HS2 prepares to excavate 21km route under London 11 Feb 2021

Six TBMs of different diameters and through varying geological conditions in three sections will be used to excavate the 21km route of the London running tunnels of the HS2 rail project in the UK (Fig 1).

• 8km x 8.8m i.d. Northolt West tunnel between West Ruislip portal and Greenpark Way ventilation shaft.
• 5.5km x 8.1m i.d. Northolt East tunnel between Greenpark Way and Old Oak Common Station.
• 7.2km x 7.5 m i.d. Euston tunnel between Old Oak Common Station and Euston Station.

Fig 1. HS2 London tunnels from Euston to West Ruislip

A seventh TBM will excavate a 1km x 6.1m i.d. logistics tunnel that will provide access from the main Atlas Road construction compound to Old Oak Common Station, the launch site for the Northolt East and Euston TBMs. It will be used to bring construction materials into the tunnels, including the precast concrete lining segments, and to remove muck.

The first TBMs procured and to be launched are two Herrenknecht EBPMs to excavate the Northolt West drives, the longest of the three alignments. These will be followed by procurement of the Northolt East TBMs in early 2021 and finally of the two machines for the Euston drives in late 2021. The main TBMs will be procured new and designed specifically for the project; a refurbished TBM is likely to be used to bore the logistics tunnel with procurement underway.

Changing and challenging geological conditions

Advancing from the West Ruislip portal, the Northolt West drives are expected to face challenging mixed-face geological conditions, according to Terry. “The location of the portal means that the TBMs soon encounter Seaford Chalk, with water pressures up to 3 Bar. For the first 3km of the drive, the TBM will negotiate a mixed face of chalk and the variable sequences of the Lambeth Group, with some significant sand channels. As the drive proceeds, the chalk drops away from the alignment and the TBM continues through the Lambeth Group with the sand channels present up to the shaft.”

To assist excavation through the challenging ground conditions, EPBMs will be used with a spherical bearing to allow for translation or tilting of the cutterhead, providing a greater degree of accuracy in terms of steering the machine, as well as controlling face pressure, cutter load, and settlement, “all of which are an advantage when you have mixed ground conditions,” said Greiner.

Fig 2. Launch location and advance direction of the six main TBM London drives

The Northolt West TBMs will also feature advanced automation technologies. Instead of traditional manual transfer of the segments from the quick unloader to the segment feeder at the back end of the TBM, the transfer will be operated automatically, as will the removal of the wooden spacers that are placed between the segments when stored and transported. At the front end, sensors and cameras will assist one operator to build the rings. Steering of the gantries will also be automatic.

Automation will continue to be pushed through the project to “further innovate, learning the lessons of the earlier drives and applying them to later TBMs,” said Greiner.

“One of the priorities is to take people out of the underground excavation environment,” said Woods. With a 40-year career in the industry, his previous projects include the twin tube TBM drives on the Channel Tunnel rail link, part of the HS1 project, under the suburbs of north London. “When we were constructing the HS1 TBM drives, we looked at every activity that took place underground and mechanised whatever we could. As a result, we achieved a much lower accident rate than other major tunnelling projects at the time. Now on HS2, advanced automation is allowing us to reduce the number of people working underground even further.”

Reducing the number of workers also helps when undertaking projects in areas where operatives are either in short supply or in demand. “It will get quite busy underground in London over the next few years and so competent professionals may become more difficult to find,” said Greiner. “Automation helps to de-risk this element of the project.”

Delivery of the Northolt West Herrenknecht EPBMs is expected by the end of 2021 for start of excavation by mid-2022.

Northolt East and Euston alignments

In addition to the Northolt West machines, the Greenpark Way working site will also receive those excavating the Northolt East TBMs at the end of their twin 5.5km x 8.1m i.d. drives from Old Oak Common (Fig 2).

The geology along the Northolt East alignment is again mixed but more benign than the Northolt West drives, without the hydrogeological challenges posed by the chalk. “From London Clay around Old Oak Common, the TBMs will encounter, the Harwich Formation and into the variable deposits of the Lambeth Formation as the reception shaft at Greenpark Way is approached,” said Terry.

The London terminus of HS2 at Euston Station

The 7.2km x 7.5 m i.d. Euston drives between Old Oak Common Station and Euston Station present the most consistent geological profile with the entire route lying within London Clay.

As the geology of the route changes, so the specifications for the TBMs will change and not all may require the spherical bearing of the first two machines. “We will have to look in detail at this, when we come to it,” said Greiner. “Because of the ground conditions, a spherical bearing lends itself best to the Northolt West drives through the chalk but it might also be an option for Northolt East.”

Although the geology is mixed along the route, it is “nothing that has not been experienced before,” said Terry and is similar to the geology of some elements of other projects in London. Some sections of the Tideway CSO drives under the Thames in London, for example, have faced similar conditions, as did the TBM and SCL excavations for the Crossrail project. “The TBMs will be designed to manage the mixed face and hydrogeological challenges, but it does pose much more of a challenge for the cross passages.”

There are 52 cross passages to be constructed along both Northolt and the Euston alignments. These are planned to be SCL construction, with a waterproofing sheet membrane and a secondary cast-in-place concrete lining. “These excavations present one of the greatest challenges in the London HS2 works”, said Terry.

The cross passages will be constructed as the main tunnel proceeds, “which raises the first of the challenges, as we effectively have to maintain a tunnelling site within a tunnelling site,” he continued. To facilitate construction, innovative openings in the segmental lining design will be used. These mean that “we do not have to separately prop up the running tunnel. We just take out the opening and work through that to construct the cross passage,” he said.

The hydrogeological conditions along the Northolt West section, “will require either dewatering, depressurisation or, in some cases, ground treatment, to construct the cross passages. With most of the route located under an operational railway, any ground treatment will need to be applied from inside the tunnel, posing an added challenge in terms of logistics,” Terry continued.

Planned UK high-speed rail development

Segment rings and diameters

The running tunnels will be lined with a reinforced precast concrete segmental lining with seven main segments in each ring. In the Northolt West alignments, the segments will be 350mm thick x 1.9m wide and are rebar reinforced. Rebar is also currently specified for the Northolt East and Euston alignments, but there is “an ongoing value engineering exercise to explore if it may be possible to change to a steel fibre solution in part of those tunnels”, said Greiner. The Northolt East segments will be 340mm thick x 1.9m wide, while those for the Euston drives are smaller again at 325mm thick x 1.8m wide.

The varying internal clearance of the segmental lining raises the question of why each of the three alignments has a different internal diameter and all are smaller than the 9.1m i.d. of the 16km Chiltern tunnel drives also on the HS2 route. “It is basically aerodynamics,” explained Woods.

“The pressure generated when a train passes through a tunnel increases with its speed but can be mitigated by increasing the internal diameter of the tunnel,” he explained. “With trains running through each of the HS2 London tunnels at different speeds, as they accelerate out of or decelerate into Euston Station, different internal diameters are specified.”

The largest internal diameter is therefore needed for the Northolt West tunnels, where trains will be travelling fastest, reducing in diameter towards Euston. In the Chiltern tunnels, which are the longest on the route, trains will run at up to 320km/hr and the larger diameter of this route reflects the need to manage higher internal aerodynamic pressure.

Reducing the diameters also saves in excavation and material costs with the smallest diameter Euston drives costing less to drive and line per ring than the largest diameter Northolt West alignments.

Integrating with station construction

Although station construction is not within the SCS contract, there is significant interaction with the station works contractors Balfour Beatty Vinci Systra JV at Old Oak Common and Mace/Dragados at Euston.

From the Old Oak Common Station box, SCL stub tunnels will be constructed for the Euston drives to allow full assembly of the TBMs before launch and, at the other end of the box, the Northolt East TBMs will launch from an ancillary shaft connected to the station box by SCL headings. The Atlas Road logistics TBM tunnel will also break through into the Old Oak Common Station box.

Fig 3. Stacked-drift construction of Euston Station cavern

At Euston, SCS is responsible for construction of the 16m wide x 12m high cavern to accommodate the turnout that will enable the HS2 service to run with the required frequency alongside what is already one of the busiest mainline stations in the UK.

“We are constructing the cavern using a stacked-drift method, whereby a series of smaller diameter SCL headings are excavated around the cavern crown profile to form an arch, under which the main cavern is then excavated to the required dimensions,” explained Terry (Fig 3). “This provides much lower potential for ground movement, which is vital at Euston, as we are excavating either under or next to the approaches to the existing main station.”

Constructing under intense public scrutiny

The ability of the project to control ground movement and construct the various underground elements safely came under scrutiny in 2020 during a court case brought by a local resident, who claimed that parts of the design were unsafe. HS2 ultimately won the case with the judge commending the highly skilled team assembled by HS2 Ltd.

“We are pleased that this matter has now concluded and we can move forward with the detailed design of the railway,” HS2 Ltd said in a written statement. “SCS and HS2 Ltd have gathered a team of experienced professionals to both guide the design and to ensure constructability.”

This collected expertise and experience will be a key factor in making sure HS2 is a success, not just in London but along the whole route. Monthly collaboration group meetings involve all of the HS2 main contractors, as well as external experts. “The group is probably the best meeting I go to,” said Woods. “It is positive. It is about solving problems, which is what we do in tunnelling.”

With construction of HS2 still controversial and numerous high-profile critics of the project, including some MPs, ready to highlight any perceived mistake, this level of skill and collaboration will be invaluable to the success of the project in meeting its target completion date of 2028-2031.