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suitable cover for culvert pipe

suitable cover for culvert pipe

suitable cover for culvert pipe

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

RE: suitable cover for culvert pipe

-if you are crossing a roadway then you have the depth pretty much set up, +/- 1.5' that you can pick if you used a V.C, If the pipe has less than 3' of cover, encase the pipe in concrete class"A",

RE: suitable cover for culvert pipe

note that backfill, compaction and depth of cover will differ based on the type of pipe you choose such as concrete, steel, polyethylene etc.  Reinforcement and pipe class for concrete will also dictate the method of installation.  Normally, however I wouldn't use a gravel over the top of the pipe. Depending on the native soil, you may want to use that for backfill instead.  Depending upon the depth to the roadway subgrade, you might want to use road base material over the top.

RE: suitable cover for culvert pipe

Okay, thanks for the info.  There will be more than 3ft. of cover.  The pipe will be galvanized steel and sitting directly on bedrock.  So I will look for a road base material.

RE: suitable cover for culvert pipe

since you are sitting on bedrock, you will need to either
a) overexcavate into the rock and place some type of bedding material
b) bed the culvert in a concrete cradle

I wouldn't recommend placing it directly on the rock.

RE: suitable cover for culvert pipe

Did they do the hydrology?  Is it common practice for a developer to designate the size of pipe, or are you determining how many barrels there will be?  You can flow fill for lateral support - it's really hard to compact under the haunches, particularly of metal or plastic.  Then again, if a contractor isn't very experienced he may float the pipe with a flowable fill.

RE: suitable cover for culvert pipe

The developer did specify the diameter of the pipe of 500mm.  What do you mean be flow fill?  Thanks for the reply.

RE: suitable cover for culvert pipe

Flowable Fill.  A sand slurry with exceptionally high slump.

RE: suitable cover for culvert pipe

flowable fill sometimes is also specified as a low strength concrete such as a 1/2 sack or 1 sack mix with high slump.

RE: suitable cover for culvert pipe

Engineering Folks.......
I'm a homeowner with a question on a driveway. Can you help, please??
Is "flowable fill" suitable to put under a 12" diameter non-concrete culvert pipe? The culvert pipe is near the road-end of the driveway to carry drainage water. The driveway guy originally put a 15" pipe because the Town told him to, but it was too large and caused our brand new driveway to start lifting within 6 months.
The driveway guy agreed to re-do it, with a 12" culvert pipe (Town ok'd), and will put "flowable fill" as the base if we want it. Neither the Town nor the driveway guy have any experienece with it. We heard it is a good product which will help prevent the lifting caused by freezing and thawing.
Can anybody tell me if this is recommended??
Thanks so much for the help, really...


RE: suitable cover for culvert pipe

you have frost susceptible soils and bedding your pipe on native material will allow frost heave.  you need to replace the bedding with either an impermeable material such as concrete or with a gravel which will promote good drainage.  The bedding depth should extend to the approximate frost depth.  If you use a gravel, you may also need a filter fabric around it.

RE: suitable cover for culvert pipe

Thanks so much for responding but, as I said, I'm a homeowner and I have no idea what your recommendation is!  I'm sorry.  
Because this culvert pipe is sitting underneath our driveway to carry the draining water, the depth of the pipe is restricted by the height of a culvert 20 feet away.  If we bury our pipe too low, it will be lower than the pipe 20 feet away which, of course, will not allow for good flow.  I'm just wondering if this "flowable fill" stuff is a good medium to use instead of gravel or anything else.
Thanks a million to whomever answers...

RE: suitable cover for culvert pipe

Flowable fill is a type of low strength concrete.  It could solve your problem if done right.  However, it is difficult to advise you without know a lot more details.  It doesn't sound like your contractor is very knowledgeable or he would have installed the culvert properly the first time.  You should contact a contractor or an engineer locally who can assist you.  You might talk with the town engineer and ask for his (free) advise.

RE: suitable cover for culvert pipe

Hi Guys:

This is interesting.   C1323 apparently is in frost heave country.

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. The larger the culvert, usually the greater this difference.   In winter the roads have a dip at the culvert.

I'd think reducing the culvert would tend to make the culvert act along with adjacent soil and everything then raises and lowers together.

I have my suspicions that frost is what caused the "lifting", unless it was water from the ditch line that fed the frost lenses, a most unusual situation.  You get the least frost heave (depth of freezing) where there is plenty of water (take a lake for instance)  Maybe it was just settlement of the backfill alongside the culvert making it look like it raised up.

If the action is not positively frost action, I'd wait a year or so before I do anything to be sure.

Sometimes we have dug out natural soil at sites where some significant differential frost heaving takes place with tapered up edges of the cut on each end. Then backfill the whole area with non-frost susceptible material, such as sand with no more than 5 percent passing the Number 200 sieve.  Concrete fine aggregate will meet this.  In this case dig down at least 2 feet at the culvert, bed it at the ditch flow line.  The most fancy job would be an excavation to 4 feet at the culvert area, which usually is OK in most northern states.

Not knowing the site, I'd start at the nearby road with the end of a taper and end up about 10 feet beyond the culvert with the other end of taper.

 I see no need for fancy work like flowable fill here for a job this small.  This treatment (undercut and sand fill)  then creates a gradual transition of normal frost heave at the ends of the underut, to little or no frost heave at the culvert (making a dip).   It will be necessary to go below the culvert some with the undercut to minimize the frost heave there (due to the cold air in the culvert)  

RE: suitable cover for culvert pipe

Hi Debbie,

Was curious if you know exactly what kind of pipe was your original "non-concrete culvert pipe", and about how much cover was over the top of same to your driveway level?  Also, is the driveway paved?

RE: suitable cover for culvert pipe

Thanks for the comments, guys!

The culvert pipe is currently black plastic, 15" in diameter, installed under a newly paved driveway.  This culvert carries rainwater drainage from the street.  It is not a large quantity of water, just an average amount, carrying only water run-off from the road.  

Before the driveway was paved and it was just gravel, the culvert pipe that had been in there for many years was 12" concrete.  When the driveway guy was preparing it for paving, he dug up the concrete pipe in hopes of burying it a little deeper.  We discovered that the concrete was really two separate pieces, at one time connected, which had come apart.  We replaced it with 12" plastic.  It looked great, and it was buried deep enough so that we thought there would be no "frost heaving" issue.

Well, at the last minute, the Town comes and tells us to dig it up and replace it with a 15" culvert because "that's the rule."  The problem with a 15" pipe is that we cannot bury it deep enough since this culvert drains to another culvert about 20' away which is HIGHER in grade.  It was late in the season, the weather was going to turn on us, the Town said it was required, and so the driveway guy re-did it at his own expense. (The Town insisted that the driveway guy should have known the rules because the Town sends the rules to all the local driveway installers, so they say.)  Anyway, the installer rushed, slapped it together, and between his haste and losing 3" in depth (going from the 12" pipe to the 15"), it was buried too close to the surface of the asphalt.  Within 6 months it had cracked.

Come to find out, it is NOT a Town requirement to have a 15" pipe, and we could have used the 12", as originally done.  So now it is being replaced again with the 12" black plastic pipe, and the best way to bury it is in question.

We heard about "flowable fill" and because there isn't as much depth as we'd like, we thought this product might help keep the pipe from lifting in the frost.

The driveway guy said he'd do whatever we want; He's been cooperative.  

The pipe was originally buried at least 3-4" from the surface, as deep as possible with the 15" pipe.  It was placed on gravel with stone under it....a generally accepted practice from a driveway installer with a GOOD reputation.  Our problem seemed to be the last minute change to an unnecessary 15" pipe.

But now we're leaning toward using flowable fill rather than gravel, thinking it might help stop some of the lifting associated with frost in this part of the country (New England.)

So, if this clarifies the situation any better, are there any other opinions about using flowable fill in this application??

Comments are very much appreciated from you fine folk...

Thanks to all!


RE: suitable cover for culvert pipe

It sounds like you will have 6 - 7 inches of cover over your new 12" pipe when you reinstall it.  Typical requirement is 24".  This helps to explain why your first concrete pipe was broken.  It was crushed by repeated traffic loads.  The plastic pipe is flexible and won't crack.  However, it may flatten over time, so I wouldn't assume it will last longer than the last one.  You didn't mention whether you had used a reinforced concrete pipe.  A well reinforced concrete pipe (maybe Class V) might have been a better choice.  With the plastic pipe, it is very difficult to get the soil compacted tightly underneath the haunches.  Then, with the shallow depth of cover and poor compaction, you can expect it to fail quickly.  A flowable concrete backfill might be a better choice for either type of pipe in this circumstance as it will provide additional support which helps to prevent pipe cracking or flattening.

RE: suitable cover for culvert pipe

Dear CVG,
Is the "flowable concrete backfill" you mentioned the same thing as "flowable fill?" (So that I can construe this to mean you believe the flowable fill would be better than regular old gravel/sand stuff?)

I sure do wish we had 24" to bury the pipe, but we are going down as far as we can in order to keep the water flowing without incident.  I will ask about the new 12" plastic pipe "crushing" in the future.  But isn't it better to have plastic with some "give" than concrete which would crack??

Thanks for your prompt response,


RE: suitable cover for culvert pipe

In your comment "Within 6 months it had cracked", is the "it" you refer to the new pavement?

RE: suitable cover for culvert pipe

The flowable fill that I am referring to is a lean concrete mixture. It is concrete which consists of about 1 sack portland cement per cubic yard of concrete.  It should have a "slump" of about 7 or 8 inches which means it is very thin and will flow around the pipe and fill all the voids.  If you decide to use it, be aware that it can cause the pipe to float when you pour in the flowable fill.  You will need to anchor the pipe down to prevent that from happening.

RE: suitable cover for culvert pipe

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,


RE: suitable cover for culvert pipe

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!)                   

RE: suitable cover for culvert pipe

A few comments on the thread:
"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.

"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.

RE: suitable cover for culvert pipe

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.  

RE: suitable cover for culvert pipe

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.

RE: suitable cover for culvert pipe

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. smile)

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

RE: suitable cover for culvert pipe

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.  

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