A modeling/engineering question

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kjd

Go make something!
Several years ago there was a single track line that saw maybe 1 or 2 trains a day (the far track in the photo). It passed through a narrow valley next to a small stream. Then a large industry, a coal mine, was built in the canyon. Most of the infrastructure is above the tracks on the hill but the loadout is obviously over the tracks. About the time the mine was built there were several mergers and abandonments which increased traffic on the line to about 15 trains a day. The loadout couldn't be over the main so the main was moved over (the near track) and the old main (far track) became the siding for the loadout. There was a short bridge, about 25 feet, over the stream at this point as well. To keep the loadout facilities out of the 100 year flood plain, the track was raised several feet. The mining company also needed an access road to the loadout.
Between the extra track, the road and elevating the existing track, the stream banks were over-steepened. What would be done there? A concrete retaining wall? The stream curves around from the left on the far side of the roadbeds and it running downhill towards the camera. It is not apparent in the photo but the abutments are not perpendicular to the track. How are bridge ties arranged in such a situation? I have seen photos of ties both across the abutment and parallel to the abutment and not perpendicular to the rails. Is tie placement in such a situation a railroad or site specific call? The tracks are about 22ft center to center, how would the abutments be built because there isn't quite enough room for two abutments with wing walls but it is a little far for a single abutment? Close to what I have imagineered with the new (closer) abutment running into the old? How much support would be required under a ballasted deck bridge 25ft long, ie the Micro Engineering ballasted deck left over from a previous project? I know there are some engineers here who work on similar projects and could perhaps offer insight. Thanks,
Paul in Seattle
 

RCH

Been Nothin' Since Frisco
I'll take a crack at answering this, Paul, but it will take some time. What is the approximate year of the reconstruction?
 

RCH

Been Nothin' Since Frisco
Okay, I'll take a stab this evening. FWIW, I am a land surveyor with a background in railroad construction staking, roadway design and retaining wall construction. I hope my answer won't come off as too "technical" but I'll give it my best shot.
 

kjd

Go make something!
Technical is fine. That is interesting to me. The original line was built early in the 1900s as a main then downgraded and then saw a resurgence in the 90s.
Paul
 

RCH

Been Nothin' Since Frisco
The world's longest answer...

“The three most important things in civil engineering are drainage, drainage and drainage.”
- Bryan Duff, P.E.



As to the question regarding tie placement on ballasted deck bridges with a skew built in, in the few cases I’ve seen the angle of skew is not very great and only a single tie on either side of the gap between bridge and terra firma is askew. Generally, the angle of the askew ties matches the angle of the bridge. It wouldn’t take much of an angle, perhaps 15° or so, before placing ties across the gap would become the only option.

As far as the layout of your abutments is concerned, I think the volume of water the stream sees at peak periods plays a key role in the position of the abutments and the shape they take. The direction of flow and any redirection that occurs would change the shape of the abutments as much as the surrounding terrain, slope, soil types, etc.

From your description of the stream, it sounds like some change in direction occurs as the stream approaches the bridge. The outer portion of the stream's curve would need to be protected against erosion, since there is a greater probability erosion will occur there. Likewise, the inner portion of the curve should be excavated more steeply and the abutments reinforced there as well, perhaps with a concrete apron, to permit flow and prevent alluvial action.

With respect to the type of abutments to be constructed, I think a reconstruction effort on the scale you're talking about, i.e., a vertical re-adjustment of the alignment, would necessitate totally new abutments. It's possible new abutments could be constructed on top of the older ones, but based on your description of the traffic the rail line saw in the past, I doubt it would have been built to support the existing traffic seen on the line at the time of the construction project, let alone the weight of a new alignment built on top of the old one. So, I surmise that it would be cheaper to construct a single twin track abutment and embankment than two separate ones, despite the tracks being separated by 22 feet. Moving earth to create the subgrade for an elevated earthen right-of-way such as this, gaining compaction, etc., is quite an undertaking and could be simplified by treating both sets of track as a single unit for the purpose of this phase of construction.

However, all bets are off if traffic has to be allowed for during the course of construction. In that case, a temporary third track might be constructed outside the limits of the two existing tracks to allow for reconstruction of one of the tracks at the new vertical alignment. Upon completion of the new vertical alignment, the second track would be constructed. Its completion would allow for the removal of the temporary third track.

Nonetheless, a single monolithic abutment, whether constructed at once in a single pour or in a modular fashion, would be the best choice for abutments with adjacent tracks being as close together as 22 feet. A drainage swale would need to be provided between the tracks outflowing to the stream. Once the runoff reaches the abutments, it would most likely be removed by sheet flowing the water over the top of the abutment and would likely include a concrete apron to facilitate this.

Now, if your layout depicted the tracks before the vertical alignment was changed, there is definitely reason to believe that two abutments, most likely of a different construction style, if not technique, would exist in a fashion similar to what you have. I believe they are undersized, in terms of the thickness of the concrete, though. I would expect them to be no less than 12 inches in width at the top for the size abutments you have in place. The one thing that bothers me about the abutments as you have them currently is the crook between the two on the right, where the wing walls converge. That creates an area on the outside of the curve of the stream where water will swirl and rapidly erode the subgrade. The outside of the curve of a stream should be free of any impediment to directionalizing the flow of water downstream, although it’s not uncommon to find drainage structures such as gabions placed along the stream bottom to slow the speed of flow. The point here is that creating a turbulent situation which favors erosion is bad. Creating an impediment to slow the flow is good. So, even though the scene as depicted in the photo is not intended to be the earlier version of the alignment (i.e., in the floodplain), it could easily pass for such a design. For the modern version of the abutment much of the philosophy stated here was considered in my design recommendation of a single monolithic abutment vs. two independent abutments.

You mentioned the vertical alignment change affected how wide the footprint of the embankment is. You are correct in anticipating this and correct in forseeing one of the solutions to mitigate the problem could bea concrete retaining wall. There are other solutions that are cheaper than a concrete retaining wall, but sometimes what’s most effective isn’t cheap. You can do it right and pay for it once or do it wrong and pay for it twice. So, with that in mind, here’s a couple guidelines to designing prototypical drainage side slopes.

Based on the surveying I did for ATSF, there are three distinct side slope situations you can have: shallower than 1:1 (commonly 2:1 on high embankments), 1:1 slope, steeper than 1:1 slope. An earthen embankment such as what you've described can be constructed without reinforcement with a 2:1 or shallower side slope. Where water would run roughly parallel to the embankment, the portion of the embankment that will get wet would have riprap placed up to the stream bank to slow erosion. Other than that, no soil stabilization would be necessary other than vegetation.

If the catch point (where the design side slope intersects the existing ground) prevents a 2:1 side slope from being used, up to a 1:1 slope could be used with reinforcement, such as steel or concrete pilings, gabions, or a concrete apron. 1:1 is the magic number, so to speak, since the load at the top of the embankment is borne by the earth underneath it diminishing downward at a 45° angle on each side. The 1:1 slope means that the outer limits of the embankment is also where the load bearing portion of the embankment ends. A 1:1 slope isn't unstable, but since it's at the limit of design, it should be reinforced.

Any slope steeper than 1:1 requires some serious reinforcement, since the load must be borne in an area smaller than is necessary. Engineered retaining walls come into play here and they are expensive to construct. They must have a rather large, deep footing, and when they serve as a bank of a stream in addition to retaining earth and bearing a load, they must be impervious to water. If that's the case, they are generally monolithic slabs poured in place. If the catch point impacts the stream but doesn't necessarily cause the retaining wall to act as the bank of the stream, I think it would be possible to construct the wall out of an interlocking concrete material, such as what is used on modern interstate bridges. Walls can even be constructed out of gabions, but I can't think of a case where the gabion wall is load bearing (i.e., an engineered wall).

So, what do I think is the best solution for your bridge dilemma? Use your existing deck bridge and get an identical one for the other track. Construct a single abutment on either side of the stream, including aprons at the tops of the abutments to provide for between track drainage, and funnelling aprons on the upstream side of the embankment (small and low on the inside of the curve, large and high on the outside of the curve). Any portion of the track that runs parallel to the stream should have no less than a 2:1 slope from the top of the roadbed shoulder to the natural ground catch point if it is unstabilized. If it's less than 1:1 but greater than 2:1, consider gabions, riprap, steel or concrete pilings or any combination thereof. If the slope is greater than 1:1, construct concrete retaining walls. If the retaining wall stays wet, make it a paved wall. If it stays dry, consider using a modular style wall.

I hope you found this post informative and you're able to put some of this knowledge to work for you designing your bridge solutions.
 

kjd

Go make something!
Thank you very much, Ryan, for such a detailed response. It must have taken some time to write it all out. Are "funneling aprons" and my "wing walls" the same? In context it seems so. The imagined stream is not very big most of the time but I forgot to remember the planning needs to be done for the floods.

As an aside, one time I went out when there was serious high water and you could hear the boulders rolling along the stream bed. There was a plate girder RR bridge that ended up with lots of wood debris piled up in it because the stream was half way up the girder and occasionally splashing over the top.

"Nonetheless, a single monolithic abutment, whether constructed at once in a single pour or in a modular fashion, would be the best choice for abutments with adjacent tracks being as close together as 22 feet. A drainage swale would need to be provided between the tracks outflowing to the stream. Once the runoff reaches the abutments, it would most likely be removed by sheet flowing the water over the top of the abutment and would likely include a concrete apron to facilitate this."

Is the concrete apron at the bottom of the abutment? I assume when you say sheet flow you don't mean a lip at the top. Or is the apron at the top between the tracks?

The prototype loadout is in Eastern KY and I don't know much about it except what I see in the couple photos I have. I model western railroads so I made up a history and moved it out west. I haven't decided where yet. Unfortunately, I don't have huge space to fill so as most model railroaders do I have exagerated the slopes but I have to consider something for the stream side slope. The retaining wall would only be about 3-4 feet tall and about 6 ft above the stream. Also, part of the reason the near track is lower is it makes the modeled scene look bigger. There are more photos on my website at:
http://members.trainorders.com/pmack/loadout/loadout.htm
They show the modeled scene before the second track but after the loadout, which is not really correct in the timeline I presented but that is how it got built.



Thank you again and I will post more photos,
Paul
 
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jbaakko

Diesel Detail Freak
Nice looking Paul, BTW I like your GP39E project, and chance you have pics of the construction of the Elec. Cabinet? Any tips and pointers you can give me towards the cabinet, like say maybe cutting a prebuilt one from another unit, or will I have to build it?

Anyways I like the new porgress.
 

RCH

Been Nothin' Since Frisco
kjd said:
Thank you very much, Ryan, for such a detailed response. It must have taken some time to write it all out. Are "funneling aprons" and my "wing walls" the same? In context it seems so. The imagined stream is not very big most of the time but I forgot to remember the planning needs to be done for the floods.
No sweat, I enjoy this kind of thing!

Aprons and wing walls are different. Wing walls are usually vertical and their purpose is to retain the earth. Funneling aprons are intended to prevent erosion and directionalize the flow of the runoff. I'm attaching some sketches that show the inside of a concrete reinforced channel, the outside and an elevation view to show the individual components. And you're right about planning for the floods. We have some big channels around here (typically referred to as "flumes" when they're concrete reinforced from top of bank to top of bank) and they look like total overkill until we get a four or five day soaking - then they're full to the brim and rushing.

As an aside, one time I went out when there was serious high water and you could hear the boulders rolling along the stream bed. There was a plate girder RR bridge that ended up with lots of wood debris piled up in it because the stream was half way up the girder and occasionally splashing over the top.
I've never heard boulders rolling, but I've seen several bridges topped like that. Maybe 15 years ago Grapevine Lake here in north Texas ended up filling with rainwater and extended itself about 5 or 6 miles to the west, topping US 377 for maybe a half mile stretch. The UP main that's adjacent to it stayed above water, but today you can still see the old bridge abutments where the line would not have survived.

Is the concrete apron at the bottom of the abutment? I assume when you say sheet flow you don't mean a lip at the top. Or is the apron at the top between the tracks?
Hopefully, the sketches will help explain what the aprons are. They are essentially a concrete covering over the backfill that prevents erosion. Sheet flowing water means that the water is intended to move across a large area rather than funneling or channeling it into a small area. Erosion is less likely to occur in a sheet flow situation than in a channelized situation. The reason for an apron to be located between the bridge pier and the abutment is to avoid having any of the compacted backfill that the abutment is holding in place get washed away. The abutment's purpose is to protect that backfill, so while a concrete apron may not be entirely necessary at the top of the abutment, it does act as insurance.

The prototype loadout is in Eastern KY and I don't know much about it except what I see in the couple photos I have. I model western railroads so I made up a history and moved it out west. I haven't decided where yet. Unfortunately, I don't have huge space to fill so as most model railroaders do I have exagerated the slopes but I have to consider something for the stream side slope. The retaining wall would only be about 3-4 feet tall and about 6 ft above the stream. Also, part of the reason the near track is lower is it makes the modeled scene look bigger. There are more photos on my website at:
http://members.trainorders.com/pmack/loadout/loadout.htm
They show the modeled scene before the second track but after the loadout, which is not really correct in the timeline I presented but that is how it got built.



Thank you again and I will post more photos,
Paul
A completely prototypical scene might end up looking somewhat boring, so there's nothing wrong with designing the scene and backing in the engineering to fit it. There are plenty of reasons you could justify the vertical position of the near track. What the terrain looks like miles in either direction of any given point on a stretch of track defines what is going on at that particular location. Trains don't like grades, so the flatter the railroad the better. If that means it's high in some places and low in others, so be it.

Anyway, let me upload these sketches and see if they help.
 
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kjd

Go make something!
Ryan, thank you again, the drawings explained themselves very well. I have a degree in geology so have studied slopes and stability a little but usually in a natural and not man-made setting. One thing I noticed is the bridge doesn't nesessarily sit on the abutment but on piers set behind it. Maybe I should have majored in enginnering instead, this is very interesting.

The place I heard the boulders rolling was on a tributary to the Potlach River in N. Central Idaho. The rail line was the abandoned Palouse and Lewiston branch of the NP. It is a steep gradient stream so had a lot of energy. I imagine most places in Texas would be in trouble if the boulders could be heard.
Paul
 

RCH

Been Nothin' Since Frisco
The abutment and pier could be integrated into the same structure, but that seems like an older style of construction. What I'm used to seeing in modern times is cylindrical piers drilled and poured in place, so the abutment can be constructed independently of the piers. Since you indicated this was a modern era reconstruction project, the separate piers and abutments seemed appropriate. Including the pier in the drawing also seemed like a good way to convey the importance of the piers, something that seems to get lost in modeling these types of bridges, along with many other things that are "sore thumbs" to me but other people would never notice. I'll bet with your background in geology there are all kinds of no-no's with terrain modeling you see, too!
 

kjd

Go make something!
jbaakko said:
Nice looking Paul, BTW I like your GP39E project, and chance you have pics of the construction of the Elec. Cabinet? Any tips and pointers you can give me towards the cabinet, like say maybe cutting a prebuilt one from another unit, or will I have to build it?

Anyways I like the new porgress.
It is scratch built which is probably the easiest, thanks to Cannon & Co. The top of it is from the GP30 shell. I cut out the piece and built the rest around that. I don't really know how to explain how to make it except look at lots of photos and make what you see, noting the relationship between the parts.

Paul
 




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