[bow][bow][bow] Amen, RWM!
A few other points. With continuous welded rail, it is critical that the ballast have enough fractured faces to interlock strongly, and there there be a wide enough ballast section (at least 12" beyond the tie ends at the top) to resist the lateral forces that create sunkinks (also known as track buckles, thermal misalignments, etc.).
The petrographic properties of the rock are very important. It must be hard enough that it does not grind down under the passing trains, yet not so brittle that it cracks under load. During the crushing operation the raw material should result in ballast of the desired size range with enough fracture faces. The chemical composition should also be checked for undesirable constituents.
For lightly used lines just about anything can be used, including the traditional pit run gravel, because the roadbed is not highly stressed. The important thing is, as one of the other posters emphasized, that there be good drainage away from the track and roadbed. Even here though, the exception is round stones. They may provide good drainage but the track will float as if on ball bearings.
For heavy traffic lines, the ballast specification might require no more than 5% of the rock to be under 1" in size with most being over 1.5" up to a maximum around 2". For branch lines a greater percentage of smaller stones is acceptable.
There is a trade off between the higher initial cost of premium ballast versus cheaper ballast that will require earlier replacement and probably higher annual track maintenance costs. This decision is very dependent on each individual situation and requires personal judgement and local experience.
You will sometimes see “mud” pumping up between the ties. This is a sign of badly fouled ballast and not good. Traditionally it was assumed that this material is from the subgrade working its way up into the ballast material, so the apparent solution was to
" lightly used lines just about anything can be used, including the traditional pit run gravel, because the roadbed is not highly stressed. The important thing is, as one of the other posters emphasized, that there be good drainage away from the track and roadbed. Even here though, the exception is round stones. They may provide good drainage but the track will float as if on ball bearings"
Dumb statement - Dimestore lawyers, rubber-tired civil engineers and purchasing agents will never read the qualifying statements - so just don’t do it.EVER
Mainline ballast gradation goes up to 3 1/2 " (1 1/2" is yard ballast and 3/4" chips stuff stays in the yards and terminals)
The thought of using pit-run makes me ill. Yes, it was used rampantly back in the dark ages. And railroads didn’t make money and employed hundreds of thousands of section hands to tamp it every day, too.
Mud, please take a look at the rail question I answered over in the MR prototype forum and see if I got it right.
RWM
…How about using a road bed of engineered {with proper strength}, concrete with rails fastened right to it. Seems such a structure would be more permanent.
Why is such a design not suitable…? Is it cost…Is it not suitable and up to the job…Why not make the “route” a more permanent structure…? Must be some important reason as it’s not used…only at special places.
Don’t forget the ballast the Penn Central/PRR used through Indiana & Ohio. It was described as a “soft limestone” that became saturated and turned into something like a gooey slop. Here’s something I found on the Interlink:
Importance of Good BallastOne of the authors led a study in 1975 by a team of U.S. railway chief engineers for the Secretary of Transportation that evaluated rehabilitation needs for the bankrupt Penn Central Railroad.
On Penn Central’s main tracks, insufficient welded rail was laid on inadequate crossties and roadbed, except at stations where nothing had been done and speed slow orders were applied for safe operation over bad track. Rail was worn out, crusade condition was either barely adequate or unsafe; and ballast was insufficient fouled, and cemented. The subgrade had failed in many cases, causing costly derailments.
With the needs for rehabilitation this implied, a key recommendation by the chief engineers was to discontinue use of a soft limestone ballast and apply instead on principal main lines a harder, more expensive granite ballast hauled in special trains from a few high-quality quarries. The chief engineers viewed ballast quality as the key to rehabilitation, and their recommendation was accepted by Penn Central and the Secretary.
Quentin: The issue is the subgrade that “engineered” surface sits on. Pak-Trak has been around for a while (you find it in tunnels, platforms and certain road crossings). If the subgrade under the horribly expensive application fails, there is no easy (or cheap) fix. Any voids or subgrade swelling create nightmares. Along with that, your impact loading with pak-trac is not the same, will not distribute evenly and may actually make the subgrade fail faster. In short, it’s a balancing act tempered with real world experience.
…Fair enough…Now we have the answer.
A problem we’re running into right now is that we are undercutting in an area that has a lot of natural springs. We have water coming up from the ground and we have the typical drainage problems caused by coal dust. I’ve never seen mud like what we have in this area. It never even dries. The mud is churned up by every passing train to the point that is is perhaps 18 inches deep in some spots. No sooner do we dump ballast than we’re dealing with mud again…
This is quite true. In the late 1970’s early 1980’s, the former John T. Dyer Quarry - located about halfway between / just west of Birdsboro/ Pottstown, and east of Reading, Pennsylvania, off the former Reading Railroad main line there - (perhaps along with other quarries) was essentially reactivated for this purpose, and loaded and shipped literally a trainload (more or less) per day of very high quality trap-rock ballast to/ for ConRail. All of its stone production was to AREA ballast gradations - none of that road and highway stuff. It was common knowledge - and a source of some amazement to the local stone haulers
Quentin - With all respect, you’re close, but no - this [the above] is the answer.
In a perfect world, once the concrete track base is poured, it would stay within acceptably close tolerances for surface (dips and humps “up-and-down” looking ahead), alignment or line (slight swerves or kinks in tangents, and sharp and flat spots in curves, from side-to-side), and cross-level/ superelevation (one rail with respect to the other, or “banking” in the curves). However, the world is not perfect, especially not the subgrades that Mother Nature has given us to deal with. Sooner or later, the subgrade will start to move - there’s a whole litany of geological and soils engineering defects/ faults/ challenges that can casue this to happen - and then how do you adjust the track or rails to get it back to where it’s supposed to be ? That’s always the dilemma for the rigid-track designs. Conventional ballasted track construction can be resurfaced and relined for costs in the range of a dollar or two per foot = $5,000 to $10,000 per mile, depending of course on conditions, details, and available track time. Concrete track construction can cost several hundred to several thousand dollars per foot. Even if the concrete track was permanent, the economics favor the ballast track construction. More to the point, the ballst track construction could be resurfaced and relined every year (way overboard for most lines ! every 3 to 5 years is more typical) for 100 years before it equalled the cost of the concrete track.
- Paul North.
Erikem, you’re so right. For one example, a couple years ago the new cross-town expressway in Tampa, Florida had serious and expensive problems with its piers settling due to voids in the underlying limestone rock/ (sinkholes / “karst”), despite a supposedly serious and appropriate geotechnical investigation and responsive design. No reason to think that a mag-lev would be any less susceptible.
RWM - Have to disagree with you on this, at least as I understand your point. The purpose and theoretical need I think is clear - to provide high-speed ground transport between major cities where the distances/ conditions are such that automobile is either too slow or otherwise inappropriate (traffic/ pollution, etc.), and where using airplanes is a waste/ misallocation of scarce airspace/ airport capacity and too vulnerable to weather and other disruptions. Mag-lev might work in Utopia with unlimited funding, no right-of-way acqusition, environmental impact, or political issues, etc. However, aside from its inherent technical limitations - switches ? interchange ? network connectivity ? etc., with just a fraction of the money and right-of-way we could “incrementally” improve the conventional train-on-track system so much that the mag-lev wouldn’t offer enough of an advantage to justify itself. Example: Would any private enterprise seriously consider building a mag-lev to parallel one of the European high-speed lines ? Not h
Paul,
Thanks for the kind words and you pointed out my big reservation with Mag-Lev - i.e. what happens when the earth moves under the structure? There are many places in California where fault lines are slipping on the order of inches per year - and I would expect that to play havoc with the alignment of the Mag-Lev guideway. I probably should have been more explicit about my preference for high speed rail over mag-lev.
I’ve been amused by a local newspaper columnist going on about all sorts of “wonderful” alternatives to the former AT&SF Surf Line - one being a mag-lev line to be placed on the I-5 alignment (presumably as an elevated guideway). One is that the I-5 alignment would limit speeds to maybe 120 to 150 MPH. The other is that there are a lot of fills along the route and some have been notorious for subsidence (especially where I-5 and I-805 merge/split in Sorrento Valley) - and keeping the guideway in proper alignment would be hugely expensive.
RWM,
Put another way, Mag-Lev is an incredibly expensive solution in search of a problem.
- Erik
Paul: Understand comparing “perfect world” against the “real world”…