Go to content
Cadent High Pressure pipeline diversion, Hillingdon, London
Cadent High Pressure pipeline diversion, Hillingdon, London

Utility works in the Hillingdon area

National Grid Pylon Diversion

National Grid is legally required to modify an overhead power line in Colne Valley for HS2 Ltd to allow for the build and operation of the new railway. We’re rerouting and raising the height of a line of electricity pylons on land now in HS2’s possession which the railway will pass under. 

To achieve safety clearance five existing pylons, conveying a 275kV high voltage overhead line across Denham and Hillingdon, will be dismantled and the ground around each of them reinstated. Six taller pylons with support foundations will be built in new locations, diverting the line across a lower section of HS2 at a suitable crossing point. The overhead line will be diverted onto these new pylons and connected back up to the existing network. 

The work is being done by National Grid and our contractor Babcock. The power line will remain the property of National Grid and we’ll be responsible for its future maintenance too. 

Our overhead power line diversion work continues until Winter 2021 however the date is subject to change to fit with the activities of other HS2 contractors. National Grid will return to the area at a later date in HS2’s development to construct a Grid Supply Point (GSP) substation connection to power the railway. 

This work is on both the east and west sides of the Grand Union Canal; near Skip Lane, Dews Lane, Harvil Road, Denham Country Park and the golf course. During the works there will be shared access along Skip Lane between National Grid and existing users. We’re working closely with HS2 and its stakeholders to maintain access routes for specific user groups while we work. Once complete the new section of overhead wire will connect into the existing line at either end. 

During the main works in summer 2021 there will be temporary traffic management and road closures of a short duration to install and then remove scaffolding and netting. 

Footpaths and bridleways which are already closed for HS2’s main construction works are affected by our diversion.  

Preparatory work is underway. This includes ecological investigations, surveys, fencing, vegetation and tree clearance and foundation works for the new pylons. 

In Denham Country Park a temporary haul road of trackway matting from Denham Court Drive and a temporary bridge over the river will be laid so our team can access areas for work. These will be removed once we complete our work. 

This work is supported by security staff, including HS2 security and where necessary the police, to maintain the highest levels of health and safety of our workers, and the public/amenity users. This includes fencing to demarcate working areas and land which is in HS2 possession. 

The work to the overhead wire and its connection to new pylon towers will be done during an electricity outage next summer 2021. This means the power supply will be turned off one circuit (i.e., side of the electricity line) at a time for the duration of our work. 

Our people working on-site are doing so under stringent health and safety measures to protect not just themselves but also the communities in which they operate from Covid-19. We’ll keep the situation under constant review, and we’ll assess our contractors’ safe working practices and use of PPE as work progresses.

Please read the National Grid Colne Valley leflet here. You can also contact National Grid at [email protected] or freephone 0800 073 1047 (7am-7pm Monday to Friday).

Overhead electricity lines and health

We understand people have concerns about electric and magnetic fields (EMFs). EMFs are produced wherever electricity is used and there have been suggestions that exposure to these fields might be a cause of ill health. National Grid takes the issue of health very seriously and relies on authoritative and independent scientific organisations such as the World Health Organization (WHO) and Public Health England to review the worldwide body of scientific evidence on EMFs and health, as well as reviewing the science ourselves. 

Health considerations are given a high priority in the process by which we arrive at any proposals for new routes for electricity connections. Our approach is to ensure that our assets comply with the guidelines set by Government on advice from Public Health England. A vast amount of research has been done into the possibility of health effects, without establishing any risks below these levels. 

If you have any further questions or concerns, please contact the EMF helpline on 0845 7023270 or email [email protected].

Ruislip Branch Trunk Sewer Diversion

A diversion of the Ruislip Branch Trunk Sewer is needed as it cannot withstand the weight of the new portal structure at West Ruislip due to being over 90 years old. The sewer manhole will also need to be repositioned away from the new HS2 tracks. 

Thames Water will be undertaking the work to divert the sewer to the west, and this will involve having to take some land within the Ickenham Green Allotments temporarily. The work will involve sinking shafts in the ground and tunnelling underground from the Ickenham Green Allotments to Ruislip Golf Course. 

We will be diverting the sewer to the west, and this will involve having to take some land within the Ickenham Green Allotments temporarily. Our work will involve sinking shafts in the ground and tunnelling underground from the Ickenham Green Allotments to the Ruislip Golf Course. 

We are working to minimise the temporary land take required and we will carry out a full reinstatement of the area once the work is finished. 

Our work is due to be completed in 2023 and the existing sewer will be abandoned and filled with concrete. 

The current sewer

The Ruislip Branch Trunk Sewer serves a population of approximately 50,000. It was built in May 1932 and is made of concrete. The sewer is approximately two meters wide and nine meters deep, it flows down into Mogden sewage treatment works. 

It flows approximately half full even in dry weather conditions, with a flow of approximately 300 litres/sec during the day. In storm conditions its flows exceed 800 litres/sec. Due to the level of peak flows there is no possibility of diverting the flow in this sewer into another sewer or catchment. 

Accessing the allotment site

To access the allotments, we will need to put in a temporary haul road from Ickenham High Road (next to the Soldiers Return) and construct a compound area for the duration of our work. 

We have submitted a pre-application to London Borough of Hillingdon. We are in the process of discussing our plans with them. We will then submit a full planning application and publicise the application with details of how to comment on the application. 

London Borough of Hillingdon will then decide on whether to grant planning permission for the temporary haul road. 

The environment

We have carried out several environmental studies which include: 

  • Tree 
  • Bat 
  • Reptile 
  • Great crested newt 
  • Invasive plant 
  • Badger setts 
  • Water voles 

A damage impact assessment was also undertaken to ensure that the final design minimises the impact on the environment and local community as much as possible. 

We are in continuous engagement with the Environment Agency and permit consents for the proposed works will be submitted. 


Our work in the allotments will be planned to keep tree removal to a minimum and the chosen location of the haul road has minimum impact on trees. Several trees near the haul road entrance will need to be removed, but we will replace them after construction.

Questions people have asked

We have produced a frequently asked questions (FAQs) document that will cover answers to the most asked questions about this scheme.

Download the FAQs

  • Our presentation to residents of The Green on Wednesday 14 July 2021.
  • Our presentation from our community meeting held in May 2021.
  • Our presentation from our community meeting held in March 2021.


Thames Water will keep people informed of upcoming works through notifications:

Notice of Thames Water works – August 2021

Notice of Thames Water works – March 2021