Shakespeare Tunnel Design

[blimmo]

Shakespeare Tunnel has a slightly unusual design with a very tall and narrow profile. Is this due to the geology of the chalk cliffs in some way? I would have thought that a closer to circular design would be stronger.

 

[cats_five]

Shakespeare Tunnel has a slightly unusual design with a very tall and narrow profile. Is this due to the geology of the chalk cliffs in some way? I would have thought that a closer to circular design would be stronger.

I believe a catenary arch is strongest. It's like an inverted catenary curve which is the shape a free-hanging chain would assume. Obviously the pointyness of it depends on the length of the 'chain' compared to the width of the arch.

 

[J s]

A catenary arch wouldn’t have a point. A pointed arch like that would be called a Gothic arch. I can’t immediately think of any structural reason why a long pointed arch would be beneficial. Maybe it was done simply for architectural reasons?

 

[CarrotPie]

Isn't it because of the instability of the chalk?

 

[Gloster]

A bit of searching suggests that the tunnels were two separate bores because of the instability of the surrounding ground. Could the shape of the tunnel have been intended to allow the smoke and steam from the locomotives to escape upwards, rather than being forced into the restricted space around the coaches?

 

[J s]

Isn't it because of the instability of the chalk?

Unless I’m missing something, I’m not sure why that shape would be beneficial if that was your concern. If anything I expect that shape has worse resistance to lateral pressures.

 

[DaleCooper]

It looks to me like a normal arch with an additional central support. The outer curves, if extended, seem to form the arch.

 

[edwin_m]

If the ground is very fractured, I think a normal arch could end up with a pile of loose rock sitting on top of its horizontal portion, pressing it directly downwards. The Gothic arch eliminates the horizontal portion and the angle means only a small proportion of the weight of any loose rock would be pressing against the brickwork.

 

[Bald Rick]

When younger, I assumed the portals were mostly decorative,and the tunnel bores were normal size. However, when I walked through it I had to correct myself! My memory fails me (it was 20 years ago) but I’m fairly sure it’s unlined.

 

[stuving]

When younger, I assumed the portals were mostly decorative,and the tunnel bores were normal size. However, when I walked through it I had to correct myself! My memory fails me (it was 20 years ago) but I’m fairly sure it’s unlined.

Not at all! Railway Times (16 September 1943) reported that the tunnel was finished apart from laying the track. It also commented on the unusual Gothic (others said lancet) form of the arches, and the "numerous shafts and galleries towards the sea" which let in light - all unusual in tunnels.

It also said this: "The greater proportion of these tunnels is bricked in the customary way (and excellent specimens of brickwork they are), it being considered better not to trust, as was at one time intended, to the simple chalk." There were several reports of workers killed by falls of chalk during construction, and comments that this chalk was hard, but contained fractures that allowed large pieces to detach unpredictably.

 

[Pigeon]

Maybe it was done simply for architectural reasons?

I've always assumed that, since the portals are so very much taller than the height of the train. It's not uncommon for tunnels on lines that were built early to have a gigantic portal that tapers down to a more sensible size inside, Box west portal being a well-known example, so I'd just guessed that whoever built the Shakespeare tunnel was aiming for a similar effect but, having less taste than Brunel, thought that making it look like a couple of stab wounds would be the best way to achieve it.

However, this discussion seems to be implying that they're like that all the way through; is this actually the case?

 

[brad465]

While we're talking this tunnel's design, what exactly was done inside the tunnel to allow 395s clearance to use it, given previously only stock with gangway doors was permitted to use it?

 

[swt_passenger]

While we're talking this tunnel's design, what exactly was done inside the tunnel to allow 395s clearance to use it, given previously only stock with gangway doors was permitted to use it?

In general terms they enlarged a specific number of cross passages, so the escape route is through into the other tunnel.

 

[izvor]

However, this discussion seems to be implying that they're like that all the way through; is this actually the case?

As @Bald Rick states above, they are. I had assumed the same as him (that they were just ornamental portals), but a railwayman friend confirmed that they are the same profile throughout. It's rather bizarre, considering there are 2 other tunnels along this coast section (and formerly a third) which are double track and normal profile, and all presumably through the same chalk strata.

I've walked around the tunnel along the foreshore at low tide, and there are signs of the horizontal shafts up on the cliff face (also, IIRC, alongside Abbots Cliff Tunnel, the next one to the west).

While we're talking this tunnel's design, what exactly was done inside the tunnel to allow 395s clearance to use it, given previously only stock with gangway doors was permitted to use it?

That question was answered in this thread, post #44.

 

[Gloster]

As @Bald Rick states above, they are. I had assumed the same as him (that they were just ornamental portals), but a railwayman friend confirmed that they are the same profile throughout. It's rather bizarre, considering there are 2 other tunnels along this coast section (and formerly a third) which are double track and normal profile, and all presumably through the same chalk strata.

According to the .sremg.org.uk site, there were four tunnels, of which one, Martello, passes through gaunt clay and greensand, and was more difficult to construct. Shakespeare has two bores due to less stable chalk.

 

[DelW]

According to the .sremg.org.uk site, there were four tunnels, of which one, Martello, passes through gaunt clay and greensand, and was more difficult to construct. Shakespeare has two bores due to less stable chalk.

Gault clay is known in parts of southern England as "blue slipper" because of its tendency to instability. A stretch of the M25 that runs along Gault clay in west Kent has shallow 1 in 6 side slopes for that reason.

Whether or not that had an effect on the choice of tunnel profiles I can't say though.

 

[norbitonflyer]

Many years ago I saw a TV programme which said that the shape of the tunnels was because of geology.

 

[Gloster]

The latest edition of BackTrack magazine (Vol.40, No.3, March 2026 (dated)) has an article on the Ashford-Dover line by Jeremy Clarke. He says,”Cubitt [the SER engineer] made the bores of Gothic profile to reduce the pressure on the crown of the arch.”