Differential Settlement of New Culvert Extension on Existing Culvert

[jlee2752]

I am preliminarily planning the connection for a new culvert extension on an existing old culvert. The existing culvert lies beneath railway and is a concrete closed bottom footing arch culvert built in the 1940s. Unfortunately, there are no clear drawings of the existing culvert, but the condition would be considered fair. Ideally, the culvert would be replaced, but due to the huge inconvenience of interfering with rail traffic, full replacement is not possible. Therefore, the culvert will be cut back and extended; the existing culvert that is left over will be replaced in a future project. The extension will be off one end of the existing culvert and will be a new concrete box culvert.

Obviously, dowelling is the straightforward solution. However, the current arch culvert has over 80 years of consolidation and settlement; it is unlikely to settle much further. The new culvert will settle much more comparatively. No geotechnical investigation has been done yet; we are in very early stages of planning. The flow through the culvert starts at the existing side and flows into the planned box culvert extension; the box culvert will be hydraulically larger than the existing.

I was thinking that there could be an option to place some sort of material between the two culverts to tolerate some differential settlement, but was not sure if this will cause other issues such as misalignment, fill infiltration or leaking. Not sure if anyone here has worked with anything like this and came up with a good solution. While it is possible to reduce bearing pressures by oversizing the span of the box culvert, I am not sure that this will help much to compensate for over 80 years of differential settlement.

Any ideas are greatly appreciated!

 

[JoelTXCive]

Do you have a photo?

What size box culvert?

Is your proposed culvert extension going to be cast-in-place; or pre-cast?

Assuming the culvert is just a normal 1-way span culvert, the longitudinal bars that you need to lap into are just the T&S bars.

Therefore, you just need to break back the development length of those bars and lap in new bars.

Put a few feet of stabilized sand under the new culvert and you should be good-to-go. (Or get a geotech to do a settlement analysis and give you recommendations for less than 1" of settlement.)

 

[DoubleStud]

The existing is an arch? That would be hard to match. Will the extension have E-80 loading as well?

 

[jlee2752]

There is more than one culvert for this project. I would say the existing culverts are concrete arches with dimensions approximately from 800mm(H)x1200mm(W) but can go as high as 2000mm(H)x3000mm(H). Some will be extended with a larger size box culvert, some will be approximately the same as the existing, some may be larger by up to 1m in each dimension. With some of the higher jumps in size, I was thinking we would overlap the larger extension over the existing and just form up the gap left from transition from small to big culvert. With some of the similar sizes, they would probably butt up right against each other and either dowel in or use some sort of material to maintain alignment but allow some settlement. Since the new extension portion will be cast-in-place, I was thinking I could thicken the haunches in order to make the profile match the arch, at least to satisfy cover, although it will not be a perfect match.

These are single span/barrel culverts and flow is only in one direction (existing is U/S, extension is D/S). The new extended portion will also be under E-80 loading. Currently there exists one track with one culvert crossing (concrete arch) per site; the culverts will be extended without interfering with the track (will need some significant retaining structure to hold back fill, timber, etc.) and the existing will be exposed before the new portion will be built off the existing. Once extended, there will be a NEW track running above the NEW extended portion of culvert. The plan is to have two tracks, so that in the future, they can replace the existing portion and use the new track for temporary staging for FUTURE replacement of existing.


Here is: photo of a large arch culvert, photo of a small arch culvert, preliminary plans for extension. Some of these have fill cover anywhere from 1m-4m. The scope is only for the extension as drawn in green; the blue future extension is not within the project. From this example, the culvert runs below the existing track, the red line will indicate where the fill will be retained so that the existing portion can be cut back and extended. The cut back is required in order to make future replacement of the existing easier. If the culvert extension is much larger, I was thinking to use a three-sided culvert to overlap, and then transition to box once past the overlap. If the culvert extension is similar in size, we can make the haunches and profiles match enough to dowel.

The main concern is the differential settlement between old and new, and I am worried about the additional stresses it may place on the existing; the existing soil is now stiffer and has no room to settle while the new portion is sitting on less rigid soil. We are extending off a structure from the 1940s... I am not sure if it is plausible to keep settlement within 25mm (1") at least until we do a formal geotechnical investigation. Also, I would like to add that this is a very high-level early overview of the project. It is more for feasibility and making sure it is extendible. Once preliminary plans are submitted, more detailed design will be needed, and each site will of course be designed and analyzed. I am sure there will be other issues that could arise but would be great if there are some solutions to the larger obstacles. Overall, I do not see an issue with extending the culverts through dowelling but worry that differential settlement might be a plan changing issue or if there are ways to avoid, mitigate or solve the issue.

 

[HTURKAK]

You do not supply alot of info. here ..
The picture implies there is some tolerance for hydraulic design and new sizing .( Upstream inlet inv. el . 112.66 while the ditch invert el. 113.22 )
I would consider construction a new culvert ( with greater slope )inside the existing culvert, provide a short piece ( with flexible joints , say socket spigot with mastic filler ) btw old and new culvert.
You are right for your concern regarding the diff. settlement and needs a short piece to compensate .
Apparently your question is general for diff. size of culverts . IMO , every culvert should be studied case by case .






Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM