Pipe Culvert at Small Ditch at End of Residential Driveway

[jlleiber]

I am working on a project where a homeowner is installing a concrete driveway over a very small (~12" deep x 5' wide) ditch. The ditch runs around a cul-de-sac, between the asphalt paving and sidewalk and tapers up to each one. The remaining driveways currently have 6" corrugated plastic piping running through/below them. The portion of driveway I am working on is only between the sidewalk and asphalt paving. He will be incorporating the sidewalk as part of his driveway (or this is probably a bad idea too?)

As it sits, the top of the plastic pipe looks to be about 6" below the top of the driveways. At this rate, I am fairly confident I just need to cast the pipe into the concrete driveway - would you guys agree? There just isn't any room to get appropriate cover for the subgrade and subsequent drive.

So if that is the case, I am thinking to excavate 24" below the bottom of the ditch to get down below frost depth - this puts us at 36" below final driveway elevation. So then 12" compacted fill followed by a 24" block of concrete with the 6" pipe cast in to serve as the driveway.

It's obviously a lot of concrete, but it's only about 5-6' wide and I don't see how to get a suitable base layer for the driveway, considering the 6" pipe is so close to grade.

Thoughts?

 

[cvg]

first of all, if you are worried about frost jacking, what about the rest of the driveway?

otherwise, just overexcavate and remove the organics and silt, replace with good structural backfill. I dont see much reason for a 6 inch pipe. that will have approximately zero capacity and once it gets silted up, it will be useless. A trench drain might be a better idea, it would have more capacity and could be cleaned easier. or if water is only occasional, a dipped crossing would be a low tech option.

 

[oldestguy]

It all heaves together so why only eliminate heaving at one place? That results in a dip there in the winter. The trench is a great idea.

 

[jlleiber]

Good thoughts on the frost heave - if I can't stop everything then there's no point in eliminating it at the front of the driveway.

How much would you overexcavate? 12"?

I'm not sure how I would get a trench drain to work in this situation. Simply because the top of the driveway is only 12" above the bottom of the ditch. Therefore, I'd end up having concrete driveway, then a full height trench drain, then more concrete driveway. There's nothing really tying it all together at that point. I'm trying to get water from the ditch on one side of the driveway to the ditch on the other

 

[TerryScan]

If I picture your scenario correctly, I would not bother with the expense of a trench drain. The drainage dilemma is not draining the driveway surface as much as maintaining the flow through the ditch.

Given the cover issue cited, rather than using 6" corrugated plastic, I would either use 6" ductile iron pipe or use a 12" RCP set 6" deeper. (Allowing the 12" to become half filled with soil will still have better capacity and strength than the 6" plastic.)

 

[oldestguy]

Wow, Gilding the Lilly!!! No belt and suspenders construction needed! If all the cross sectional area of the half of 12" drainage pipe is OK for ditch flow, why all the concrete and why the undercut? I'd excavate the length of the bedding area down to the invert elevation of the trench (that's the bottom), lay a pipe, whether plastic or steel or two to get that cross section area, backfill to the grade of the subgrade as needed for the concrete slab (probably 5" would be all the slab thickness needed) and be done with it. The backfilling around the pipe might be compacted a little, but that slab will bridge any weak pipe that you place. Your pipe does not have to carry any load. With ditch invert 12" below slab top, 6" ID pipe and 5" slab works fine.

 

[Ron]

A crack in the concrete will develop over the pipe!

 

[oldestguy]

Hell no. That pipe is about the same support as the nearby ground. Ask for a 5 bag concrete mix and keep the amount of water in the mix to a minimum. After setting is done, maybe by evening, cover with plastic and keep it there a week. Daily add water under the plastic. Concrete is strongest if the mix water is kept to a minimum, and interestingly once it is hardened, keeping it wet helps the hardening process. If a crack will form verify this by looking at neighbors. My bet is no cracks. A decent strength concrete slab can withstand all sorts of variable support as well as different loads. Just do the best you can to make the support for the slab as uniform as you can. After raking it to the level "subgrade" you want, drive your car back and forth (yes over the pipe also) so as to make one wheel track twice over every part of the area. Then just before placing concrete wet the area to make it damp, but not sloppy. That will tend to keep the water in the slab as uniform content, top to bottom. If you place concrete on dry soil, the bottom of the slab will be dryer than top. Later on, after things stabilize (a year or so) the top will shrink more than the bottom,resulting in a slight curl. No big deal unless you drive fast over it.

 

[oldestguy]

Once more: You might be thinking of some sort of base course for under the slab. Base course under concrete slabs provide very minimal added support capability. You might want to place a layer of sand there if you have difficulty getting a smooth subgrade for the slab. Sand can be raked to smooth grade easier than some soil. Keep it simple and spend your dough no something above ground level.

 

[oldestguy]

OG again: If one wished to get very secure against the pipe causing a crack, a number of alternatives could be: Cut or form a joint there (you need joints at least every 15 feet or less; place reinforcing grid over the pipe, centered in slab; do the Marston & pangler Imperfect trench thing (do a Google search for that) using s strip of foam insulation on the top, held there by a few hoops from a croquette game or similar.

Look at page 10

http://www.eng.auburn.edu/files/centers/hrc/930-592.pdf