Breaking new ground on the first London tunnel in three decades

The Silvertown Tunnel, a new 1.4 km twin-bore tunnel under London’s River Thames, is a testament to modern engineering. Spearheaded by the Riverlinx CJV, a consortium of BAM Nuttall, Ferrovial Construction, and SK eco plant, this project represents a significant advancement in urban infrastructure. Among the high-calibre team of specialists assembled by Riverlinx CJV is Ayesa, appointed as one of the main underground structural designers. Ayesa’s responsibility spans the spectrum of temporary and permanent works, including cut and cover tunnels, TBM shafts, and other critical structures.

The Silvertown Tunnel project is not just about constructing a new tunnel; it transforms the 62-acre Silvertown site. It includes restoring the historic 1930s Millennium Mills, building a bridge to connect Silvertown Quays with Custom House Crossrail station, and establishing the Roundhouse East as a dynamic cultural event space. These developments are set to revitalise Silvertown, turning it into a thriving community hub within London. The tunnel will significantly reduce traffic pollution and congestion, improving connectivity across the city.

However, the project has faced several technical and logistical challenges, including:

• Complex Engineering Requirements: Designing and constructing the tunnel beneath the Thames riverbed while maintaining a consistent vertical gradient, launching and turning one of the largest Tunnel Boring Machines ever used in the UK.
• Environmental Considerations**: Adhering to the Tunnel Code of Construction Practice (CoCP), the project emphasises sustainable construction practices, including river transport for materials and low-emission machinery usage.
• Pandemic Challenges: Overcoming disruptions caused by the COVID-19 pandemic, such as site delays and the need for remote collaboration.

Ayesa played a key role as one of the main underground structural designers. In close collaboration with the Riverlinx CJV, Ayesa worked alongside Ferrovial’s Engineering Services department and BAM Wayss & Freytag on critical elements, including the design of the innovative “peanut shaft,” which was used to launch the largest TBM in the UK. This 1.4km twin-bore road tunnel under the Thames broke new ground in engineering. The CJV was instrumental in introducing several firsts on the project, including the award-winning “Peanut” launch chamber, a “rotation chamber,” and a “retrieval” chamber, all of which facilitated the operation of the UK’s largest Tunnel Boring Machine (TBM), nicknamed “Jill” after the first female bus driver in London.

Collaboration During the Pandemic: The COVID-19 pandemic brought significant challenges, but Ayesa successfully navigated these obstacles through effective collaboration with COWI, the geotechnical designer. Despite facing hurdles such as dispersed teams and site delays, this partnership led to optimised workflows, reduced review times, and cost savings for the client. Consequently, Ayesa and its partners were Shortlisted in the UK Ground Engineering Geotechnical Team of the Year 2022.

Geographic Coordination: Teams from Madrid, London, Vancouver, and Delhi collaborated to refine the design scope. Initially conducted in-person at the client’s UK office, meetings quickly transitioned to virtual platforms due to the pandemic. This shift to digital collaboration via virtual meetings and secure file-sharing facilitated continuous progress.
Workflow Efficiency: The collaboration streamlined workflows by leveraging team strengths and assigning expertise-based tasks. Junior team members focused on model development and coordination, while senior staff optimised solutions. The agreement on standardised procedures reduced review times, simplifying client validation and cutting costs.

Damage Assessments: Ayesa and COWI jointly conducted damage assessments, evaluating options based on complexity and risk. This included assessing the foundation movements of a DLR viaduct crossing the cut-and-cover work (handled by Ayesa) and the superstructure (managed by COWI). Their combined analysis accounted for potential ground movements from dewatering, excavation, and construction activities.

Technical Analysis: Throughout various project stages, models were thoroughly analysed, and interactions were assessed to refine solutions. Mitigation scenarios were developed to identify critical behaviours and ensure optimal performance and safety.

Eco-Friendly Practices: The project team, including Ayesa, prioritised eco-friendly practices by using low-emission machinery and transporting materials by river. Remarkably, 780,000 tonnes of spoil from the recently completed second tunnel were removed via barge. The early stages of the project faced challenges from remnants of old industrial structures and maintaining a 4% vertical gradient for the tunnel under the Thames riverbed.

Ayesa’s Comprehensive Design Contributions: Ayesa’s responsibilities encompass designing various components, such as temporary and permanent structures in cut-and-cover areas, TBM shafts, portal structures, and access ramps, as well as overseeing damage assessment, instrumentation, and monitoring. The project employs both TBM and cut-and-cover excavation methods.

In detail, Ayesa is responsible for designing critical elements like the temporary embedded walls, the Open Cuts and Cut and Covers to allow the excavation and construction of the Permanent Works, and the TBM operations. The walls are constructed using various techniques, such as sheet piles, secant piles or d-walls. The firm is also leading the Permanent Works, which links the Bored Tunnel Section to the existing road on both sides of the river, including the access ramp areas, Cut and Cover sections, and Portal structures. Ayesa has employed advanced techniques using 3D structural and geotechnical modelling for complex structures, like the shafts, and 2D models for linear structures, such as the Cut and Cover North and Open Cuts. Our comprehensive approach included designing temporary supports, construction stages, interaction between structures and dewatering plans, showcasing our commitment to delivering efficient, innovative solutions in complex, multi-stakeholder environments.

Innovations in Shaft Design: The Peanut, Rotation, and Retrieval Shafts: Ayesa played a key role in designing the award-winning “Peanut Shaft,” successfully launching the UK’s largest Tunnel Boring Machine (TBM). This innovative design features four 21.2-meter diameter shafts connected through T-shaped counterforts, curving to align with the road without needing external ground treatment or anchors. The TBM, nicknamed “Jill” is 82 meters long, has a 11.91-meter cutter face, and weighs 1,800 tonnes. It completed the first tunnel drive by February 2023, advancing at an impressive pace of 10 meters daily.

The Peanut Shaft was developed in response to land rights constraints that required avoiding anchors to the ground. This design allowed the shaft to be built using secant piles—a first for a peanut shaft in the UK. Alternative methods, like D-walls, were unsuitable due to existing obstructions and supply chain limitations, such as the unavailability of a hydro mill in the UK. As part of the innovative implementation, the team conducted a feasibility assessment, evaluating various global solutions for their viability in the UK. The chosen solution combined four shafts connected through T-shaped counterfort structures, eliminating the need for external ground treatment or anchors.

After successfully completing the first tunnel, Ayesa helped develop an oval-shaped “Rotation Chamber” in March 2023. This chamber facilitated a 180-degree rotation of the TBM in sections—a strategic move by Ferrovial to expedite the process and avoid a six-month dismantling and rebuilding phase. The chamber was 18 meters deep, 40 meters long, and 39 meters wide, featuring two openings for the TBM tunnels northward and access to the cut-and-cover tunnel section southward. The rotation was facilitated by an innovative “nitrogen skates” system using a complex hydraulic jacking system.

Finally, Ayesa contributed to designing the “Retrieval Chamber,” where the TBM was disassembled. The successful execution of the launch, rotation, and retrieval chambers demonstrated significant ingenuity in overcoming complex challenges and enabling the entire tunnel to be bored in less than one year. With the main tunnelling work completed in 2023 – the project remains on target for operations in 2025.

• The Silvertown Tunnel project brings several beneficial social outcomes that will enhance the quality of life for Londoners. The new tunnel will significantly reduce traffic congestion and pollution, improving air quality and a healthier environment in the surrounding areas.
• Enhanced connectivity between the Royal Docks and Greenwich will provide better access to jobs, education, and services for local communities.
• The revitalisation of the Silvertown site, including the restoration of the historic Millennium Mills and the creation of new public spaces, will foster a vibrant community hub, promoting social interaction and cultural engagement.
• The project also supports sustainable urban development by prioritising eco-friendly construction practices, contributing to London’s broader environmental and social goals.

The project has received numerous accolades, underscoring Ayesa’s role in delivering exceptional engineering solutions:

2023. WON: NCE Tunnelling Project of the Year (over $200m): Riverlinx CJV: BAM UK & Ireland, Ferrovial Construction & SK ecoplant Silvertown Tunnel.

2023. WON: NCE Tunnelling Innovation in Tunnel Boring Machinery and Systems – Pressure Ring Launch and Floating Rotation.

2022. Ground Engineering Awards: Innovation in Shaft Design & Construction: Recognized for the Secant Bored Pile Peanut Shaft design and construction.

2022. Ground Engineering Awards: Shortlisted: Geotechnical Team of the Year: Ayesa, Cowi & Riverlinx CJV.