Chiltern Tunnel South Portal and Colne Valley Western Slopes

Align JV is working on behalf of HS2 Ltd to build 22 kilometres of the high speed rail line, running between the Colne Valley and the Chilterns. This includes the Chiltern twin-tunnels with their four vent shafts to regulate airflow, one intervention shaft and the shaft headhouses which house electrical equipment .

This page provides information on the 'You Said, We Did' series of online information webinars and virtual exhibition we are holding for the Chiltern Tunnel South Portal and Colne Valley Western Slopes , a key part of the design for HS2 in the Colne Valley.

Since spring 2018 we have held a series of information events with the local community to share details on the progress of the designs, construction, and to seek views and respond to feedback for the Colne Valley Western Slopes, the South Portal and Colne Valley Viaduct. This online exhibition and webinars are the latest in that series.

In Summer 2019 we held a round of 'You Said, We Did” events across the valley to show how our detailed design aligned with the feedback we received, and to provide detailed information about the construction of the viaduct. The designs for the South Portal and Colne Valley Western Slopes were still undergoing design review and we are pleased to be able to share the detailed design for these key elements today.

We asked you to rank our objectives for the Western Valley Slopes for the landscaping around the portal, and you commented that reinstatement of habitat areas, including woodland and hedgerows was the highest priority. For the Tunnel Portal design, you felt that screening the railway structures with landscaping or new planting was the top priority.

Your comments are highlighted in the four sections below: Landscape and Habitat Design; Visitor Experience and Views; Earthworks; and Portal Structure and Building Design and Visibility.

You can also find more information on these, as well as the construction site and Colne Valley Western Slopes Masterplan in our virtual engagement boards:

Visualisation - View from the Old Orchard Pub at Harefield looking west (Year 15)

For Landscape and Habitat Design, your priorities ranged from planting and landscaping to hide the railway without disrupting existing habitats, connecting up wildlife habitats through integrated and sensibly planned corridors, and whether there are any guarantees that wildlife will return after being moved from its current habitats.

Existing valuable habitats including tree belts and hedgerows have been substantially retained where possible. We will be creating a far richer ecological environment than prior to construction of HS2 and this will be fully connected into the wider landscape. Our landscape proposals will provide new and varied habitats and help reduce the visual and noise impacts of the railway.

Our proposals will create linked woodlands, hedgerows, scrub and waterbodies set within substantial areas of calcareous grassland. These habitats will be connected to existing vegetation and wildlife corridors which extend around the site and into adjacent areas to establish a comprehensive habitat mosaic.

Regarding trees, you asked that we ensure there are more trees than before construction to help absorb sounds from the railway and whether established plants/trees would be used or saplings that take 20 years to grow. Noise mitigation will be provided by our earthworks and noise fencing, while the majority of the trees around the site have been retained and approximately 70,000 new one will be planted. These will be planted as small trees as they are much more likely to establish and require less maintenance.

Visualisation: Colne Valley Western Slopes North Abutment viewing point

In terms of the Visitor Experience and Views, you felt it essential to maintain the magnificent views across the Colne Valley and asked whether we could do something similar to the ‘hills’ built along the A40 by material from the construction of Wembley Stadium which had become very successful recreational spaces.

Proposals include several new footpaths, cycle routes and bridleways, together with the realignment and partial upgrade of existing rights of way impacted by the railway. These routes will be connected to the wider footpath and cycle network at the Colne Valley Western Slopes boundary and will form part of a broader recreational offer delivered by the HS2 project.

The design seeks to enhance the visual experience and provides opportunities to view the railway from ‘set-piece’ locations; and also to view the Colne Valley Western Slopes in its broader setting by establishing open vistas from higher ground. In other locations where infrastructure could potentially look out of place within its surroundings, views of the railway are concealed typically using a combination of planting and/or landscaping.

We have considered how the site looks from other locations within the valley, particularly from areas around Harefield. Our landscape proposals aim to recreate the wooded character of the valley and also introduce areas of open grassland more typical of the upper slopes to the west. We have used planting and earthworks to hide most of the railway in views from the eastern side of the valley and where built features such as the south portal are visible these are set into areas of vegetation to reduce their impact.

Visualisation: Colne Valley Western Slopes Tilehouse Lane Viewing Point

You told us that sensitive landscaping shaping must take precedence over long-term planting programmes and that it should also minimise noise, as well as visual impacts.

The earthworks have been designed to reflect the existing pattern of ridges and valleys which will wrap around the new railway footprint, with most of the material excavated from the tunnel being placed on land within the south east and south west of the site. Drainage proposals include using earthworks to create a series of crests and dry valleys to naturally direct and control water runoff, while ditches and pipe systems will manage track, landscape and highways surface water. This will ensure that the water runoff rate will be the same as before construction of the railway, avoiding an increase in flow rates down stream.

The earthworks will also help to reduce noise impacts and include the Colne Valley Viaduct North Embankment noise barrier. The design of the landscaping has been influenced by noise assessment modelling, aiming to achieve the best combination of noise reduction performance, while complementing the surrounding landscape.

Visualisation: Chiltern Tunnel South Portal and building

For the portal structures, your priorities were to strike the right balance to achieve minimal noise, visual and environmental impact, particularly when trains enter the tunnel at high speed. You wanted them to be elegant and integrate with the landscape.

The curved hood and staggered arrangement of the portal structures reduce the sound of trains entering and exiting the tunnels, while the tapered shape and the perforations along the sides of the portal structures allow air pressure created by passing trains to dissipate, mitigating the noise. The portal structures will be made of smooth and textured concrete, textured at low level to ground them within the landscape. The portals are part of the 'family' of structures along the HS2 route through the Chilterns and Colne Valley, sharing a common theme with materials used for the Colne Valley Viaduct and Tilehouse Lane Overbridge.

In terms of the portal building, which will house mechanical and electrical equipment, you requested that it also be elegant and blend into the landscape. It has been kept as compact as possible, a simple object within the landscape, designed not to distract from the portal structures. The building is clad in dark grey steel louvres, wrapped by a band of vertical aluminium fins, while doors and openings are concealed within the louvred facade to simplify its appearance and reduce perceived scale.

The portal structure and building are designed to be screened with landscape earthworks and tree cover to reduce their visibility from long distance views. Openings in tree cover at specific locations will enable snapshot views to the portal structure and building. In these locations, low level rail systems equipment will be screened from view.

Find out how HS2 & Align are working to prevent impacts on water supply in your local area.

This webinar, hosted by Align and HS2 in June 2021, provided residents the opportunity to hear and ask questions about:

For more information about HS2 please contact our HS2 Helpdesk team on 08081 434 434 or email hs2enquiries@hs2.org.uk