Kennaway Tunnel - North portal design reasons

[alexross42]

Kennaway Tunnel at Dawlish has always struck me as of odd design, I can't recall seeing such a design/technique on any other tunnel. It almost looks as if two arched portals have overlapped and another portal was going to dug through the cliffs on an inland trajectory but was abandoned!

Also, the arch of the main tunnel isn't a perfect arch, the side that intercepts with the 'fake' portal appears to drop lower than the other side.

Can anyway shed any light on the engineering benefits behind this design?

A couple of links that might help illustrate:

https://commons.wikimedia.org/wiki/File:Dawlish_MMB_09_South_Devon_Main_Line_(Kennaway_Tunnel).jpg

 

https://www.google.com/imgres?imgurl=https%3A%2F%2Fc8.alamy.com%2Fcomp%2FCY07BY%2Fvirgin-trains-voyager-train-leaving-kennaway-tunnel-in-dawlish-CY07BY.jpg&tbnid=6qNxC42j4_709M&vet=12ahUKEwiAtfPo2JuEAxWPXaQEHeIgD_8QMygAegQIARBW..i&imgrefurl=https%3A%2F%2Fwww.alamy.com%2Fstock-photo%2Fkennaway-tunnel.html&docid=MSLNj5lTtEvQtM&w=863&h=1390&q=kennaway tunnel &ved=2ahUKEwiAtfPo2JuEAxWPXaQEHeIgD_8QMygAegQIARBW

 

Thanks,

 

Ross.

 

[Edwin_m]

At a guess, the retaining wall above the "blind" arch was there before they built it, but they needed to undercut it to get the track alignment they needed.  The "blind" arch is to stop the rest of the cliff falling on the railway.  They couldn't extend the retaining wall to track level because the track would be in the way, and making the tunnel a bit longer would have been more costly.  

 

The apparent asymmetry of the arch in the second picture is at least partly because it's skewed, again because of the shape of what is has to support above.  Hence the right hand side is nearer the camera and appears lower than the left.  You may be able to zoom the photos and count the bricks on the vertical part each side to see if they are actually at different heights above the track.  

 

 

[Siberian Snooper]

It may be because the  original line was single track and it was the best that could be done to get the best alignment in confined space between the cliff and the sea, without a prolonged line closure.

 

 

[Wickham Green too]

7 hours ago, Siberian Snooper said:

It may be because the  original line was single track ...

The tunnel certainly doesn't look over generous for two broad gauge tracks !

[JimC]

I'm pretty sure I've seen somewhere that this is the result of altering a single track broad gauge tunnel ino double track standard. ISTR in other cases along there the tunnel was removed completely, but could be wrong.

[eastglosmog]

The line was single in broad gauge days and in standard gauge until c 1906 (I think).  Had to be doubled on the landward side, otherwise would have had to move the sea wall out!  With the main road at the top of the cliffs, could not cut the slope back, so had to build the oblique arch to support the slope.

[Rivercider]

21 hours ago, JimC said:

I'm pretty sure I've seen somewhere that this is the result of altering a single track broad gauge tunnel ino double track standard. ISTR in other cases along there the tunnel was removed completely, but could be wrong.

Yes, the two tunnels, one at each end of Teignmouth station, were opened out when the line was doubled, prior to that Teignmouth station was a very cramped affair,

 

cheers 

[DCB]

There used to be a book on the Exeter - Newton Abbott line in Swindon Library, which described the tunnels.   All the tunnels between Dawlish and Teignmouth are asymmetrical due to being originally broad gauge single track  and following the gauge conversion were widened by erecting an internal structure over the standard gauge track and then hacking away at the Sandstone to widen the tunnel,  while the trains continued to run.  No need for a circular cross section as Red Sandstone  a nice material for tunnelling  nice and stable bit not too hard. I can't remember what it said about Teignmouth but there is a very very long down platform for arriving tourists still in place,  For some reason I spend more time watching trains at Dawlish and Teignmouth (and Aviemore) than I do anywhere within 100 miles of home.

Edited February 18 by DCB

[Rivercider]

1 hour ago, DCB said:

There used to be a book on the Exeter - Newton Abbott line in Swindon Library, which described the tunnels.   All the tunnels between Dawlish and Teignmouth are asymmetrical due to being originally broad gauge single track  and following the gauge conversion were widened by erecting an internal structure over the standard gauge track and then hacking away at the Sandstone to widen the tunnel,  while the trains continued to run.  No need for a circular cross section as Red Sandstone  a nice material for tunnelling  nice and stable bit not too hard. I can't remember what it said about Teignmouth but there is a very very long down platform for arriving tourists still in place,  For some reason I spend more time watching trains at Dawlish and Teignmouth (and Aviemore) than I do anywhere within 100 miles of home.

It is only in recent years that I have realised that Teignmouth once had two tunnels, though it becomes obvious once you know where they were. When the tunnels were opened out about half a dozen bridges had to be built, three of them at the east end. A number of low quality workers housing was removed at the time, and one of the Teignmouth churches had to be rebuilt on another site to make room for the additional cutting.

 This is a view from the lengthy down platform, it had been extended at least once to avoid down trains requiring to draw up to allow passengers to alight. This section of down platform is now disused and overgrown. The three 'new' bridges can be seen. 3/7/82

 

cheers

[DCB]

The  NLS   (National Library of Scotland)  Website has "Georeferenced"  maps of the whole of the UK  and has pre 1888 and pre 1914    25" to a mile maps for the Teignmouth and Dawlish area.   The dates are a bit elastic,  butthe earlier one is Broad gauge single track and the later Standard gauge double track.  There is no tunnel West of the sea wall at Teignmouth on either.  The double track broad gauge ran out along the sea wall and ended at the first tunnel towards Dawlish  There was a signal box there  just by that first tunnel.    I can't find any older maps,

[Rivercider]

16 hours ago, DCB said:

The  NLS   (National Library of Scotland)  Website has "Georeferenced"  maps of the whole of the UK  and has pre 1888 and pre 1914    25" to a mile maps for the Teignmouth and Dawlish area.   The dates are a bit elastic,  butthe earlier one is Broad gauge single track and the later Standard gauge double track.  There is no tunnel West of the sea wall at Teignmouth on either.  The double track broad gauge ran out along the sea wall and ended at the first tunnel towards Dawlish  There was a signal box there  just by that first tunnel.    I can't find any older maps,

The book you may be remembering is 'Exeter to Newton Abbot - A Railway History' by Peter Kay, which is packed full of history, dates, and photos.

Kennaway Tunnel at Dawlish was widened, and opened to double line in October 1905.

 

Parsons Tunnel signal box at the west end of Parsons Tunnel opened in 1884 when the line was doubled from there west to Teignmouth, it was later replaced by another box in 1906.

 

Teignmouth West Tunnel was opened out between 1879-81.

Teignmouth East Tunnel was opened out 1880-84.

The 'temporary' station at Teignmouth dating from the opening of the line in 1845 and was still in use when the broad gauge was replaced in 1892, the new station being constructed in 1893-95,

 

cheers 

[DCB]

3 hours ago, Rivercider said:

The book you may be remembering is 'Exeter to Newton Abbot - A Railway History' by Peter Kay, which is packed full of history, dates, and photos.

 

The 'temporary' station at Teignmouth dating from the opening of the line in 1845 and was still in use when the broad gauge was replaced in 1892, the new station being constructed in 1893-95,

 

cheers 

That's the book.  Not sure if they still have it, Swindon Central Library is not the easiest place for me to get to, anything over 100 yards from the car is a challenge for me these days.

Was Teignmouth's Temporary station on the same site as the present one?

[Rivercider]

1 hour ago, DCB said:

That's the book.  Not sure if they still have it, Swindon Central Library is not the easiest place for me to get to, anything over 100 yards from the car is a challenge for me these days.

Was Teignmouth's Temporary station on the same site as the present one?

Yes, the same site, but of course enlarged lengthways, I think when the tunnels were present shunt moves at each end of the station were into the tunnels.

The South Devon Railway stations at Starcross Dawlish and Teignmouth were all apparently meant to be temporary.

They were built out of wood in 1845/46 for the opening of the line.

Dawlish station burned down and was replaced in 1875.

Teignmouth station was rebuilt in 1893-96 when the tunnels opened out and the line doubled.

Starcross main station building lasted until it was demolished in 1981!

 

cheers

 

 

[Edwin_m]

How did single track sections work when this line first opened as an atmospheric railway?  A socking great pipe between the rails would make points rather difficult...

[JimC]

I've seen ropes and/or horses mentioned to handle points on atmospheric lines. I should have thought the inconvenience and delays would have been rather significant.

[Edwin_m]

2 hours ago, JimC said:

I've seen ropes and/or horses mentioned to handle points on atmospheric lines. I should have thought the inconvenience and delays would have been rather significant.

Indeed.  I guess the pipe would have to be gapped, so the train would have to detach from the piston and connect to a new one on the other side of the gap.  Someone would then have to remove the piston from the dead end of the pipe and re-position it for the next train.  If it failed to detach then the piston would slam into the end of the pipe and almost certainly break something.  

[Artless Bodger]

13 minutes ago, Edwin_m said:

Indeed.  I guess the pipe would have to be gapped, so the train would have to detach from the piston and connect to a new one on the other side of the gap.  Someone would then have to remove the piston from the dead end of the pipe and re-position it for the next train.  If it failed to detach then the piston would slam into the end of the pipe and almost certainly break something.  

According to Charles Hadfield in his book Atmospheric Railways the pipe gaps had self acting valves at the ends which would admit a piston to enter or leave without destroying the vacuum, the pistons remained attached to the piston carriages. On the South Devon there was an auxiliary 8" tube alongside the track to pull the train forward so its piston entered the main pipe on starting from stations. Gaps had bypass pipes so engines could exhaust either length of pipe.

[Rivercider]

4 hours ago, Edwin_m said:

How did single track sections work when this line first opened as an atmospheric railway?  A socking great pipe between the rails would make points rather difficult...

I think it was all rather inefficient.

As I understand it when the line between Exeter and Newton Abbot was opened as a single line there was no facility to pass passenger trains, there was only a single platform at each station, the loops at Starcross, Dawlish, and Teignmouth were all goods only.

Then there was always the possibility of a passenger train being braked too early and stopping short, or over-running the platform and then having to be manhandled back.

The atmospheric system might have worked tolerably well as a passenger only service with a regular service between two stations, but there seems to have been no easy way to provide points, or any kind of junction. The telegraph had not been installed at any of the pumping stations which meant that delays could not be advised, so fuel was burnt uselessly to create the vacuum before the next train was ready to depart.

 

cheers

[DCB]

There wouldn't have been much shunting back in atmospheric days and what there was would have been  by human and horse power, right up to the 1960s a lot of shunting was by a couple of porters putting a shoulder to a wagon.  Even the heaviest coaches were only a few tons and were habitually moved between tracks using wagon turntables  and man power even at large stations.  Atmospheric had a lot going for it.  The Piston carriage was far lighter than any steam or electric unit as it didn't need adhesion to haul the train as the force was applied by the piston direct to the carriage, it just n3eded to be heavy enough to stop it.    The pipe had gaps, so do electrified lines. but it was fast and that made it efficient, reducing track occupancy  to a level where Brunel believed a single track could pass the traffic which required  double track north of Exeter and with greatly enhanced safety as Atmospheric  trains could not run towards each other on the same track.  Nothing else (except cable haulage maybe) has this feature, and back before before block working this was a massive deal.   Dawlish station and maybe others was raised up so the gradient slowed approaching trains and speeded up departing ones.  Its just a shame the seals didn't work.  It's interesting to see  the way the original single track was widened, there are some  real examples of advanced bodgery  between Newton and Totnes nice stone bridge on up and tatty iron one beside it on the down line.