suitable cover for culvert pipe

[lot51]

I am installing a culvert pipe to get across a subdivision drainage ditch. The subdivision has specified the culvert pipe diameter as 500mm. What type and how much cover should I put over this pipe? It was suggested to me to use a granualar A type gravel, and then compact it after. Does this sound right. TIA

 

[c1323]

Dear rconnor & cvg,

Yes, the "it" I was referring to was the asphalt driveway. That is, the driveway cracked right over the culvert pipe within 6 months of the driveway being installed.

You had also asked, rconnor, about the "original non-concrete culvert pipe" material. It was originally installed with a 12" diameter black plastic; then it was changed to a 15" diameter black plastic when the Town erroneously told us to change it because the 12" would not be approved. It is soon to go back to the 12" black plastic (because the Town was wrong) just to be able to get the extra 3" of depth.

THANK YOU, cvg, for the comment about anchoring the pipe before pouring in the flowable fill! Good to know!

Also, what do you think about the plastic vs concrete? Isn't the plastic better because it will "give" a little??

Thank you so much,

Debbie

 

[rconner]

When this thread started I wondered if "frost heave" as reported was in fact what had caused whatever problems had occurred here. I knew that some contemporary "black plastic pipes" now used as culverts in some areas have very low effective "stiffness". A rough definition of stiffnesss is a unit load per unit length of pipe that will deflect the pipe ring (normally in the downward direction) a significant amount in a laboratory test. In the field load can be created by the weight and/or settlement of earth over and around the pipe AND by live loads, such as vehicles driving over the pipe. Though soil at normal burial depths normally supports flexible pipes pretty well and limits the deflection amount, a problem that I could see one getting into (particularly at very shallow very few inch cover as you report) is that dynamic live/wheel etc. loads become quite large, and additionally a larger percentage of the total load on a flexible pipe. This might cause the pipe (and whatever is over same, including a pavement) to undergoe more dynamic/repeated and magnitude of downward "flexing". At some point I wonder if even asphaltic pavement that is repeatedly flexed a certain amount might crack? While very lightweight plastic pipes with a relatively smooth exterior surface are perhaps also known to move/float in the ground more than heavier pipes, I doubt that flotation force per Archimedes is what has damaged your driveway. If too much dynamic flexing has however been the case, I suspect your situation might be improved (lesser magnitude dynamic flexing) with one or more of the following:
1. Selection of at least somewhat stiffer pipe.
2. Contruction of firmer pipe support and surrounds.
3. Burial of the conduit as deeply as is practical to satisfy hydrology/flow demands.
With regard to item 1, some of these 12" black plastic pipes are designed and manufactured with a "pipe stiffness" of only about 50 pounds per inch per inch or so. On the other hand, a minimum class ductile iron pipe culvert (e.g. in accordance with ASTM A716) would have a calculated pipe stiffness based on nominal thickness of maybe 20 times that value. Of course a ductile iron culvert would probably cost some more, unless you could snag some left over from a jobsite (as there is very little "free lunch"!) On the other hand, I'm not sure that a very rigid concrete pipe laid very shallow immediately underneath your pavment is necessarily the answer either, per reasons stated by cvg and additionally as very rigid might act as a fulcrum point if the pavement subgrade yields more on either side of the pipe than it does immediately over same (if you then get pavement flexing in a different, negative beam moment fashion?).
I guess an ideal situation would be a finite amount of flexing of the asphaltic pavement over the pipe that is precisely the same as it is anywhere else on your driveway as vehicles roll over same, and that said flexing locally damages neither the pipe nor asphalt (but this may require some quite fancy Engineering!)

 

[TerryScan]

A few comments on the thread:
Oldestguy:
"What is odd about this is I have seen lots of frost action (in northern states) and most of the time the land on either side of the pipe raises with frost action, but the culvert stays there"

I suspect this more likely the ground did not heave next to the pipe, but the paving over the pipe sunk. This is often the case when the pipe is not properly bedded or backfilled.

cvg:
"This helps to explain why your first concrete pipe was broken."

c1323 could probably clarify this, but I read his original statement as two sections of concrete pipe were removed vs. a broken pipe.

I think the most important thing here is proper installation regardless of the type of pipe used. The most commonly neglected procedure is backfill and compaction under the first half of the pipe (the haunches) as cvg mentioned. Because the installation is on bedrock, the flowable fill sure sounds like the best bet (if not just poured concrete if you want to be done with this.)

If you do go with plastic pipe (High Density Polyethylene or HDPE), I would use double wall(ADS N-12 for example

http://tinyurl.com/r85va)

vs. single wall for added strength and stiffness.

 

[cvg]

I think rconner has a good suggestion. ductile iron pipe might be a better choice in the long run even though it would be more expensive up front.

 

[oldestguy]

Isn't this fun?

This poor guy has many sugggestions and thoughts, all based upon what each of us thinks the case is.

From my perspective, I like the thoughts of rconner as being the best way to go.

Good luck.

 

[Boczech]

Hello All and sorry if this is a redundant question. I am the owner of a property in Northern Calif south of Red Bluff,CA. I want to put a culvert in to access the property from the asphalt county road. I was looking to put a steel 30" culvert which has 34" from the flow line to grade. In the near future I am looking to build a ICF concrete house and need some advice on what to do so this culvert will support the weight. Steel or black plastic ? The county permit says steel or black plastic no concrete. Any ideas or recommendation would be appreciated. )

Thanks once again and sorry about the redundant question
Pulling out hair (Boczech)

 

[cvg]

4 inches of cover will not be sufficient, especially for construction loads. Large trucks, such as the concrete truck which will deliver concrete for your home construction will crush the pipe. Raise the road and try to provide at least 12 inches (or more) of fill over the top of your pipe. You will probably need a permit for your grading and drainage plan. The county or city engineer will review the plan and issue the permit. They can also advise you on a correct size and installation method for your culvert. The permit will cost $, but the advice is free.