HS2 London

The Colne Valley Viaduct Crosses The Grand Union Canal

A quick visit to the Grand Union Canal between Harefield and Denham, at the point where the Colne Valley Viaduct, part of the HS2 railway, crosses it.

The construction of the two-mile-long viaduct has been creeping towards the canal for the last two years, and new the red construction girder has slid across the canal itself, and is installing the concrete segments of the permanent structure below.

The construction means that the east bank (which is part of the London LOOP walk) is closed for a couple of weeks, while the towpath on the west bank has now reopened, but closes for 20-30 minutes several times a day for each concrete segment to pass over. The canal itself has been closed all winter to boaters but is tentatively going to start opening from tomorrow, daytime only.

HS2 London

Colne Valley Viaduct: 1 Year In

It’s just over one year since I last had a detailed look at the worksite for the forthcoming Colne Valley Viaduct, and I was back last week to see what’s changed.

Launch Girder

The viaduct is being built north to south, and this striking red structure, the launch girder, has just been assembled on the earthworks leading to the northern end. It will soon take a slow journey, sliding the couple of miles across the viaduct, atop the newly created piers, building the main bridge deck from the precast sections being created at an onsite factory.

The launch girder sitting on the northern viaduct embankment.
Unusual site speed restriction sign.


These are now appearing regularly, in various stages of completeness stretching along the first part of the viaduct. The plan I believe is to create one a week.

The second pier.
The sixth pier, with concrete surface texture detail visible.
The seventh pier, being shuttered while the concrete sets from being poured. The ex-Denham Water-ski Club hut is behind.
Rebar for the eighth pier in place, nearly ready to be shuttered.

The pair either side of the A412 (Nos. 3 and 4) have a different design, as they are angled, to support the viaduct which will cross the road here at a severe skew angle.

Grand Union Canal

The Grand Union Canal is one crossing that needs to stay open as much as possible – so the haul road will not cross it. Instead, two haul roads will meet either side of it. Here, the photo shows the southern limit of the northern haul road pontoon on Savay Lake, with the forthcoming piers of the viaduct due to appear on the left.

Southern end of the northern haul road, by the Grand Union Canal towpath.

Reflectors for a survey robot have been embedded beside the canal here on its towpath here, at the point that viaduct will cross, to check there’s no movement or subsidence of the canal or towpath here when the piers or viaduct deck go on – particularly important as the land and lake to the west of the towpath is slightly below the canal level.

Survey reflector on the towpath.

Haul Road Pontoon

On the other side of the canal, another haul road pontoon, coming across Broadwater Lake from the south and London. It may advance a little bit more, across the track and cycle route here, and up to the other side of the Grand Union Canal. The viaduct piers will be on the right. It will likely be over a year before these ones get constructed, as this is the southern end of the viaduct. The closed, but not yet relocated, Hillingdon Outdoor Activity Centre, is just to the right of the far end of the pontoon here – spared a direct hit by a late design change in the HS2 route, but still likely to move to be on a different lake.

The haul road crossing Broadwater Lake.
Southern haul road pontoon – current end.

HS2 London

HS2: The Colne Valley Western Slopes

Satellite imagery of the Colne Valley Western Slopes worksite in early April 2021, from Sentinel. The tunnel portal is at the top-left (NW corner), just before the M25 motorway. © Sentinel Hub.

High Speed 2 (HS2) will leave London by curving away from the Chiltern Main Line, across the Colne Valley on what will the the UK’s longest railway viaduct. It crosses the Greater London boundary before briefly coming back to ground level and then on into the Chiltern Hills in a long tunnel.

The place between the viaduct and the tunnels is called the Colne Valley Western Slopes. It is here that Align JV, who are building the viaduct in one direction towards London, and the Chiltern tunnels in the other direction towards Birmingham, have created a huge construction site to assist in building both. The site covers several square miles of what were arable fields, and is highly visible from Harefield, a village on the other side of the valley:

The Colne Valley Western Slopes construction site, from Harefield village to the north-east.

The Colne Valley Western Slopes site is very large for several reasons – it houses factories to construct the concrete segments both for the tunnels and the viaduct deck and also the launch podium for the tunnel boring machines (TBMs). It will also be able to comfortably accommodate the chalk spoil coming from the TBMs. Having the production and spoil deposition both on site will substantially cut down on lorryloads bring concrete in and chalk out of the site. A private “junction” has been created on the adjoining M25 motorway, to provide a suitable route for the lorries that do need to arrive.

The upper part of the construction site (focusing on the tunnel itself) viewed from the south-west. The two TBMs (tunnel boring machines) are being reconstructed by the two huge cranes on the left, prior to launching the Chiltern Tunnels boring later this spring. The large factory on the right is the concrete tunnel lining creation plant.

The opportunity is being taken, post construction, to not just restore the arable farmland beforehand, but instead create a large-scale nature habitat – to partially offset HS2’s undeniable impact on the natural environment. HS2 have recently unveiled their detailed plans for the site, which include reconnecting two public paths that have been severed, and plans for new forest to help screen noise and improve biodiversity. A number of set-piece views of the line are also being designed.

Plan for the restored and added path network on the Colne Valley Western Slopes site. Drawing © HS2.
The middle part of the site. A balancing pond, to manage drainage, is being constructed on the right. The concrete “roadway” running through the centre of this picture, with a crane on it is the route of the railway. It will launch the tunnel boring machines to the left, and the viaduct construction girder to the right. The lower part of the site (to the right of the photo) will be mainly used to place the spoil from the tunnels.

A common design style for the tunnel portals, the viaduct, and a new road bridge to accommodate a rerouted road, Tilehouse Lane, that sits in the centre of the site, has been adopted. The design is modern and sophisticated, without distracting from what should return to a natural, rural environment post-build.

HS2’s common design for the three structures in the area – white concrete structures with distinct (~30 degree) angles, forming the tunnel entrance, bridge parapets and viaduct ends and piers. Drawing © HS2.
HS2’s plan for the eventual look of the site (looking from the south-west). The diversion of Old Shire Lane, to pass under the start of the Colne Valley Viaduct, causes the U-shaped path seen at the forefront. Tilehouse Lane’s new alignment results in the new bridge, crossing the line in the distance. The tunnel portal is at th far end. Drawing © HS2.

HS2 London

HS2: The Colne Valley Viaduct

HS2 drawing of the Colne Valley Viaduct crossing the Colne River (slightly realigned here) with a planned foot crossing of the river within the Broadwater Lake Nature Reserve here, which will be added. This will be point 5 on the enumerated satellite image below. @HS2.

In the first post in a mini-series, I looked at how High Speed 2 is impacting West Ruislip and Ickenham. In this second piece, I move slightly west, to look at the Colne Valley Viaduct. At 2.2 miles long, this will be one of the biggest structures on the entire line. The viaduct is what carries the line away from the existing Chiltern Line it has been sitting beside, and outside of London altogther, into the Chiltern Hills. As a symbol of leaving/arriving in the capital, and the scenic nature of the valley it crosses, the Colne Valley Viaduct has the potential to be a “tourist attraction” of the railway – both for the people on it, and also users of the natural park that sits underneath it.

The Colne Valley is a very “wet” valley – it was extensively mined for gravel, and as a result there are numerous lakes. The viaduct crosses a number of these, as well as clipping some natural woodland that has gradually built up. A slight planned route change (moving the curve around 100m north) since the initial plans a few years ago, has resulting in less ancient woodland being destroyed, but the line does still pass close to a number of nature reserves, including through one – Broadwater Lake Nature Reserve – that it has unfortunately closed for many years, and outdoor leisure facilities (two of which are in the process of moving).

Harefield No. 2 Lake, the pylons to be moved, and Hillingdon Outdoor Activity Centre, are visible from the National Cycle Route 6 track. The viaduct will pass right across this picture. Points 13-15 in the satellite image below.

It’s not the only railway viaduct in the valley, here’s the Chiltern Main Line viaduct just to the south – incidentally the electricity pylon here will be going soon as part of a power rerouting for the new viaduct, so this particular view will improve soon:

Older viaduct carrying the Chiltern Main Line across the Colne Valley, seen from National Cycle Route 6 beside the Grand Union Canal.

Piling for the piers of the viaduct begins this month and is set to take a number of years, the work moving from west to east, with haul roads currently being built underneath the viaduct. The concrete structures are being built on site. The haul roads will build out from each end, meeting either side of the Grand Union Canal. This break will mean as short as possible a closure for the canal itself – it will only need to close when the deck building girder is directly above it.

The viaduct beside the A412 North Orbital Road, and Denham Waterski Lake (point 3). A strip of trees, forming part of Battlesford Wood, stood here once. There will be a new path and maintenance road here, formed from the haul road underneath the viaduct, which will connect across a new bridge to Broadwater Nature Reserve, opening up more walking routes across the Colne Valley here.

The large, curving and highly visible nature of the viaduct has meant that it has been possible to justify creating a design for it that is not just a simple set of vertical piers. Instead, sweeping arches will carry the line through the woodlands and across the waterways and lakes.

HS2 illustration of the crossing of Korda Lake (point 8) by the viaduct, viewed from south of point 7. © HS2.
The viaduct will cross Moorhall Road (from where this photo is taken) and Korda Lake, here, directly overhead. A test piling rig is visible in the middle of the lake. Looking towards point 8 from point 9.

The viaduct will have sound barriers but they will mostly be transparent, except for the section facing a new housing development at Denham Film Studios, so that passengers will be able to enjoy the extensive views across the lakes at ~200mph. At this speed, it will take less than a minute to complete cross the valley on the viaduct. As the viaduct is “bookended” by long tunnels at either end, it will be the most obvious visual clue to south-bound passengers that they are about to arrive in London.

Route of the Colne Valley Viaduct, on OpenStreetMap. The Greater London border follows the River Colne here and so crosses the structure.
Satellite photograph of the construction worksites along the Colne Valley Viaduct, in early April 2021. © Sentinel Hub. Number key below.

From west to east, the main crossings and affected waterways and buildings are:

  1. Old Shire Lane, a bridleway that also forms the Hertfordshire/Buckinghamshire border. It is currently closed. It will be diverted in a loop to pass under the start of the viaduct. It is due to reopen in towards the end of 2021. A diversion route is further south, via Wyatt’s Covert.
  2. The A412 North Orbital Road. An unpleasant, fast road, which is made even worse at the moment by high dust levels and HS2 construction trucks.
  3. A narrow strip of woodland beside a lake, and the Denham Waterski clubhouse, which is about to be demolished and has a brand new replacement on the opposite shore.
  4. Battlesford Wood, just north of Denham Film Studios.
  5. The River Colne, which will be rerouted slightly here to flow more perpendicularly between two piers of the viaduct. A bridge will also be built here, to connect Battlesford Wood and a new path under the viaduct to the A412 and Wyatt’s Covert, to a nature reserve and Moorhall Road.
  6. Almost the full length of Long Pond (called Long Lake on some maps).
  7. A narrow nature reserve, Broadwater Lake Nature Reserve, which is unfortunately closed for the duration of HS2 construction but will have improved access links afterwards (see above).
  8. Korda Lake – HS2 goes diagonally right across this fishing lake.
  9. Moorhall Road, which goes between Denham Green and South Harefield.
  10. A corner of Savay Lake.
  11. The Grand Union Canal and towpath. A new pier on to Savay Lake will be built here, opening up a view.
  12. A track just east of the canal, that forms National Cycle Route 6.
  13. Diagonally right across Harefield No. 2 lake.
  14. Under a line of pylons that is getting rerouted further east, as it will be too close to the viaduct structure.
  15. Right over the top of Hillingdon Outdoor Activity Centre, which is also closing and moving to an as yet announced location.
  16. The viaduct finally touches down in fields just before Harvil Road.
Here’s where HS2 crossings National Cycle Route 6, between Harefield No. 2 Lake and the Grand Union Canal. Point 12 in the satellite image.

For the last piece in this mini-series, I focus on the huge worksite for both this viaduct and also the Chiltern Tunnels – the Colne Valley Western Slopes.


High Speed 2 in Hillingdon

There’s a big building project underway at the moment, High Speed 2 (HS2), and it’s taking a big dent out of my local borough, Hillingdon in west London. Hillingdon is where the forthcoming railway line will burst out of its tunnels from central London, before curving away from the Chiltern Mainline it has been running beside, onto a long viaduct across the Colne Valley, and out of the borough into another set of tunnels by the M25 orbital road, passing beneath the Chiltern Hills on its way to Birmingham and beyond.

Related major works currently include rerouting a large power line that would otherwise run across the viaduct, rerouting gas mains and water mains, straightening and rerouting several roads, building new electricity substations, building or enlarging various access roads for the above, building a “haul road” to run underneath the viaduct for constructing it, creating a factory for the concrete parts for the tunnels and viaduct, and building a long electrical supply cable to power the tunnel boring machines. Plus associated patching up of the existing infrastructure such as footpaths, bored out earth storage and the start of post-project landscaping.

It’s surprisingly hard to find out exactly the true extent of what is going on, at the current time. It’s a hugely complex project, and HS2 comms are therefore having to run local engagement activities in many different sections, all at the same time. The Hillingdon version is a bit of a mess, with updates coming as an incoherent set of PDFs and maps of various formats.

To see what’s going on, I took several trips around the area, crossing most of the places where the line will pass. There is no unified map showing the current areas under HS2’s control, so there were some surprises, with the “orange army” and green security fencing appearing in unexpected locations.

My intention is to visit the construction areas fairly regularly over the next few years, and see how they are changing. It’s fairly easy for me to get there and it’s a pleasant enough cycle.

Starting at the point where HS2 emerges from tunnels, opposite West Ruislip station on the Central Line (and beside and just to the north of the Chiltern Mainline). There will be a tunnel portal here but work here is much less advanced than at the tunnel at the other end of the Colne Valley.

The western tunnel portal for the tunnels from Central London, at West Ruislip.

Ruislip golf course is closed, with HS2 taking a bite out of its southern edge (although the rest of it seems intact, so it’s odd the whole thing is closed). So, I took a route on the south side of the Chiltern Mainline insetad. It was rather a pleasant route, meandering through suburbia, woodland and grassy fields, passing a cricket pitch at one point, before suddenly you are at the River Pinn (of Pinner fame) and on the Celandine Route, a waymarked long distance through the borough. HS2 hoarding appears immediately on the other side of the nice bridge here, but the paths have been upgraded by the company contractors, having turned into a quagmire earlier in the winter.

The Celandine Route passes underneath the Chiltern Mainline (and then a future bridge under HS2), beside the River Pinn. HS2 have kindly upgraded the path here after churning it up.

There is a short public footpath, U46, which I took here to get onto Brakespear Road South. This footpath runs directly underneath the HS2 route, so will vanish soon and will only reappear after works are finished, rerouted slightly to the north. For now, the path has HS2 fencing on both sides, accompanied by various dire warnings about a High Court injunction being in force to stop protestor camps (re)appearing in this area.

U46 public footpath, directly underneath HS2, and hemmed in by construction fences. It’s not long for this world.

Crossing underneath the existing bridge for the Chiltern Mainline, I took another right of way (U50) which goes through rather waterlogged fields, before ending up at Harvil Road. This is not a pleasant road to cycle along, as the HS2 construction camps run for almost a mile, and the road itself is narrow and twisty (it is getting straightened and rerouted as part of the HS2 works).

Sheet piling at the beginning of the Harvil Road straightening works.

First you pass the worksite for the straightening work and a major water main rerouting, on the right. Then you pass a huge cutting (at least 20m deep) being dug out, then space for a power plant for powering the line, then the line itself, and finally further up on the left an access road being created for the the viaduct works, and one for the powerline rerouting and substation building.

There is, I think, an old public footpath that will be gone for ever, as it will sit under the line and the power plant. Another one nearby, across Harefield Moor, has been rerouted.

Works at Harefield Moor, with a half-built pylon, the start of the diversion, visible to the left of the current one. There should be a reinstated public bridleway here, soon, giving access to the Colne Valley lakes.

Finally, down to the Colne Valley itself and its forthcoming viaduct, but that’s for the next post…

HS2 in Hillingdon (west) – 1: Future portal for the tunnels to central London. 2: Existing bridge (and future bridge) on the Celandine Route at the River Pinn. 3: Path U46. 4: Harvil Road straightening. 5: Future power substation for HS2. 6: Concrete plant being used by HS2, and power line being diverted through. 7: Huge cutting. 8: Start of the Colne Valley Viaduct. 9: Access road for the viaduct construction. 10: Power line rerouting and new substation. Map © OpenStreetMap contributors.