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embankment dimensions (GWR)

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Whilst thinking about building a short section of embankment, I've been rummaging around for suitable dimensions. So far I've found little other than some old drawings reproduced in 'The Making of a Railway' by L. T. C. Rolt, a fascinating book about the building of the Great Central. These show a two track standard gauge main line with a width of 21' 6" between the outer balast shoulders, and a flat top to the embankment 31' wide. The slope of the sides is said to be variable.

 

What I would really like to know are

  • the equivalent dimensions for an embankment on the original GWR broad gauge main line (for example, think of the section between Twerton tunnel and Saltford)
  • typical and maximum slope angles of the embankment sides

Any ideas?

 

Nick

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Hi Nick,

 

whilst not specific to GWR practices, this came up on the old RMweb. See:

 

http://www.rmweb.co....t+angle#p494427

 

There may be something there that will be of some help

 

Thanks Steve. Plenty of disagreement there :icon_rolleyes: I'd searched for dimensions, but not slope, as that only occurred to me when posting. There was a post by Miss Prism including some dimensions, unfortunately not dual track broad gauge.

 

Nick

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In 1838 Brunel submitted a report to the Directors comparing the land take of his broad gauge with standard gauge lines. In it he stated that the maximum width (I take this as the loading gauge width) to be allowed for a single rail (=track) was to be 13 feet and to be 26 feet under bridges and through tunnels (for a double line) which resulted in the width between tunnel or bridge sides coming out at a constant 30feet.

 

The 30feet width dimension also applied to embankments - which hopefully answers one of your queries? According to a drawing in MacDermot the original scheme ballasted the track to top of longitudinal sleeper level and carried on at that level to the top of the embankment slope. The distance between adjacent running lines, measured from the running edge of the rails, was 6ft 2.5" which would have left a total of 9ft 9" outside the cess side running rails evenly divided between the two sides - i.e. a cess 'path' 4ft 10.5" on each side (measured from the running edge of the cess side rail).

 

Nothing on the slope of the embankment I'm afraid but as I understand it that would in any case depend on the material being embanked.

 

And hopefully I'll get back to tail traffic etc this coming weekend :D

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Thanks, Mike, that is very helpful :icon_thumbsup2: I had most of the basic dimensions, but that overall width of 30', and hence the width of the cess on either side, were what I was missing. As to the slope, Yes, it would depend on the material. I've been checking some of my own photos and those in various books and, despite some of the arguments on the old RMweb that Steve pointed me to, the embankment near Newton St Loe looks very steep, possibly around 1.5:1. On the other hand, some of those near Keynsham appear closer to 2:1. The trouble is, no one ever seems to photograph them from the right direction :rolleyes:

 

Looking forward to the next episode of tail traffic :D

 

Nick

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Nick, FWIW, here's the double-track version of the standard gauge sketch, which was posted on the old forum, but I'm not sure where. The ballast depth is an idealised form, as discussed in the old thread.

 

post-133-12568653111881_thumb.gif

 

Here's the single-track version, for the sake of completeness:

 

post-133-12568657629018_thumb.gif

 

Thanks to Mike for the interesting BG notes.

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Thanks, that adds confirmation of the maximum recommended slope :icon_thumbsup2:

 

Nick

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The actual slope of embankment or cutting depends on the material being used to form it and for embankments the underlying strata for stability ie. the natural angle of repose. Slopes made of chalk are about 30 deg from the horizontal or less wheras rock can be near vertical in a cutting and about 45 deg on an embankment. HTHs

 

Cheers Phil

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The actual slope of embankment or cutting depends on the material being used to form it and for embankments the underlying strata for stability ie. the natural angle of repose. Slopes made of chalk are about 30 deg from the horizontal or less wheras rock can be near vertical in a cutting and about 45 deg on an embankment. HTHs

 

Cheers Phil

To aid stability, and reduce land-take, I believe Brunel actually installed a core of chained-together timbers in some locations. Unfortunately, with the passage of 150+ years, these materials have decomposed, which may be why there have been some embankment slides, and others averted by pre-emptive works, on the Western. Before he retired, my brother-in-law was involved in quite a lot of work on this around Bristol, Swindon and Gloucester.

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The actual slope of embankment or cutting depends on the material being used to form it ...

 

Yes, indeed. I remember once seeing a table of the natural angles for a wide range of materials, but can't remember where it was. If anyone, maybe a civil engineer, knows of one and can post the details here, I think it would be very useful.

 

One thing I wonder about is whether embankments like that I mentioned earlier between Twerton tunnel and Saltford were built with some form of stone core. It's difficult to imagine how they would achieve such steep sides otherwise. Of course, my hazy memory may be exaggerating the steepness as I've only seen it from a distance in recent years and probably haven't been really close to it since they stopped running steam engines over it :rolleyes:

 

Nick

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To aid stability, and reduce land-take, I believe Brunel actually installed a core of chained-together timbers in some locations. Unfortunately, with the passage of 150+ years, these materials have decomposed, which may be why there have been some embankment slides, and others averted by pre-emptive works, on the Western. Before he retired, my brother-in-law was involved in quite a lot of work on this around Bristol, Swindon and Gloucester.

 

Not quite - Brunel laid the original section of line from the London end with the transoms between the longitudinal sleepers attached to piles driven into the sub-base. The idea was to make for smoother running but the result was the opposite so the piles were abandoned in later construction (west of Taplow I think) and the original ones were either removed or driven down out of the way. And as far as I know there have not been any major problems with embankments on that section.

 

The embankment slip problems occur in a number of areas on ex GW lines including the Badminton cut-off which was built long after IKB popped his clogs and also on bits of the line down to Pilning which came after his time. The problems are, I understand, mainly down to ground conditions and not constructional issues although the major slip area at the top of Dauntsey Bank (which was finally put right some years back) was due to problems with a stream and an inadequate culvert although some of that might not have entirely been constructional as teh stream had reportedly moved. The slips between Uffington and Shrivenham which have also received a lot of attention in recent years were, I suspect, probably more to do with later quadrupling work rather than the original building and the slip at Twyford back in the late '60s was put down to gravel extraction in the area as the bank had been stable up until then.

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Civil engineering text books will tell you about what slope angles a given material can be formed in, and embankments will of course be made of reworked material - often from adjacent cuttings so that the net requirement of fresh material during earthworks is minimised. Even a 1v:1.5h slope is really steep (try walking up one), and many embankments will be shallower than that. If land take is a major issue, e.g in urban areas, then the answer would have been to construct a viaduct/arches of some kind instead.

 

Cuttings on the other hand will almost always be through natural in-situ material. This might be competent rock, in which case sides can be almost vertical, or superficial materials (sand, boulder clay etc) or weathered rock which is weaker than its fresh equivalent. Drainage and superposition are also important (c.f. Chalk on clay in Folkestone Warren). So every site will have its own characteristics and design criteria.

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Yes, indeed. I remember once seeing a table of the natural angles for a wide range of materials, but can't remember where it was. If anyone, maybe a civil engineer, knows of one and can post the details here, I think it would be very useful.

 

One thing I wonder about is whether embankments like that I mentioned earlier between Twerton tunnel and Saltford were built with some form of stone core. It's difficult to imagine how they would achieve such steep sides otherwise. Of course, my hazy memory may be exaggerating the steepness as I've only seen it from a distance in recent years and probably haven't been really close to it since they stopped running steam engines over it :rolleyes:

 

Nick

 

One point in that are Nick is that there was rock waste from the tunnelling in that section of route and presumably it would have been used in the embankments unless it couln't be physically got to the site.

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Yes, indeed. I remember once seeing a table of the natural angles for a wide range of materials, but can't remember where it was. If anyone, maybe a civil engineer, knows of one and can post the details here, I think it would be very useful.

 

 

Something from Google

 

http://www.civl.port.ac.uk/britishsteel/media/BSCM%20HTML%20Docs/Angle%20of%20repose.html

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One point in that are Nick is that there was rock waste from the tunnelling in that section of route and presumably it would have been used in the embankments unless it couln't be physically got to the site.

 

Yes, there would have been plenty of waste from Twerton tunnel, as well as Nos 1, 2 and 3 tunnels near Bristol, and the short Saltford tunnel. All of these would have been cut at about the same time as the embankment was built. If I remember correctly, the Bristol-Bath section was built as one operation.

 

Something from Google

 

http://www.civl.port...f%20repose.html

Thanks, Rod. That's just the sort of thing I was thinking of. Of course, the raw figures need to be tempered by consideration of load-bearing capacity when compacted, but it's a start. Interesting to note an angle of 45 degrees (i.e. a 1:1 slope) for rubble and most rock types.

 

Nick

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Just to round this off as my original question has been effectively answered, at least until someone comes up with some real measured drawings... Thanks to all for your input :D

 

By the way, the 30' figure for the trackbed width is supported by the caption to plate 419 in Vaughan's 'A Pictorial Record of Great Western Architecture', where he notes that the span of Brunel's bridges between Bristol and Bath varied between 29' and 31'6". He also notes that the distinctive pointed arch shape of these bridges leaves no room for track slewing, confirming that the broad gauge required no more room than standard/narrow gauge.

 

Nick

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