Frequently asked questions - prepared by HS2 Ltd :-

How long is the Chiltern Tunnel and where does is start and end?

The tunnel is 15.8km long, and starts at the south portal just inside the M25 south of Junction 17 and just south of Chalfont Lane. The northern end of the tunnel comes out northwest of South Heath, near Gt Missenden.

How deep will the tunnel go?

The depth of the tunnel varies. At its deepest point it will be over 60m below the ground surface. At its shallowest point it will be approximately 20m below the ground surface. The depth of the tunnel at any point is also factored by the above-ground topography.

What and where is the South Portal?

It is where the TBMs will be built to dig the Chiltern Tunnel. It is next to the M25, near junction 17 in between the motorway and the A412 Denham Way. The south portal will also be the place where staff are based who are building both the Colne Valley Viaduct and the Chiltern Tunnel. Some staff might live there some of the time, or commute to work there. All of the contractors working there have been asked by HS2 to provide travel management plans so they don't impact on local traffic. This might be car sharing for workers living in the local area, or minibuses picking up staff from local stations. They will not be allowed to park on local roads.

What is a TBM?

It is a Tunnel Boring Machine. Two will be used to create the two tunnels needed under the Chilterns. They are 10 metres wide and around two football pitches long (210 metres). When will you start digging the tunnels? Align, the consortium who will be digging the tunnel, is still working on its programme but currently estimates they will start tunnelling in late 2019. The first TBM will starting digging followed by the second one three months later. They will come out at South Heath in late 2023.

So who are Align and who are EK?

Align is the consortium of contractors responsible for the design and construction of the Colne Valley Viaduct, the Chiltern Tunnel and the north and south portals. At the north portal they hand over to the next consortium of contractors, EK, who will build the line from there onwards including the Wendover Dean viaduct.

What happens to the soil (spoil) they dig out?

It goes back along the tunnel to the M25 where it is largely spread across the fields there, immediately adjacent to the M25. Align is currently developing a landscaping scheme which proposes using the spoil to create new chalk grassland habitats, woodland and public amenity. It may be necessary to remove some unsuitable material via the M25. HS2 Ltd are its contractors are doing all they can to ensure as much of the movement of spoil is kept off the public highway which is why they are building the slip roads on and off the M25 at this location.

Where are the vent shafts are and what do they do?

There are five intervention and ventilation shafts which are Chalfont St Peter, Chalfont St Giles, Amersham, little Missenden and Chesham Road (near Hyde Heath). The intervention only shaft is at Chesham Road. The Intervention and ventilation shafts both allow for the emergency services to access the tunnels and also allow cooling of the tunnels and smoke extraction in the event of an accident. The intervention shaft at Chesham Road near South Heath (only) shaft has no ventilation function.

What about the vent shafts and the soil they dig out, where does that go?

The consortium of contractors who are building the tunnel (Align) are constrained by limits that are identified in the Code of Construction practice. Align is working on the traffic management plans for all the sites however they are aiming to do all they can to ensure the least impact on local roads. For EK, it is working on moving as much spoil along the line of route rather than use local roads.

If there is an emergency in the tunnel what happens to the passengers? Will they be evacuated through the vent shafts?

No. They will be evacuated from the train in the tunnel and moved to a safe area. There are cross passages within the tunnel to allow for this.

There are rumours the tunnel is going to be extended beyond South Heath?

Following a C1 and C2 review of the current location of the Chiltern Tunnel north portal, both ALIGN and EK provided HS2 with their indicative costs/savings. In summary; if the portal was moved 300m northwest, away from South Heath, it would cost the Project an additional £9M for the civil works alone. The ground conditions are such that extensive additional ground treatment would be required. There would also be no clear environmental benefit. Materials that the C2 contractor would have excavated from the portal, and reused elsewhere on C2, would now need to be sourced elsewhere. Chesham Road shaft would also be changed to a ventilation shaft, with increased local impacts. The additional railway systems costs have not been calculated, but could potentially double the civils figure. Consideration is currently being given to other ways of reducing environmental impacts at the north portal, such as a reduction in approach tunnel separation and hence track separation, to try and reduce the width of the excavation and the extent of the portal cutting. The design teams are also looking at slope gradients with the same intention to reduce the overall width of the Works.

Does HS2 own Annie Baileys pub near South Heath/Gt Missenden?

No HS2 does not own this pub and currently has no intentions in that regard.

Will staff be living at the vent shaft sites?

No. The only permanent accommodation site is at the south portal. There will be 24 hour security at each site, but no permanent accommodation, although there may be some overnight stays by staff if necessary. All main accommodation is at the south portal.

With the Chiltern Tunnel build what is the impact on the chalk aquifer and the water?

There will be no short, medium or long term impacts on the chalk aquifer. Tunnelling methodologies and materials are chosen to mitigate all geological and hydrogeological risks, with the end result that water quality, water quantity and water flows below ground and above ground are protected during the excavation and construction works and afterwards. Extensive ground investigation and groundwater monitoring works continue to gather information to inform the choice of methodologies and materials. Along with the extensive ground investigation which has already taken place, Align has commissioned an additional 43 boreholes, and geophysical surveys including sonar and ground penetrating radar studies. This information will allow Align to design its tunnelling technology to protect the aquifer and water supply during the tunnelling operation.

Has HS2 given any more consideration to the dangers of increased traffic flows to users of the many side roads/junctions off the busy main roads in the area and what about ran running by HGVs?

Safety is paramount, both for our staff and drivers as well as the general public and other road users. Align and EK are looking to raise the standards for our vehicles on the public highway. All drivers will be professionally trained and, we will put similar requirements on our supply chains. Drivers on the HS2 project will be subject to 'Rural Driver Training' which focuses on recognising the specific hazards of driving in rural environments, and how to mitigate those. All vehicles and suppliers will be fitted with safety equipment that complies with the Transport for London Freight Operators scheme (FORS). This includes on board dash cams, blind spot cameras and sensors and audible warnings for cyclists and pedestrians when the vehicles are making turning manoeuvres. Our highway engineers are also currently looking at junction layouts where we know they could be improved for safety. All HGVs will have clearly visible signs in their windscreens stating they are working for HS2. Any rat running HGVs must be reported by residents to the HS2 number - 08081434434 which operates 24/7. They are not allowed to use local roads through villages and town centres.

So when will HS2 be open to the public?

In 2023 the contractors building the line, including the tunnels, shafts, portals, viaducts from Euston to Birmingham will finish work then the team known as Railway Systems will fit out the line, with track, signalling, power supply etc. They will also test the trains built especially for HS2. The line is due to be operational in 2026.