Update on tunnelling and cross passage construction in Hillingdon

What we are doing

Tunnels and cross passage construction update
The tunnels’ route broadly follows the London Underground Central Line and the Chiltern Mainline. Since both tunnel boring machines (TBMs) were launched from the West Ruislip Portal site in autumn 2022, activities supporting tunnelling works at the Portal are ongoing. These include removing excavated material, supplying materials and maintaining tunnelling equipment.

The TBMs are progressing at a rate of approximately 150 metres per week. We expect them to have reached the Hillingdon/Ealing border as shown on the map on the pdf attachment.
We have now started building the cross passages near to Herlwyn Avenue and Roundways/West End Road. Cross passage locations and key dates are shown on the map on the pdf attachment.

Cross passage construction
We are now progressing with the construction of cross passages, which are approximately 500 metres apart along the route. While invisible to the travelling public, the cross passages will have a key role in providing a safe operational railway. Each tunnel has an emergency walkway that allows passengers to safely evacuate the train and walk through the cross passages into the other tunnel where they can be rescued on a passenger train.

The ground above the tunnel is mostly clay and approximately 16m (52 feet) thick, with a sand channel running through the tunnel’s crown. Because of the ground conditions at these locations, we will freeze the ground which is the proven, safest way to control the ground during cross passage construction. The side of the completed upline tunnel will be broken out and then a remote-controlled robotic digger will be used to dig out the frozen ground and a sprayed concrete lining will form the interconnecting tunnel. This phase generates some ground-borne noise and vibration, but because of the clay layer and the ground conditions it will be less noticeable.

Keeping you informed
Due to noise transmission paths that occur naturally in the ground, it may be possible for people living approximately 100 metres away to hear or even feel something intermittently as the tunnel boring machines are passing. Not everybody will experience this but if you do, this tends to be a very low-level rumble and will only last a few days as tunnelling progresses past properties.
Our community engagement team have been contacting residents and businesses whose properties are along the line of route to let them know when to expect tunnelling near their properties. We’re also checking in with residents and businesses after each TBM has gone past ensuring that you are being kept well informed and updated.

We have also been contacting the occupants of properties who are closest to the cross passage locations. There is a period of the construction works that involves breaking through the side of one tunnel to connect it to the other. This might generate some vibration which may be heard as ground-borne noise in nearby buildings. Whenever possible, in order to minimise disruption to residents, we will limit this activity to extended daytime hours (Monday to Friday, 7am to 10pm and Saturday 7am to 1pm).

Monitoring taking place during tunnelling
We have installed specialist equipment in multiple locations along the tunnelling route to monitor ground movements before, during and after construction. The surveying team will be out and about in Hillingdon to take regular readings from the studs located in the pavements or to take readings from the equipment mounted on buildings. You may see members of the team assembling a tripod to mount monitoring equipment in order to take readings. This activity will be quiet, for a short duration and will not affect access to footpaths and driveways. We apologise in advance should this activity inconvenience or disturb you.
Boreholes in nearby open spaces
As part of our continued investigative work, we are routinely checking the boreholes along the route. Some of the boreholes are located in open spaces in the area and you may see our operatives carrying out checks on a daily basis.

How we are managing the impacts of tunnelling
TBMs are the best method for safely excavating tunnels of this type because they build the tunnel as they dig. This reduces the risk of settlement (the way ground moves around a hole after it has been dug) and minimises potential for any damage to nearby properties. The TBMs can operate 24 hours a day, 7 days a week until the construction of the tunnels is complete.
Continuous tunnelling by TBMs has been used successfully on major projects such as Crossrail, Thames Tideway and the Northern Line Extension. Extensive knowledge and data gathered from tunnelling in similar conditions enables us to accurately predict ground-borne noise and vibration levels. Our assessments did not predict significant ground-borne noise and vibration impacts. However, it may be possible to hear a low-level sound from the TBMs during quieter periods. It is unlikely vibration from tunnelling will cause damage to nearby buildings.
How deep are the tunnels?

The depth of the Northolt Tunnels West in these locations (from the ground surface to the crown of the tunnel) varies between 13.6m at its shallowest and 18m at its deepest (approximately the height of three and four double decker buses stacked on top of each other respectively).

How are properties protected from the potential impact of our tunnelling works?
There are legal provisions under the HS2 Act to ensure protection should any damage occur from HS2 tunnelling works or excavations.
If, during or following tunnelling, you believe that damage has occurred, you can contact us to let us know. As part of the assessment of a damage claim, a second comparison survey of a property may be undertaken. You can contact the HS2 Helpdesk on 08081 434 434 or email [email protected]

Keeping you updated
We will provide updates on the TBMs’ locations at https://www.hs2.org.uk/in-your-area/map/
As tunnelling progresses eastwards, the team will ensure people living at the locations shown on the map are kept up to date on approximate timings and location of the TBMs and when cross passages are being constructed.
If you would like any further information on tunnelling activities, please get in touch by contacting the HS2 Helpdesk on 08081 434 434 or email [email protected]

Download information relating to these works

• Tunnelling FAQ
• Update on tunnelling and cross passage construction in Hillingdon

Find out more about

• Hillingdon

If you have a question about HS2 or our works, please contact our HS2 Helpdesk team on 08081 434 434 or email [email protected]