Anchor down Culvert pipe to prevent heaving

[DDBR]

I would like to replace a 40 foot long, 12" dia., culvert pipe that runs across my drive way. This pipe is shallow with less than 2 feet of cover and every season it heaves up causing my driveway to crack. I am going to repave my driveway this summer and will be replacing this pipe and resetting it at the lowest grade possible, which is with about 20 inches of cover. I have a plan that I hope will keep the pipe from heaving through my new driveway surface but I'm looking for other possible options. I will be installing 20' sections of 12" dia. ductile iron pipe instead of using 12" dia. concrete pipe in 4' sections which is in there now. I feel that using a stiffer pipe will reduce heaving in itself. I also plan to install mooring anchors about 5' in length along the pipe at either end and one in the middle. The anchors are rated at 3000lb of break out force when placed in firm clay soil. I will use steel straps to attach the anchors to the pipe with an anchors on either side of the pipe. Can anyone give me an idea of what forces are happening on the pipe during freeze-thaw?

 

[jayrod12]

A stiffer pipe may exacerbate the issue as the heaving is the soil itself not the pipe. a stiffer pipe will just transmit the soil forces more efficiently. You need to make sure whatever ground anchors you are using extend below the frost level so they don't move in conjunction with the soil. I don't think you're going to find a winning solution to this in the long run. Estimating uplift loads from frost is probably as close to a guessing game as there can be.

 

[CarlB]

I suggest placing rigid insulation below the culvert (say 4' wide, 2" thick) to reduce the depth of frost penetration below the pipe 7 related heave.
Non-frost susceptible (NFS) fill below the culvert would also help.
I don't believe use of DIP or other pipe material will make any difference.
If anchors are deep/strong enough to not move, pipe will bow up between them.

 

[cvg]

it is possible that reducing heave in the pipe trench may also exacerbate the problem if the rest of the driveway is not also treated with NFS or insulation. this would be a case of differential heave

 

[CarlB]

The culvert area now heaves more than the adjacent driveway, likely due to the wetter condition below the culvert, and the deeper frost from cold air in the culvert. The insulation should lessen this, doubtful that it will eliminate it.
Yes, differential movement may well still occur, whether it be there is still some movement below the culvert, or the culvert treatment is so effective the culvert doesn't heave, but the adjacent ground does (unlikely). If you are clairvoyant enough to predict where the movement will occur, you could provide a transition area of varying depth of NFS material, and/or varying insulation thickness.

 

[DDBR]

Thank you all for your input on the topic. I am going to do more research in the NFS fill as my method of choice.

 

[civilman72]

I would consider matching the cross-section of the main road when you rebuild your driveway. The main road likely has a deeper pavement section than your driveway and the moisture from the granular basecourse under the road is soaking the less permeable material under your driveway entrance. Differential settlement is likely being caused at the interface of these different materials. When you make the vertical cut along the main road you should be able to see the pavement section. Plan to match (or go below) the depth of this section and extend the granular materials from the main road past your culvert location.

 

[oldestguy]

One factor not mentioned is that the cold air in the pipe probably causes heaving there to go deeper than elsewhere.

Heaving by frost action is pretty difficult to resist. I'd say you are asking for problems with the anchors and likely will break the pipe between anchors by that uplift force.

I'd look to the insulation alternative or also consider an alternative of replacing under and nearby soil that is below the road base subgrade with a tapered section of non-frost susceptible granular material,

For example if the base of the culvert is at 2 feet depth. I'd dig down there to 3.5 feet =/- and taper up the excavation to base course depth in a 6 to 8 foot distance each way out from the pipe. Fill with granular material, such as concrete sand, ASTM-c33 fine aggregate. Any such granular fill should have less than 5 percent passing the number 200 sieve (0.075mm as I recall?) That's the silt and clay fraction. Many natural deposits meet that, such as beach sand.

However, perhaps combining the replacement alternative with the closed cell insulation allowing less depth of all the work at the center, but still tapering up to subgrade depth.. That's the belt and suspenders treatment.

As a side note pay attention to road heaving at culverts. Most larger culverts do not heave, but the road nearby does. Maybe due to much available water. Remember when water freezes it gives off heat. An example of this is a lake. The ice thickness usually is less than frost depth on the shores nearby, all factors the same.

Oh and re-use the old pipe.

 

[bimr]

Agree with oldestguy.

Removal of wet and weak material underneath the pipe and replacement with a clean and draining aggregate would transform the ground into non-frost-susceptible ground and minimize the frost heave. See the attachment.

 

[CarlB]

Here's a case study presented by Dow insulation, for insulation placed under a culvert to mitigate heaving.
Same principles that have been presented here-use NFS soil under culvert, and in tapered transition areas. Depth of replacement below culvert can be minimized with use of insulation.

http://msdssearch.dow.com/Published...foam/pdfs/noreg/179-04501.pdf&fromPage=GetDoc

 

[DDBR]

Thank you CarlB!