I’ve seen where the BNSF has laid asphalt for a roadbed for the ballast to rest on. Does this really help in drainage and keeping the ballast clean? If it’s a great idea, why haven’t railroads used it before and in large applications?
Nate
I saw that photo in Trains recently, pretty cool. In that situation it looked like they were putting down a completely new line, something that isn’t done very often anymore. This technique seems like it would add a fair amount of expense to a mile of track. It’s not something that can easily be done when working on existing lines, even if it is effective. I’m also not sure how well it would hold up under different climatic conditions.
It will be interesting to hear how well it’s working in a few years. Once it has seen a fair amount of traffic and a couple of summers worth of heat. Will the ties and ballast have squished their way into the pavement? Will the ballast keep enough heat off the asphalt to prevent that from happening?
I wonder if this would really work. It seems the weight of the trains would crumble the asphalt quickly even with the ballast helping spread the weight. Trucks, roughly 1/2 the weight of loaded freight cars, seem to do a number on the Interstates, which are paved very thickly with asphalt. Hmmm, maybe if the asphalt was very, very thick…
Jay
I think that was a temporary thing due to the specific spot on that jobsite. It also isnt like the track is being put right on the asphalt, just like track isnt on the dirt that is a few feet under every right of way.
Adrianspeeder
Remember that the rails will help to distribute the weight to some extent. On the other hand my pickup truck can leave marks in my driveway on a hot summer day. [:0][:p]
The photos are in the October issue, specificly on page 47. The caption states that they are using 6" of asphalt, and that they expect the ballast to become partially embedded in the asphalt.
For this project they are using concrete ties. during the construction process, the ties were set directly on the asphalt. But, once completed and ballasted, the track is lifted, and is resting only on the ballast. The ties are no longer in contact with the asphalt.
Remember that with out the asphalt - the ballast would be sitting on dirt. Also, last summer I watched the construction of a new highway crossing connecting the UP with the Niles Canyon RR. There a concrete slab was used in the same way as BNSF is using asphalt for about 20 feet on each side of the highway.
dd
The asphalt functions essentially as a self-healing subbase for the track structure, with a primary purpose keeping fines from contaminating the ballast proper. I had thought that something like roller-compacted concrete (RCC), which is a good method for making subbase for structures like airport runways, would be a logical basis for a new railroad line, but the asphalt is a more-reasonable-cost method…
Keep in mind that the pressure the asphalt ‘sees’ is distributed over the area of the broken bottom surface of the ballast prism – an advantage of the soft material is that these bottom faces ‘lock’ into place and have some resistance against lateral displacement. It’s not as if the ties are sitting on a blacktop road!
A funny thing about that: Back in the mid-1970s, there was a contest for college engineering students, IIRC sponsored by the Asphalt Institute, for the best use of asphalt in railroad applications. I believe, from the wording of the contest, the idea was to see whether paving equipment, perhaps slipform type, could make a meaningful bonded grade that would support trackwork directly. THe BNSF approach makes much better sense…
I concur that you won’t see much application of the technique to in-use mains – you essentially have to excavate the entire ballast prism, then ditch down for drainage, backfill, and compact, before the asphalt layer would go down. Operations to fill, seal, and bond low spots in the subgrade would require extensive ballast removal, but perhaps cheaper backfill – this is the Macadam principle with a vengeance!
…I believe we’re seeing a new railroad put down because it looks like they are constructing a new fly-over…
There is an old saying in the geo-tech/civil engineering business when construction standards and costs are discussed, in short “PAY ME TRUE COST NOW FOR QUALITY WORK, OR PAY ME A LOTS MORE LATER FOR YOUR SHORT SIGHTEDNESS AND CHEEPNESS OF THE PRESENT DAY.” Having done my fair share of work in both concrete and asphalt my feelings as to the merits of materials used in foundations and their construction would be something like this; The use of portland cement concrete in the building of railways, buildings, roadways and foundations of any kind is a long and established practice, I myself find its use to be a sign of high intellegence and good charater in the purchaser of the work and its structures. Yet when Concrete is not affordable (which is more often than not the case) asphalt is a very good material to use. It holds moisture out after fill has been given its final compaction roll when dry density is at an acceptable level (this is especially important where there is an expandable clay or silt in the geo-matrix) , and makes for a solid base for additional fill or balast, asphalt also has give and flexability properties that are excellent in high weight and directed pressure situations.
A few years ago there was a track washout after a rainstorm near San Juan Capistrano. When I went to check out the rebuilding effort, they had put down asphalt before rebuilding the track bed. (MP 195)
Stupid question - sorry to ask - What are FINES? Fines to me are something I have to pay when I park in the wrong spot.
Plain old dirt (and variations thereof)…
As in, its texture is finer than the ballast.
There are no stupid questions.
Fines = small size material (e.g. fine-grain). Think sand, silt, etc.
Thanks. OK, makes sense now. Fines would “upset” the flow of water through the ballest?
That’s one thing they do. You might think they’d ‘wash’ out, but not so…
More important, in my opinion, is that they get in between the pieces of ballast and spoil the integrity of what the ballast does.
Remember that plain old dirt has lots of rocks in it. Too much dirt in ballast essentially gives you track that might as well be laid on… dirt.
Fines in and around the ballast can physically work in between the rocks, and (via a variety of mechanisms) makes the trackwork less stable. There are aspects of physics which make migration of fines upward into ballast happen (I won’t bore everyone going into them)
There are a number of approaches to separating the ballast, which is a coarse material (think big gravel) which stabilises the track structure and spreads the load out to something the underlying soil can take and the underlying soil which, in most cases, can’t take the load of a train, especially when it’s wet. Asphalt happens to be a very good one, and it’s cheap and easy to lay to a reasonably exact grade. Roller compacted concrete works, too. So do certain types of ‘construction’ fabrics – but they are very expensive. I’ve used them all, here and there – depends on how much it costs and what you are going over (I’ve ‘floated’ a siding on a swamp once, some years back, on a rather difficult mix of finer and coarser ballast all wrapped up in a construction fabric. Wasn’t cheap, but it worked).
The discussion of materials rarely goes very long with lab practice getting into the mix. thanks folks, I still do miss it from time to time.
OK. If the FINES get in the ballast can the track start to shift/float as if on ice? How often does the ballast need to be cleaned or replaced?
With that being the case, during the railroad resturation going on here, there is a section of track under water from a large ditch running over it. Although the track workers were able to lift up the rails a bit and clean off the huge mess of sediment left by a storm, I was amazed that they didn’t do more. With such an obvious problem, why haven’t they done much more about this, and won’t the sediment from future storms washing over the track cause massive problems? Speaking of which, in winter, when that whole thing freezes, what will they do then?