HS2 contractors build 93m-long service tunnel at Euston for Northern line

Traction substations are used to convert electrical power to a form suitable for a rail system. A new TSS is being constructed to house the equipment needed to provide services and ventilation for the safe operation of the Northern line at Euston station, as the existing one on Melton street will be demolished to make space for the construction of the new HS2 station.

The Melton street building originally opened in 1907 as an entrance and exit to what is now known as the Northern line and was repurposed for its current function in the 1960s.

In order to connect the new seven-storey TSS – three stories below ground and four above – with the London Underground Network a 93m-long, 6.5m wide tunnel has been built.

Construction works started last April, with the Mace Dragados JV (MDJV) sinking a 20m deep shaft. “The shaft is split into two sections,” MDJV senior project manager for the Traction Substation Rob Williams said. “The top 10m section is formed of a caisson, which are curved precast elements that form a ring when you connect them together. The bottom section is formed of spray concrete lining.”

The diameter of the top section is 8m, while the bottom section has been expanded by 2m to facilitate tunnelling operations.

HS2 Ltd Euston project client Andy Swift explained that the two different techniques were used because of the variations in ground conditions. The caisson was used for the top section because the ground consisted of made ground and gravel. Once Clay was reached, he said “traditional underpinning techniques were used where you dig down a meter and spray”.

Williams added that the team faced a challenge when constructing the shaft. “The construction industry ran out of fibres which hold up the spray concrete lining. We all [Transport for London (TfL), HS2, MDJV and supply chain] had to get together to come up with a technical change proposal, so we replaced the tensile capacity from fibres with mesh reinforcement.”

When the shaft was completed, the construction of the tunnel began. The team carved out the underground passage using an 8t excavator and coated it with a primary sprayed concrete lining (SCL).

After applying waterproofing, the team reinforced the tunnel with steel and concrete to give it the strength and structure needed ahead of the construction of the HS2 Station that will be taking place above it.

Working with sub-contractors Cementation Skanska, Careys and JG, MDJV designed the tunnel in a way to reduce the construction’s carbon impact. Carbon savings of 140t were achieved by reducing material and water use, vehicle movements and energy consumption. Efforts to reuse material already on site resulted in over 1,000 fewer lorry movements and an associated saving of 76t of carbon dioxide from vehicle emissions.

Throughout the tunnelling works, the shaft has been enclosed by an acoustic shed to reduce noise pollution. This enabled 24 hour working which was required to meet the demands of the programme.

The tunnel is split into two sides separated by a reinforced concrete wall, one side is the cable route and the other is the ventilation route. Due to the existence of water infrastructure assets, a small section of the tunnel has a smaller diameter.

For the construction of the TSS, 104 piles were installed which allowed the excavation of a 20m-deep box. Earlier this month a 20m x 20m, 1.5m thick base slab has been installed made of 500m³ of concrete. “It's the biggest pour that has been done at HS2 Euston so far,” Williams highlighted.

He said that the main civil and structural works will be finished by mid next year, with the commissioning of the new substation expected at the end of 2024 or start of 2025.

After that the existing substation will be demolished. HS2 Ltd will be carefully removing some of the iconic historic features and tiles donating these for reuse and heritage displays.

TfL lead sponsor Lorena Naylor said that TfL is working with HS2 “to ensure the heritage features from the original building, including the oxblood tiles from 1907, are salvaged and re-used at other stations such as Oxford Circus.”

“Alongside the new traction substation here, HS2 will deliver a capacity upgrade to Euston underground station ticket hall as well as a new bus station,” Naylor added.

Refreshed designs for HS2’s Euston station were revealed in March, seven years after initial drawings for the London terminus were first tabled.

The updated designs reflect the government and HS2’s decision to reduce the number of platforms at the station from 11 to 10 in order to allow construction to take place in one single phase.

The HS2 station itself will be set across three levels, with 10 450m long subsurface platforms, which will at peak operation will be used by up to 17 high speed trains per hour serving destinations in the Midlands and the North.

Concerns about reducing the number of platforms have previously been raised by rail professionals, with one HS2 insider telling NCE that having one less platform gives the service less “wiggle room” should delays occur elsewhere on the line.

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