drivers for tunnelling
Key cost drivers for tunnelling works are related to geology, design and method:

  • Geology and ground conditions will partially determine the tunnelling method and will introduce risks and constraints associated with the construction method. Obstructions and other aspects of ground conditions will also impact on the detailed design of the tunnel solution.
  • Key design factors include tunnel diameter, depth and constraints on geometry such as a maximum radius or gradient. It may not be practical to optimise the tunnel alignment due to obstructions and interfaces with other elements of the tunnel network. Other design related costs include those associated with work areas at the tunnel head.
  • Method is the area where there is the greatest opportunity to optimise outturn costs based on the best balance between programme and time-related costs. On larger projects, programmes can be accelerated by increasing the number of tunnel bores, or through the fairly standard practice of 24-hour working – both of which will change the balance of fixed costs associated with investment in plant and logistics. Other variables include disposal and transport costs as well as the residual value of tunnelling plant.

Table 2 sets out indicative tunnelling costs for bored tunnels using either a Slurry TBM or an Earth Pressure Balance (EPB) TBM sourced from the HS2 programme.

Principal cost elementDescriptionCost
Tunnel boring machine (TBM)Purchase cost only – operating cost elsewhereSlurry TBM £16m
Earth Pressure Balance TBM £18m
Tunnelling supportFixed cost: For instance, slurry treatment plant, gantry cranes
Time related cost: TBM operation
Fixed: Slurry TBM £45m each
EPB £35m each
Time-related: £1.1m per week
Tunnel constructionComprises excavation, waste conveyance, cost of linings, linings transport and installationSlurry TBM £25,000 per route metre
Earth Pressure Balance TBM £22,000 per route metre
Disposal of excavated materialsCosts for sustainable re-use or disposal. Assumes no contamination.Commercial tip disposal: £4,500 per route metre
Sustainable placement: £3,000 per route metre
Tunnel portalsApproximately 30m-wide structures to bored tunnel headwalls£20m-£65m each depending on topography
Tunnel shaftsFor ventilation and emergency intervention access, as well as operation and maintenance£10m-£30m each
Mechanical and electrical systems in tunnelsSystems for tunnel operation only – no fit-out related to tunnel function.£4,000 per route metre

Logistics
Logistics has a critical impact on programme and the impact of the project on its neighbours. The most significant aspects of logistics involve getting machinery into position and supplying materials including linings and removal of spoil. In some instances, tunnel boring machines will need to be assembled in access shafts – introducing further constraints into where tunnel drives can be run from. There are strategies for avoiding road congestion, with Crossrail using rail deliveries and the Lee Tunnel using the river to save about 80,000 lorry journeys. Muck disposal is also a consideration. Crossrail has created a new nature reserve in the Thames Estuary with their excavation waste.

Significant challenges are likely to be encountered when tunnelling in city centres, where the above-ground footprint may be very restricted. For example, the recent Bond Street Station expansion has access through two 9m diameter shafts to the tunnelling site 20m below. Sites for Crossrail and Thames Tideway are also constrained by available sites, which inevitably places a limit on the rate at which tunnelling works can be undertaken – extending the programme and increasing overall project costs.

Contract strategy
Contract strategy is an important consideration, due not only to the inherent risk involved in tunnelling but also the extensive up-front costs associated with design, plant purchase and logistics. Contracts in tunnelling tend to be design and build, based on a reference design provided by the client team, which is adopted by the contractor. The reference design will typically be about 30-40% complete but will be the basis for most of the approvals and licences. NEC contracts are often used as they allow for a clear allocation of risk between contractor and client. It is standard practice to include a geotechnical baseline report defining the ground conditions accepted by the contractor. Deviations from expected conditions may form the basis of a compensation event.

In general, the NEC Option C variant is used, enabling ready valuation of the contractor’s expenditure and the basis for a pain/gain incentivisation. With the target cost approach, effective project controls including earned value analysis are essential to provide assurance that delivery performance is on target.

Procurement
Tunnelling projects typically have long lead in times. Even after procurement, it typically takes nine months to manufacture and a further three-plus months for site assembly and set up of a tunnel boring machine. Given the need for specialist skills, it is essential to understand capacity in the market and how to structure the works contracts and communicate any preference for joint ventures.

This can be done by engaging the supply chain at an early stage.

Early market engagement, will typically involve a review of the packaging strategy as well as the envisaged contract strategy and risk allocation. Early engagement also enables contractors to form joint ventures with suitable experience. The packaging strategy may be structured to take account of capacity as well as the impacts of geology basis. For example on TTT, there are three main tunnelling contracts of a reasonable size, each awarded to a different joint venture. By splitting the works contracts up, TTT secured better competition, reduced levels of risk per contract and also accessed specialist skills for different ground conditions. Early engagement and a considered contract strategy are essential to securing the right mix of capacity, on the appropriate balance of risk and reward.

Though tunnelling skills tend to be a global resource pool there are still some shortages of designers and civil structural engineers, meaning that contractors often have to pay a premium for these skills.

To mitigate this, the UK has established Europe’s only dedicated tunnelling and underground construction academy, with capacity to train up about 200 tunnelling staff at a time. Long term investment is set to continue, with the UK government contributing £1.1m of funding, to match £1.7m of industry investment in creating a legacy of engineering jobs and skills.

Tunnel boring machine at Lee Tunnel

Tunnel boring machine at Lee Tunnel

05 / Lee Tunnel Case Study

The Lee Tunnel is the UK water industry’s largest project since privatisation in 1989 and currently the deepest tunnel built in London, delivered at a total cost of £635m. The project will stop the pollution of the nearby river Lee, which currently receives an overflow of sewage and storm water when the capacity of London’s 19th century sewage system is exceeded during storms. It is the first of two tunnels to be built as a part of Thames Water’s London Tideway Improvement system.

Every year some 39m tonnes of sewage mixed with rainwater flow through CSOs (combined sewer overflows) into the Thames; the Thames Tideway Improvement programme is designed to minimise these overflows as well as improving sewage works. The first stage was an upgrade to all sewage treatment works, the second is the construction of the Lee Tunnel, and finally the third is the TTT, which will run 25km from Acton to Abbey Mills where it will connect to the Lee Tunnel.

The Lee Tunnel is designed to convey and store sewage and storm water from Abbey Mills pumping station Combined Sewage Outfall in East London to the Beckton sewage treatment works. It needs to be deep enough to meet the Thames Tideway Tunnel at its lowest point, 75m below ground level. The project constructed by the MVB JV has posed unprecedented challenges in dealing with high groundwater pressures in the deepest sections, where the groundwater pressure is up to 8 bar, eight times the air pressure found at sea-level.

One of the greatest innovations on the project was the construction of five shafts, the largest ever sunk in the capital, with the UK’s deepest diaphragm walls. These shafts are a world first in the building of inner linings to a diaphragm wall as huge, stand alone, concrete chimneys, enabling the shafts to withstand the huge water pressure acting upon them. The largest shaft is 38m internal diameter with walls 98m deep. Lessons taken from the Lee Tunnel will be adopted by the larger TTT project. Further innovations minimised the steel content and carbon footprint of the tunnel, as well as significantly improving its durability over its full lifetime.

TTT tunnelling starts in 2017, with Thames Tideway project completion in 2023, and the project will make good use of the benchmarking and innovative techniques sourced from the Lee Tunnel.

06 / Conclusion

The UK’s strong pipeline and continued demand for tunnels will support capacity in the industry for the next decade at least. When tunnelling is the most cost effective way to achieve desired socio-economic outcomes, projects will move forward while seeking to minimise the costs and significant impacts on the local environment. This can be achieved in optimising the route, and construction, based on ground and groundwater conditions, and the function of the tunnel, as well as looking to use innovative methods to increase safety. Early engagement of the supply chain and transparency surrounding procurement and contracts are essential to ensuring capacity adapts to the future pipeline and UK tunnelling continues from strength to strength.


Acknowledgements
We would like to thank Richard Stoodley, Martyn Court, Andrew Merry, Carlos De Freitas, Ian Hughes, Mark Shaw, Doug Clayton, Wyn Roberts and Chris Pike of Arcadis for their contribution to this piece.