Moderator: Site Management Team
As at Hatfield there are cross connecting passages allowing users to move from one tunnel to another in case of fire.
Well of course.
KeithW wrote: ↑Mon Jun 25, 2018 09:58 Most such tunnels are under mountains or water of course and DO have either refuges or other escape routes. The Channel Tunnel has the third bore for evacuation and firefighting access. The Hatfield Tunnel has emergency doors to let drivers escape into the other tunnel in case of fire. Looking at the Stonehenge Tunnel plans I don't think an escape to the surface is practical for several reasons. At its lowest point the tunnel will be 20 m below the surface, the exists would be within the WHS site area and last but not least such routes would become in effect chimneys drawing hot air and fumes up them. Some form of escape route really will be necessary however on a new construction.
I guess that's why the proposed 200m concrete canopy at the western portal has these vents in it? However, English Heritage & the National Trust aren't too keen on the design. I'd not thought about them acting as 'chimneys' letting out car exhaust fumes, but I guess you could well be right. As for the substantial plant with a longer tunnel, this is something I've never heard the anti-tunnel protesters ever talk about. You've made a very good point here. Also, I don't think there will be any escape route inside the vented section?WHBM wrote: ↑Mon Jun 25, 2018 03:23 You are not going to get a 3.2km tunnel without some very substantial ventilation plant, with significantly noisy fans, as those who live near recent urban tunnels (Limehouse Link etc) know only too well. Can't do it any other way. I also suspect that there must be a need for emergency evacuation points to the surface along the way.
I think air quality rather than pressure the reason why road tunnels over a certain length need ventilation points. Pressure pulses are a problem in high-speed double track rail tunnels.
We might see a big increase in the number of electric vehicles by the time the tunnel is scheduled to open in about 2026. This might help with the air quality issue? http://www.nextgreencar.com/electric-cars/statistics/I think air quality rather than pressure the reason why road tunnels over a certain length need ventilation points.
That doesnt seem to be a problem at Hatfield, that is somewhat larger of course . The open roof section will also help in accustoming the drivers to adjust to daylight again, on the downside being rained on in a tunnel will be novel.
It will be interesting. And that will be a positive. Our infrastructure should be interesting.KeithW wrote:That doesnt seem to be a problem at Hatfield, that is somewhat larger of course . The open roof section will also help in accustoming the drivers to adjust to daylight again, on the downside being rained on in a tunnel will be novel.
The National Trust's response to the same location;Western Portal EH welcomes the mitigation Highways England has included in the scheme at the western portal location to protect the OUV of the World Heritage Site and largely supports the current design although would seek more detail about the size and exact location of the land bridge to ensure it maintains visual and physical connectivity between barrow groups in the western part of the WHS.
English Heritage considers that this is the most appropriate solution to removing the sight and sound of moving traffic from the WHS is to locate the western approach to the tunnel entrance in a deep cutting. This will be mitigated by a partial canopy and a green bridge which will help to maintain connectivity between monument groups. These new elements are to be welcomed. However along with other heritage partners, English Heritage has asked Highways England to consider a further or wider green bridge in this area.
An independent outline assessment of the potential impacts on OUV, jointly commissioned by Historic England and National Trust suggests that the proposed green bridge will do little to mitigate the impacts of the scheme upon the WHS at this location. It identifies the need for a wider land-bridge at a position east of the proposed green-bridge to provide meaningful physical and visual landscape connectivity between the Winterbourne Stoke and Diamond monument groups.
It's this very last bit from the National Trust which refers to the design of the 200m concrete 'grassed-over' canopy vents at the western portal4. Central section: Please provide us with any comments you may have on our proposals for the green bridge (No.4) at or near the western boundary of the World Heritage Site.
We consider that it is a positive addition to the scheme design to have increased physical connectivity in this part of the WHS. However, we consider that more work needs to be done with regards to the design of the road as it leaves the western portal to include mitigation measures which would best protect the OUV of the WHS and designated heritage assets. Based on the information that is currently available, our joint assessment work with Historic England indicates that due to the proximity of the new road to both the Winterbourne Stoke and Diamond barrow groups, and because it severs the visual link between them, the impact of the proposed scheme on these two monument groups, both of which convey attributes of OUV, is not acceptable without further mitigation.
Specifically, we do not believe that the land bridge options as illustrated in the consultation documents adequately mitigate the impact of the road as it passes between the Winterbourne Stoke and Diamond barrow groups. The first proposed location for this bridge on the line of the old A360 would provide no mitigation to the adverse visual impacts of the road on these two groups. While the second proposed location (as illustrated in Figure 5.18) is in an improved location, this bridge is currently of insufficient width to provide acceptable mitigation. Our assessment has suggested that a land bridge with a width of at least 150 metres would be required, but the exact length and design would need to be modelled and assessed to ensure it provided acceptable mitigation.
If a longer land bridge requires internal lighting beneath its length, we would expect Highways England to honour their commitment to no external lighting of the road within the WHS. We expect any land bridge to be designed in such a way so as minimise its visual impact on the landscape while helping improve habitat connectivity and access to the newly created restricted byway (if appropriate). For the avoidance of doubt we believe that the newly proposed byway that runs from north to south along the western edge of the WHS must be a restricted byway not open to motorised use. We look forward to separate discussions with Highways England about how agricultural and landowner access will be maintained and managed across the affected part of the WHS.
5. Central section: Please provide us with any comments you may have on our proposals for the cutting on the western approach to the tunnel. We welcome the proposal to place the road in a cutting on the western approach to the tunnel. Based on our current understanding we believe the proposed route of the cutting for the new road should have minimal impact on buried archaeology. It avoids the known sites and monuments that embody the special qualities of the WHS and lies in an area where (based on current survey work) we expect there to be few archaeological features.
Our previous assessment work has shown that were the new dual carriageway to be on the surface, it would have a highly damaging impact on the WHS. Our preferred design of a cutting, with vertical sides and rounded, grassed shoulders will minimise its physical presence and visual impact on the relationships between and settings of key monument groups within the WHS. The highly intrusive noise and visual impact of the traffic on the current A303 is damaging to the WHS and a sensitively designed cutting could reduce those impacts. However, while this approach does much to mitigate the impact of this section of the road on many of the monument groups, in order to protect the OUV of the WHS more work is required to mitigate the impact of the road as it runs between the Winterbourne Stoke and Diamond barrow groups (see Question 4). The proposed vertical sides will reduce the visual impact of the traffic and the road itself from outwith the cut. The sloped version would be more visible and increase the visibility of both the road and traffic from the key monument groups that convey attributes of OUV of the WHS. It would also require a fifty percent greater land take than the vertically sided solution with grassed top, and so would have a much greater physical impact on the WHS with the increased possibility of impacting on unknown archaeology.
6. Central section: Please provide us with any comments you may have on our proposals for the western entrance to the tunnel.
We consider that the proposed alignment of the road and the associated portal position presents the best opportunity to ensure the protection of the WHS – but only if appropriate mitigation is incorporated in the design. While the consultation document proposes a bored tunnel ‘of at least 2.9km’ our assessment has been carried out based on a 3km bored tunnel. Despite the extra 100m of bored tunnel length, our assessment work has shown that even with a bored tunnel length of 3km, the proximity of the tunnel portal to the Normanton Down barrow group would have a damaging impact on the OUV of the WHS without additional mitigation.
Highways England has proposed installing a further 200 metres of cover beyond the western tunnel portal. This could be either a 200m cut-and-cover tunnel extension or an extension of a similar length with a vented canopy at the end of the extension. A 200m extension has the potential to mitigate the impact of the road on the views between the Winterbourne Stoke, Diamond and Normanton Down barrow groups and as such our assessment work tells us that this 200m extension beyond the entrance to the bored tunnel is essential in order to protect the OUV of the WHS.
The canopy proposal could provide benefits of moving more of the tunnel buildings within the canopy as well as allowing for reduced vertical height at the tunnel entrance. However, we are concerned by the visual impact of the current proposed design of the ventilation openings. Even if an acceptable design could be achieved we would need to understand the practical and visual impacts of any fencing and additional safety measures that might be required around such openings as well as the impacts of any light emitted from those openings.
It does make me wonder if English Heritage & The National Trust are the major players here and most of the other consultation responses were just academic? It will be interesting to see the details of the revised plan and whether EH & the NT will effectively say 'yes' to the project moving forward? I guess it will now hinge on the length of the proposed A360 green land-bridge (which is intended to become a bridleway due to the road's realignment with the new junction) & whether the heritage bodies will continue to request an even longer one at this point and an extra (smaller one) one near the western portal?
However, along with other heritage partners, English Heritage has asked Highways England to consider a further or wider green bridge in this area.
And responded to. But no doubt EH and NT are the respondents looked for.jackal wrote: ↑Tue Jul 17, 2018 09:57 Supplementary consultation now open:
https://highwaysengland.citizenspace.co ... july-2018/
I read that as well. To get printed in the magazine it must be a reflection of the organisation position, if not the membership. Did say EH would continue to push for further improvements where necessary. Generally sensible approach.Tinpusher wrote: ↑Tue Jul 17, 2018 18:18 In the current edition of the EH members magazine, there is a piece written about the tunnel by an EH staff member. This person is pro tunnel. I think they've engaged positively this time. Say your piece but respect that the 303 needs to move...then accept the compromise.
Problem with a lot of UK decision making is, the all or nothing approach combined with belief that a shrieking minority represents millions of people.
