After 16 years of planning and £1.2 billion already spent on development, the UK government last month approved the £9 billion Lower Thames Crossing project. Transport Secretary Heidi Alexander granted the development consent order on March 25, 2025, paving the way for constructing the UK’s largest road tunnel.
The Lower Thames Crossing will consist of a 14.5-mile (23km) road linking the A2 and M2 in Kent with the A13 and M25 in Thurrock, Essex. The project’s centrepiece is a twin-bore tunnel under the River Thames between Gravesend in Kent and Tilbury in Essex.
At 2.6 miles (4.2km) long, these tunnels will become the longest road tunnels in the UK. Each tunnel will carry three lanes of traffic in one direction with a 70mph speed limit. Approximately 80% of the route will run through tunnels, cuttings, or embankments to minimize visual impact on the surrounding landscape.
The tunnels will be 16.4 meters (54 feet) in diameter, making them the longest among the country’s most extensive road tunnels. The significant diameter is required to accommodate three full traffic lanes in each direction while maintaining safety standards for modern highway design.
National Highways, which will oversee the project, plans to begin construction in 2026, with completion targeted for 2032. The project has been divided into three main construction packages:
- Roads North of the Thames: Balfour Beatty secured this £1.2 billion contract in January 2023 to construct 10 miles (16km) of highways connecting the M25 at Junction 29 and the A13 with the northern tunnel entrance.
- Kent Roads: In July 2023, Skanska was awarded this £450 million contract to build 3.7 miles (6.0km) of roads connecting the A2/M2 to the southern tunnel entrance.
- Tunnels and Approaches: In December 2023, a joint venture between Bouygues and J. Murphy & Sons won this £1.3 billion contract. To save engineering costs, the consortium plans to use a single tunnel boring machine for both tunnel bores, turning it around and reusing it after completing the first tunnel.
According to the Department for Transport, the construction phase is expected to support up to 22,000 jobs across the supply chain.
The Dartford Crossing, which has been the only road crossing of the Thames east of London for more than 60 years, currently handles approximately 160,000 vehicles daily—far exceeding its designed capacity of 135,000. This congestion creates significant delays, particularly during peak hours, and generates substantial air pollution that impacts the neighbouring communities of Thurrock and Dartford.
National Highways projects that the new crossing will reduce traffic at the Dartford Crossing by approximately 20%. Matt Palmer, National Highways Executive Director for the Lower Thames Crossing, stated that the project will “unlock growth with quicker, safer, and more reliable journeys.”
Traffic modelling indicates that while the number of vehicles crossing the Thames will increase with the new infrastructure, the distribution across two crossings will create a more efficient traffic flow. This would transform the daily commute for thousands of motorists and improve freight transport efficiency between the UK’s busy Channel ports and the rest of the country.
Engineering Challenges
Building tunnels of this scale under the Thames presents several significant engineering challenges:
The project requires extensive ground investigation work to understand the geological conditions beneath the river. Site teams must account for varying soil types, potential archaeological findings, and groundwater issues.
The tunnel boring operations must be precisely controlled to prevent settlement or disturbance at the surface. This is particularly important where the tunnel passes under environmentally sensitive areas.
The substantial diameter of the tunnels necessitates specialized tunnel-boring machines. The Bouygues-Murphy joint venture’s approach to use a single boring machine for both tunnels introduces added complexity to the logistics and timing of construction but could result in significant cost savings.
Connecting the tunnels to the surface roads requires carefully engineering the portal structures and approach roads to manage the transition from underground to surface traffic flow.
National Highways has described the Lower Thames Crossing as “the most environmentally sustainable road project ever delivered in the UK.” The engineering approach includes several sustainability measures:
The project aims to reduce its estimated construction carbon footprint by around 50%, with a target of a 70% reduction. This will be accomplished by using alternative fuels, such as hydrogen, to power the construction fleet and specifying low-carbon concrete and steel in the design.
The project will deliver six times more green space than the road, with plans to plant over one million additional trees, create a new community woodland, and establish two new public parks overlooking the Thames.
Seven green bridges blend the road into the landscape, providing safer crossing points for wildlife and pedestrians.
For every mile of new road, almost three miles of new or improved routes for walkers, cyclists, or horse riders will be created.
The government is exploring private finance options for the £9 billion project. Chancellor Rachel Reeves indicated in January that the private sector could help “to deliver the infrastructure that our country desperately needs.”
While the specific funding model has not yet been finalized, it is understood that utility works would be publicly funded alongside significant private-sector finance. Road tolls, similar to the existing Dartford Crossing, are expected to be implemented to recoup some of the costs.
The Long Road to Approval: A Historical and Technical Deep Dive
Origins and Evolution of the Crossing Concept (1989-2009)
The concept of an additional Thames crossing downstream from Dartford was first proposed in the “Roads for Prosperity” white paper back in 1989. The document identified the need for relief on the eastern side of the M25 between Kent and Essex. However, it wasn’t until the 2002 ORBIT Multi-Model Study, which examined orbital transport problems around London, that the Lower Thames Crossing (LTC) was formally recommended for further investigation.
In 2008, an ambitious alternative emerged when Metrotidal Ltd. proposed the “Medway-Canvey Island crossing” – a £2-4 billion combined road and rail tunnel including a surge-tide barrier and tidal power plant. This proposal gained initial support from Kent and Essex County Councils, the Thames Gateway South Essex Partnership, and the Department for Transport, representing an early attempt at integrating infrastructure with renewable energy generation.
The Department for Transport published a comprehensive study in January 2009 examining potential solutions to Thames crossing capacity issues. This study evaluated five potential corridors:
- Option A: Additional capacity at the existing Dartford Crossing
- Option B: Swanscombe Peninsula linking the A2 to the A1089
- Option C: East of Gravesend, linking to the M20
- Option D: M2 link to Canvey Island
- Option E: Isle of Grain linking to the east of Southend-on-Sea
Interestingly, the study noted a lack of demand for additional passenger rail or rail freight capacity across the river, citing then-recent projects like High Speed 1 and Crossrail as providing sufficient cross-river capacity between London and Kent.
Route Selection and Refinement (2010-2020)
In October 2010, Kent County Council commissioned a study proposing that the northern end of the crossing should bypass the M25 entirely and continue directly to the M11 and Stansted Airport – a significant expansion of the original concept that would have created an actual outer orbital route for London.
The route selection process became more complex in October 2012 when plans were announced for the London Resort theme park near Swanscombe. This development eliminated Option B, which would have utilized the Swanscombe Peninsula.
After years of deliberation, in April 2017, Transport Secretary Chris Grayling confirmed Option C as the preferred route. This decision set in motion detailed design work, though the project’s scope continued to evolve. By November 2017, National Highways (then Highways England) announced several design modifications:
- Rerouting to avoid a landfill site near Ockendon
- Redesigning the junction with the A13
- Removing planned junctions with the A128 and A226
- Widening the A2 from its junction with the new crossing approach to Junction 1 of the M2
These modifications demonstrate the iterative engineering approach necessary for a project of this scale, with each refinement addressing specific technical, environmental, or community concerns.
The project faced a significant setback in November 2020 when National Highways withdrew its Development Consent Order (DCO) application after the Planning Inspectorate requested additional information regarding environmental impact and construction plans. This highlighted the increasing importance of detailed environmental assessments in significant infrastructure projects. A revised application was submitted in November 2022, beginning an 18-month planning process further extended due to the July 2024 general election.
Engineering Technology and Innovation
Tunnel Boring Technology
The twin-bore tunnels planned for the Lower Thames Crossing will utilize state-of-the-art tunnel boring machines (TBMs). These massive pieces of equipment, often custom-built for specific projects, can cost upwards of £10 million each. The Bouygues-Murphy joint venture’s decision to use a single TBM for both tunnel bores represents an innovative approach to cost management. This method requires:
- Completing the first tunnel bore
- Extracting the TBM at the endpoint
- Disassembling, transporting, and reassembling the machine at the starting point
- Reconfiguring the cutting head, if necessary, based on geological findings from the first bore
- Beginning excavation of the second tunnel
This approach saves substantial capital costs but extends the timeline for tunnel completion. The technical challenge lies in the precise extraction and reinstallation of the TBM without damage.
The TBM will need to handle the specific geological conditions beneath the Thames, which include:
- Chalk formations on the southern side
- Alluvial deposits in the river basin
- London Clay and Thanet Sand formations
- Potential pockets of groundwater and methane from historical industrial uses
Each tunnel will be 16.4 meters (54 feet) in diameter – large enough to accommodate three full traffic lanes, emergency access routes, and ventilation systems. The TBM must excavate approximately 42 cubic meters of material for every meter of tunnel advanced.
Low-Carbon Construction Techniques
The project’s ambitious target of a 70% reduction in carbon emissions during construction requires implementing multiple innovative engineering approaches:
Alternative Fuel Technology: The construction fleet will utilize hydrogen-powered vehicles where feasible. This requires establishing hydrogen refuelling infrastructure at key construction sites and potentially modifying standard construction equipment to accept alternative fuels.
Low-Carbon Concrete: Traditional concrete production is a significant source of carbon emissions. The project specifications call for:
- Portland Limestone Cement (PLC) with reduced clinker content
- Geopolymer concretes replace Portland cement with alkali-activated materials.
- Concrete mixes incorporating recycled aggregates and industrial byproducts like Ground Granulated Blast Furnace Slag (GGBS) and Pulverised Fuel Ash (PFA)
Engineers must ensure these alternative concrete formulations maintain the necessary structural properties for a major infrastructure project with a 120+ year design life.
Green Steel: Reducing emissions from steel production involves:
- Specifying steel with high recycled content
- Sourcing from electric arc furnace production rather than basic oxygen furnace methods
- Potentially utilizing emerging hydrogen-reduced iron technologies
- Optimizing structural designs to minimize total steel quantities
Digital Twin Technology: The project will employ comprehensive Building Information Modeling (BIM) and digital twin technology to:
- Optimize material usage through precise quantity takeoffs
- Reduce rework through clash detection
- Enable carbon calculation at the design stage
- Simulate traffic flows and environmental impacts
- Plan maintenance activities throughout the structure’s lifetime
Ground Engineering Challenges
The tunnel route presents several geotechnical engineering challenges:
Groundwater Management: The tunnels will pass below the water table and the Thames itself, requiring:
- Compressed air working environments within the TBM
- Sophisticated tunnel lining systems with multiple waterproofing layers
- Continuous pumping operations during construction
- Permanent drainage systems for the completed tunnels
Settlement Monitoring: Tunneling inevitably causes some ground movement that must be precisely monitored and controlled using:
- Automated Total Stations (ATS) to monitor surface movement
- In-ground instrumentation, including extensometers and piezometers
- Real-time data systems allowing immediate TBM parameter adjustments
- Compensation grouting in sensitive areas, if necessary
Contaminated Land: The industrial history of areas along the route necessitates careful materials handling:
- Comprehensive pre-construction ground investigations
- Materials classification systems for excavated soils
- Treatment facilities for contaminated materials
- Engineered barriers to prevent mobilization of existing contamination
Environmental Engineering
Beyond standard road construction, the project incorporates specialized environmental engineering elements:
Noise Reduction Technology: To minimize operational noise impacts, the design includes:
- Low-noise road surfaces using polymer-modified asphalt
- Acoustic barriers designed with computational fluid dynamics modelling
- Earth berms integrated into the landscape design
- Tunnel portal designs that minimize noise propagation
Air Quality Engineering: Given concerns about vehicle emissions, the tunnels incorporate:
- Longitudinal ventilation systems with jet fans
- Air treatment facilities at tunnel portals
- Monitoring stations integrated with traffic management systems
- Emergency smoke extraction capabilities
Water Management Systems: To protect the Thames and local watercourses:
- Sustainable Drainage Systems (SuDS) with multiple treatment trains
- Retention ponds designed as ecological habitats
- Oil interceptors and filters at discharge points
- Automated monitoring and shutdown systems for pollution incidents
Green Bridges: The seven planned green bridges represent advanced bioengineering, incorporating:
- Soil depth profiles sufficient for tree establishment
- Specialized lightweight growing media
- Integrated drainage and irrigation systems
- Wildlife guidance features funnel animals toward crossing points
Construction Methodology
The construction sequence for a project of this scale requires careful planning and coordination between multiple contractors:
- Enabling Works (2026-2027):
- Site clearance and establishment
- Archaeological investigations and mitigation
- Utility diversions
- Ecological translocation activities
- Civil Engineering (2027-2031):
- Construction of approach cuttings
- Establishment of tunnel portal structures
- TBM launch preparation
- Road earthworks and structures
- Tunneling Operations (2028-2030):
- TBM assembly and launch
- Continuous tunnelling operations (24/7)
- Tunnel lining installation
- Cross-passage construction
- Tunnel Fit-Out (2030-2031):
- Roadway construction within tunnels
- Mechanical and electrical systems installation
- Ventilation systems
- Safety and monitoring equipment
- Approach Roads (2029-2032):
- Pavement construction
- Bridge and structure completion
- Signage and safety barriers
- Integration with existing road network
- Commissioning (2032):
- Testing of all systems
- Emergency response drills
- Phased opening to traffic
- Monitoring and adjustment period
The entire construction sequence must be orchestrated to minimize disruption to existing transport networks and nearby communities while maintaining the planned construction timeline.
Technological Legacy
The Lower Thames Crossing isn’t just significant for its physical infrastructure and potential technological legacy. The project’s commitment to low-carbon construction is creating a new blueprint for sustainable infrastructure that is already influencing other sectors:
- The nuclear industry is adopting similar carbon reduction techniques for new power station construction
- Water utilities are implementing comparable approaches for reservoir and treatment plant projects
- Railway projects are examining the procurement and material specification methods
- Aviation infrastructure developments are adapting digital carbon calculation tools
This transfer of engineering knowledge and practices across sectors represents one of the most valuable long-term impacts of the project, potentially influencing British civil engineering approaches for decades to come.
The Lower Thames Crossing represents a significant advancement in UK infrastructure engineering. If the project progresses as planned, it will alleviate congestion at the critically overloaded Dartford Crossing and establish new standards for sustainable infrastructure development.
The project balances immediate transport needs with long-term environmental considerations through innovative engineering solutions. As construction begins, the technical approaches may influence future infrastructure projects across the UK and establish new benchmarks for sustainable construction practices.
The crossing stands as evidence of the UK’s commitment to investing in critical infrastructure, though the accurate measure of its success will depend on whether the completed project delivers the promised improvements in traffic flow, economic growth, and environmental sustainability.
TLDR Summary:
- UK government has approved the £9 billion Lower Thames Crossing project
- The project includes a 14.5-mile road with twin tunnels (2.6 miles long) under the Thames
- Construction is to begin in 2026, with expected completion in 2032
- Balfour Beatty, Skanska, and Bouygues-Murphy JV are the leading contractors
- Aims to reduce Dartford Crossing traffic by 20%
- Targeting 70% carbon reduction in construction through innovative engineering
- The financing model is still being determined, with private-sector involvement likely
