Track construction ballast depth

[Junior]

Looking at the current track construction standards it suggests that an extra 50mm of ballast should be specified in areas when sand blankets & geotextiles are required.

Is this to provide a layer of protection for the geotextile from an over zealous tamper operator?

Why else would you need the extra depth of stone?

What is the likelihood of a tamper operator dropping the tines too far these days?

Seems a bitdaft to protect the blanket but not worry about the formation in other areas.

thanks in advance.

 

[Joseph_Locke]

Deeper ballast spreads the load from the sleeprs wider. Modern machines are more or less fully automated.

 

[Romsey]

A "skim" renewal is 300mm deep from finished ballast top / sleeper tops.
A "deep dig" is 600mm normally done where the ballast is contaminated and therefore the sort of location which would have blanketting.
This gives some idea of the ground conditions at the bottom of a "deep dig", top soil mixed with coal dust and sand. The last full renewal was thought to be in the 1950's when no one had dreamt of 25t axle loadings let alone stone trains of 1700t trailing load over this route.

 

[BigB]

The last full renewal was thought to be in the 1950's when no one had dreamt of 25t axle loadings let alone stone trains of 1700t trailing load over this route.

Of course LNER had 25t axle loads for some of its locos, so not unheard of from the 1930s, but the increase in trainload has been significant, as has train speed.
Even the difference in practice since the early 70s is notable.
Network rail have some good videos on YouTube around tamping which are worth a look to understand it from that perspective.

 

[Romsey]

Of course LNER had 25t axle loads for some of its locos, so not unheard of from the 1930s, but the increase in trainload has been significant, as has train speed.
Even the difference in practice since the early 70s is notable.
Network rail have some good videos on YouTube around tamping which are worth a look to understand it from that perspective.

Yes a good point about LNER overweight express locos. However A3 and A4 class were not normally seen between Eastleigh and Salisbury. S15's did their bit on rolling out the substructure. RA9 is 23.0 to 24.1 tonnes in modern standards

 

[Bald Rick]

A "skim" renewal is 300mm deep from finished ballast top / sleeper tops.
A "deep dig" is 600mm normally done where the ballast is contaminated and therefore the sort of location which would have blanketting.
This gives some idea of the ground conditions at the bottom of a "deep dig", top soil mixed with coal dust and sand. The last full renewal was thought to be in the 1950's when no one had dreamt of 25t axle loadings let alone stone trains of 1700t trailing load over this route.View attachment 78353

Different to my experience. All measurements taken from sleeper bottom.

Full ballast renewal was 300mm below sleeper bottom. New ballast should have been compacted in 2x150mm layers (but rarely was). Geotextile membrane, where applied, was about 10mm, so discounted in the measurements.

‘Deep dig’ was 400mm, occasionally 450mm. That allowed for 100/150mm of sand and/or some track drainage.

‘Skim’ was, effectively, get the track up, take off the ballast around the sleepers (but not much below it), rough it up a bit, then put new track straight down (ie little / no new bottom ballast), top ballast on top, and then lift the new track up through the ballast to the required design. I never had a skim with more than 100mm dig beneath sleeper bottom. A

 

[Romsey]

Different to my experience. All measurements taken from sleeper bottom.

Full ballast renewal was 300mm below sleeper bottom. New ballast should have been compacted in 2x150mm layers (but rarely was). Geotextile membrane, where applied, was about 10mm, so discounted in the measurements.

‘Deep dig’ was 400mm, occasionally 450mm. That allowed for 100/150mm of sand and/or some track drainage.

‘Skim’ was, effectively, get the track up, take off the ballast around the sleepers (but not much below it), rough it up a bit, then put new track straight down (ie little / no new bottom ballast), top ballast on top, and then lift the new track up through the ballast to the required design. I never had a skim with more than 100mm dig beneath sleeper bottom. A

Actually I think we're in the same magnitude of digging allowing for measuring from the top or base of the sleeper. The terminology also varied with different renewals managers and contractors. So much for "one way of doing things" from IP Plain Line renewals, which normally meant how LNE did it.
I just sat there mentoring contractors staff on inputting worksites into PPS and train requests into varying Network Rail systems and picked up a bit of renewals terminology on the way!

 

[CJK64]

A "skim" renewal is 300mm deep from finished ballast top / sleeper tops.
A "deep dig" is 600mm normally done where the ballast is contaminated and therefore the sort of location which would have blanketting.
This gives some idea of the ground conditions at the bottom of a "deep dig", top soil mixed with coal dust and sand. The last full renewal was thought to be in the 1950's when no one had dreamt of 25t axle loadings let alone stone trains of 1700t trailing load over this route.View attachment 78353

Never have I seen a ‘dig’ determined from the top of the sleeper as there are so many sleeper types with varying depth measurements. You would need different standards for wooden sleepers, a different standard for F27 sleepers, a different standard for G44 sleepers and so on..
There is only one part of the sleeper that is the same no matter which type, and that is the bottom of the sleeper and that’s why all standards work from the bottom of the sleeper.
A skim dig is generally 50mm below sleeper bottom and a full or deep dig is either 250mm or 300mm and then your drainage.

 

[civ-eng-jim]

Looking at the current track construction standards it suggests that an extra 50mm of ballast should be specified in areas when sand blankets & geotextiles are required.

Is this to provide a layer of protection for the geotextile from an over zealous tamper operator?

Why else would you need the extra depth of stone?

What is the likelihood of a tamper operator dropping the tines too far these days?

Seems a bit daft to protect the blanket but not worry about the formation in other areas.

thanks in advance.

I've seen details where the 50mm is required for geogrids. From the bottom up, the geotextile is laid on the formation, then 50mm of ballast, then the geogrid and then your minimum ballast depth for line category. The school of thought is that sandwiching a combined geotextile-geogride between ballast and formation doesn't allow the geogrid to lock ballast together as effectively. Whether the installer does this is a different matter....Time constraints on site might make this an impractical approach.

Other reasons for additional ballast depth (in general) is that the ballast needs to be free draining - Every time you tamp, it creates about 4kg of ballast dust which works its way to the bottom of the ballast. Overtime, this clogs the ballast. A higher category line (Faster trains, more tonnage) the ballast will deteriorate more quickly and likely be tamped more regularly and therefore more ballast dust is created. The additional ballast depth for higher category line provides longevity to the ballast before it needs cleaning or renewal. On High Speed 1, ballast depths are 500-700mm deep. Mountaineering equipment is needed to cross the track.

If tamping tines hit bare formation it's not going to affect the integrity of that formation. If you punch through geogrids and geotextiles then their function is...well, knackered with holes every 600-700mm. So it's not protecting the sand blanket, but the membrane separating sand from ballast. I can't think of time where I haven't specified at least a geotextile membrane. There wouldn't have been much (if any) control over the materials used for earthwork when the railway was built. A lovely free-draining crushed-stone subgrade would be marvelous!

While the ballast does spread load to some degree, an increase of 50 - 100mm (From Cat6 lines to Cat 1A lines) isn't going to make a lot of difference. You can see during a plain line renewal that you're able to dig a near vertical face in the ballast and the adjacent track stays happily in the same place, often with an engineering train sat on it - so it isn't actually that great in spreading load.

Here's an example from Cardiff where tamping tines have clobbered the base slab (over a wide culvert). And the track on the right illustrate how shallow the ballast is - An extreme example.

 

[Joseph_Locke]

I've seen details where the 50mm is required for geogrids. From the bottom up, the geotextile is laid on the formation, then 50mm of ballast, then the geogrid and then your minimum ballast depth for line category. The school of thought is that sandwiching a combined geotextile-geogride between ballast and formation doesn't allow the geogrid to lock ballast together as effectively. Whether the installer does this is a different matter....Time constraints on site might make this an impractical approach.

The aim of all this being to spread the load more evenly over the formation, reducing localised high load areas and thus overloading, pumping and other formation symptoms

....

While the ballast does spread load to some degree, an increase of 50 - 100mm (From Cat6 lines to Cat 1A lines) isn't going to make a lot of difference. You can see during a plain line renewal that you're able to dig a near vertical face in the ballast and the adjacent track stays happily in the same place, often with an engineering train sat on it - so it isn't actually that great in spreading load.

Agree with you under a static load you can more or less stack ballast vertically, but run a DTS on the open track next to your hole and I you'll find the dynamic case isn't quite so rosy. Ballast has a repose angle around 1 in 1-ish, so raising the ballast depth from 200mm to 300mm more or less doubles the area at the base of the pyramid (taking the packing area to be 200mm square).

Here's an example from Cardiff where tamping tines have clobbered the base slab (over a wide culvert). And the track on the right illustrate how shallow the ballast is - An extreme example.
<image removed>

And a poor arrangement - a tamper trying to tamp a bridge deck does the tamper no good either! Ballast that shallow isn't tamper country and should have a big sign saying so.

 

[Junior]

Many thanks for all of your responses to my post, I appreciate it.

 

[mwmbwls]

How are the tests for asphalt based ballasting progressing - has it advanced from the university test bed stage?