GWSR General Discussion and Operations

The line is contemporary with the Bicester cut off (the Chiltern line). The latter has suffered with embankment issues. This hasn't been helped by soil moisture deficits over dry summers with bands of clay being most susceptible. There is a band which runs across the Chilterns from east to west and I wouldn't be surprised if it reaches to the Gloucestershire and Warwickshire line. When a railway is renewed it is unusual to replace the earthworks, (rails, ballast, signalling systems, bridges are normally what is replaced). Also the knowledge of what makes up an earthwork is not well known surveys can be taken but I am aware of instances where the material suddenly changes.

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The question that has been in my mind since Chicken Curve has been why this line has been so prone to problems

 

Hey, that's our railway you're talking about there!

I think it's accepted wisdom that the line was built quickly, and on the cheap. Combined with the particular local geology and geography, and lack of understanding at the time of construction, we are where we are. The good news is the impact is much less severe than in the past, in part I think we can put that down to the heroic efforts of our drainage team over the last decade or so.

 

Prompted by this thread, I spent some time trying to find out some details of the local geology. For anyone who has a little knowledge of soil mechanics or geology, these links might be of some interest.

https://www.researchgate.net/public...and_outcrop_patterns_from_temporary_exposures

https://webapps.bgs.ac.uk/Memoirs/docs/B01641.html Ch.2 in particular.

http://www.glosgeotrust.org.uk/downloads/Cotswolds LGAP.pdf

 

As Alex said, it's a combination of factors: local geology and geography, and lack of geotechnical engineering knowledge (i.e. the behavior of structures constructed of piles of earth) at the time of construction. Of these, I would rate the last as the most significant - although the geography, which required lots of embankments, played a significant role. Not so much the geology; there have been a few minor issues with the sides of cuttings, but nowhere near as bad as the embankment issues.

I've been thinking about the geotechnical engineering issues, wondering if I could have done a better job, with the knowledge available at the time; I'm not sure I could/would have. Take the example I gave, where, when they dug down in a slumped embankment, they found a buried grass layer, where the earth of the original embankment had just been dumped straight onto the pre-existing turf (thereby providing a shear plane for the embankment to slide on). If I were given the job of building an embankment today, even without studying geotechnical engineering, I'd cut a trench in the turf, to tie the embankment in - because I now know of that failure mode. But in 1904? There's a good chance I wouldn't have.

Several other lines have also had geotechnical engineering issues on embankments: e.g. the Bluebell (section of embankment South of Kingscote station); the NR N-S line at the Robertsbridge end of the RVR (of which they said "As part of the process of stabilising the bank [the contractor], dug out much of (what they call) peat packing material, being that used in the original construction back in the late 1800's, before replacing this with a more proven rock based packing/fill. Such were the construction methods of the age"). I vaguely recall some others, but can't be bothered to look for them.

I think the GWSR has seen worse than these others only because it has a lot of high embankments.

Noel

 

The GWSR Honeybourne line was a late build, one of the last in the country. It is pretty straight and level, meaning long cuttings and long embankments.
It wasn't built by men with wheelbarrows ( eg Tring cutting) but by mechanical steam excavators and a temporary railway line with tippler wagons. So, lots of soil was moved around.
Slipping soil has been an issue for a long time. There is both anecdotal and physical evidence of previous slip repairs, certainly during BR times, and probably earlier.

 

All fair, yet these embankments are 50-60 years younger than many on the national network seeing much greater use (I’m listening to them go by from my desk!). If what you write is correct (and I’ve no reason to doubt it), the question then is what the earlier builders got right and later builders didn’t.

 

I wonder what the contractual defects liability period was? Is any warranty remaining?

 

Maybe those heavily-used lines on the national network got better maintenance; maybe they had improvements during those long lives. You'd need to study the detailed records for each.

Noel

 

Many mainline embankments have had significant works done over the years to be fair, much larger scale than we can contemplate affording proactively.

 

Interesting is that the section north of Broadway to Honeybourne West Loop (say 4 miles) is perfectly flat, with no cuttings or embankments.

 

Well, that's the Vale of Evesham for you

Tom

 

I don't think there is a "one size fits all" answer - you would have to look at each structure and dig into its history, the local geology, subsequent maintenance etc.

One issue on some lines - certainly an issue on the Bluebell - is the extent to which ash was used in construction. Normally when surveying a line, you would try to balance the cuttings and embankments, so that earth removed from a cutting could be used on a nearby embankment without having to be transported too far. But where that wasn't possible, you would have to bring in material, and as the railways developed, one material they had in abundance and which was essentially free except for the transport was ash. So land would be built up using ash, but I doesn't necessarily form a very stable structure. In heritage railway terms that problem is exacerbated by the fact that frequently, the lines we now have as heritage railways were often - not universally, but often - lightly used and built on the cheap.

Tom

 

I suggest that a valid comparison in terms of contemporaneous main lines would be the GCR London Extension?
This was also built post the pick and barrow era. I’m not aware of any history of structural failures there?
Not sure what conclusions can be drawn - different geology, ground works design, contractor?

 

Fair comment - and part of my curiosity

 

Ash may have been a factor in the most recent GWSR slip - "the soil here .. appears to be mostly ash and spent ballast."

Odd that there's spent ballast in there too - that all can't date to the original construction of the line, then! As @Breva mentioned, "There is both anecdotal and physical evidence of previous slip repairs, certainly during BR times, and probably earlier." So not just used in construction, but also in 'repairs'.

Noel

 

I recall that there was a reference some years back (might have been in this forum or possibly the GWSR's own web site) to an account by GWR engineers of remedial works carried out in the 1920s. Two pieces of work were highlighted. Firstly a rebuild of the "chicken curve" embankment, which alas did not prevent mulitple recurrences in later years of problems at this location. Secondly, the construction of a retaining wall on the east side of the cutting just north of Toddington station. That retaining wall is visible today and, as far as I am aware, has done its job successfully.