Step Plate Junction

[Lucan]

Can someone please tell me what a step plate junction is? Seems to be particular to tunnels.

 

[swt_passenger]

Can someone please tell me what a step plate junction is? Seems to be particular to tunnels.

Mostly used as a term by London Underground. Think of the diameter of a deep bored tunnel formed by iron or concrete segments. At a junction they have to gradually increase the overall diameter, and eventually split into two separate tunnels, so every so often there’s an increase and a need to adapt to the sizes of the different adjacent segments. This might happen two or three times over the length of the junction. The point where the diameter increases can be seen from within the larger space as a flat plate with a hole the diameter of the smaller tunnel. This is a step plate.

Much of a step plate junction can be built around an existing running tunnel, this was done recently in order to tie the Northern Line extension into the existing Kennington loop tunnel, there are pictures here:
https://www.globalrailnews.com/2018...ls-over-new-northern-line-extension-junction/
In this case there seems to be only two steps and the most visible ‘plate’ is formed from concrete, but in earlier constructions there’d often be more steps and a steel or iron step plate.

Hope this has helped.

 

[swt_passenger]

Another article here has a very good photo showing the ability to build the new large diameter junction tunnel around the existing running tunnel, and keep trains running through while it was being done:

Over the weekend of the 15 - 18th September, a major milestone in the delivery of the London Underground Northern Line Extension (NLE) was achieved. Having constructed the enlarged tunnel around the existing cast iron lined tunnel of the operating Northern line tunnel (Charing Cross branch), at each of the two locations where the NLE connects with the current railway, the existing cast iron tunnel rings were removed above existing track level. The newly created enlarged sections of tunnel, known as step plate junctions, then became integral in the operating Northern Line.

http://www.infoworks.laingorourke.c...ec/lifting-the-lid-off-the-northern-line.aspx

 

[Lucan]

Thanks for that. I knew the feature without knowing the name for it (I have worked in a few LU tubes).

 

[Taunton]

I've never uinderstood how these junctions often managed to get built around the operational railway, with just a weekend possession to knock the last bit through, but when the Piccadilly Line extension to Terminal 5 was built, the loop round through Terminal 4 which the new work tied into in this manner had to be closed for nearly two years 2005-6 to achieve the same.

 

[Emblematic]

I've never uinderstood how these junctions often managed to get built around the operational railway, with just a weekend possession to knock the last bit through, but when the Piccadilly Line extension to Terminal 5 was built, the loop round through Terminal 4 which the new work tied into in this manner had to be closed for nearly two years 2005-6 to achieve the same.

It always sounded quite hairy, with the old tube tunnels propped up on jacks and trestles within the new ones. Just not sure you'd be allowed to do it quite the same way today, hence the longer closures. To quote L P Hartley, “The past is a foreign country; they do things differently there.”

 

[swt_passenger]

It always sounded quite hairy, with the old tube tunnels propped up on jacks and trestles within the new ones. Just not sure you'd be allowed to do it quite the same way today, hence the longer closures. To quote L P Hartley, “The past is a foreign country; they do things differently there.”

The recently modified northern line running tunnels were held up with poured concrete AIUI, the first picture in the linked article in post #3 shows the existing tunnel operational within the enlarged. I don’t think the closures were much longer than historically?

There’s a YouTube video that shows what was done:

 

[Bald Rick]

I've never uinderstood how these junctions often managed to get built around the operational railway, with just a weekend possession to knock the last bit through, but when the Piccadilly Line extension to Terminal 5 was built, the loop round through Terminal 4 which the new work tied into in this manner had to be closed for nearly two years 2005-6 to achieve the same.

Presumably because the Junction is directly beneath the T3 apron and one of the piers, and ground movement there would have very significant implications. One imagines Heathrow were also cognisant of previous tunnelling efforts in the area.