Culvert with shallow cover frost jacked

[LLigetfa]

This is my first post to this forum. I design computer networks for a living, not roadways. When I built my home, I had the contractor strip off the foot or so of black clay topsoil down to grey clay and replace with gravel. Unfortunately the gravel was of a poor grade with clay fines in it.

I have a 10 inch steel culvert with only a couple inches of cover going under the walkway and every Spring, the frost heaves it up. The culvert never returns to the same grade as material seems to migrate under it. Every few years I dig it all up and reset the grade. The walkway is made up of stepping stones with grass growing between them so the stones move up and down with the frost.

My real problem now is the wife doesn't want grass between the stones any more but rather wants the stones set in concrete with exposed aggregate surface. The frost heaving at the culvert will break up the concrete.

The frost in this area can and does go down about 6 feet and I am not about to dig up the entire walkway and redo the base. No matter what material I put there, it is still a low spot and will fill with water for the frost to act on.

At one point, I replaced the steel culvert with a 6 inch plastic one thinking the smaller diameter would allow for more cover and might heave less. Unfortunately, it filled with water which froze and blocked the flow resulting in water overflowing the walkway. She made me put back the 10 inch culvert which generally only half fills with ice so the top half still flows.

Is there any way to set the steel culvert in a triangular concrete base/surround or would it still move more than the walkway on either side of it? Should I set the smaller plastic culvert in concrete and leave a heat trace cable inside it over the Winter to energize in the Spring when the thaw/freeze cycles fill it with ice?

I'm guessing that a culvert will always be a problem and that I should convert it to an open swale and build an arched bridge over it. Hopefully then the bridge footings won't get jacked, but rather simply rise with the frost and return to their orignal grade.

I also have questions about the exposed aggregate and the need for expansion joints, but will leave those until I solve this culvert issue.

 

[LLigetfa]

Is there any way to set the steel culvert in a triangular concrete base/surround

Actually, I should have said D shaped laying on the flat side. If that could work, perhaps putting rigid foam board under the concrete to keep the cold inside the culvert from going deeper than under the walkway might keep everything rising and falling at the same rate.

What is the R value of a foot of soil? I was thinking two layers of 2 inch foam, the bottom layer a little narrower than the top one, and the top one a little narrower than the concrete.

 

[civilman72]

An arched bridge with open swale is your best bet, but also most expensive.

A cheaper version of the bridge concept would be to replace the existing culvert with a sidewalk "chase" or "scupper," which is simply a three-sided (U-shaped) concrete channel with a metal cover (you can get decorative covers if your wife desires). The wider the channel the better, and the addition of heat tape along the bottom of the concrete channel will help freeze/thaw maintenance. You can tie the concrete channel into the adjacent walk with dowels and a small toewall to discourage uneven settling/jacking.

 

[LLigetfa]

Yes, I had considered a scupper but SWMBO would never approve it. She also refuses to discuss the smaller culvert/heat tape option.

You did not express an opinion on using foam insulation under concrete. Would 4 inches of foam under 6 inches of concrete below the 10 inch culvert be a close equivalent to 18 inches of base under the path (+4" of concrete/stone path)? I only get one chance at doing this right. If it doesn't work, I would have quite a mess of concrete and rebar to jackhammer out.

She is still very much on the fence WRT an arched bridge in this location. The problem there, is to aesthetically fit it into its surround. She keeps showing me pics of arched bridges anchored with vertical interest of flora/rocks/whatever but then she's not committed enough to invest the effort to maintain it. If she simply wants an arched bridge somewhere on the property, I have 18 acres and lots of other places to consider.

Thanks for your input.

 

[oldestguy]

I'd say your plan should be more along the lines of having everything heave up and down together. In my neck of the woods roadway culverts (being larger) don't heave up, while adjacent ground does. Does not sound like you.

If you wish to insulate, you should run that under and to each side of the culvert in a manner to taper its effect. Even one inch of closed cell stuff does an amazing job.

Thus, to get a tapering effect you would vary the insulation thickness from max. (say 2" or 2-1/2") to maybe one inch about 2 feet off to the side, maybe farther if you are able.

Your gravel may be doing the heaving of the sides of the pipe, and instead of being lifted by the stuff under the pipe. Thus any soil above the insulation should be non-frost susceptible, meaning no more than 3 or 4 percent passing the Number 200 sieve or simply the sand and gravel mix coming from a concrete plant (or equal). Taper that thickness also farther beyond the insulation. The insulation must be closed cell so it won't take on water.

Interesting to see you have water in the pipe sometimes. That being there at times would indicate that it gives off its heat of fusion as it freezes, while adjacent ground freezes more rapidly and deeper, but less freezing occurs beneath the pipe. This may also explain the sides doing the lifting. Remember, the areas of least frost depth are those where there is water plentiful, as at a lake.

Frost heaving also can go sideways against your pipe, popping it up.

I have noted that the most frost differential heaving of roadways occurs when there are changes in temperature to the point of some frequent melting and freezing cycles, generally tied to use of salt (lowering the freezing point) and at the open joints. Is there salty water involved?

 

[LLigetfa]

Yes, I understand how there are side forces as well. That explains why the largest nuts in a tin of mixed nuts come to the top. That is also why I want to encase the round culvert in concrete that is wider at the bottom. My thoughts were to make it 2 feet wide at the base with 6 inches of concrete (with rebar) below the culvert.

I understood the concept of tapering the insulation which was why I mentioned making the bottom layer narrower than the one above it. Yes, getting everything to move up and down at the same rate is key. Too much insulation and the path may raise more than the culvert but that should not result in jacking. I will have to put keyed expansion (control) joints in the path to allow for possible differential movement. I figured one control joint above the culvert and the others, 12 feet away. I'm hoping one keyed joint every 12 feet will work. I plan to use something like

http://www.connollykeyjoint.com/keyjoint.htm

The amount that frost acts on clay is dependent on how much water is present. There is clay under my front stoop which is a concrete slab poured onto 2 inches of foam insulation. The perimeter weeping tile keeps it dry enough so it doesn't move while the paved approach in front of it does. Obviously there will be more water at the culvert than anywhere else.

Ice forms inside the culvert over Winter while there is thaw outside partly because it is colder in the shade. The cold air inside the culvert freezes the ground deeper than it does under the rest of the pathway. This cold thermal mass then refreezes the water on a mild day and the ice waxes.

 

[oldestguy]

I suspect that any concrete anchoring or enclosing will not result in a happy reception by the XYL (check out ham radio lingo for that one).

Since water stands and freezes, do you have sufficient fall for the drainage ditch? Perhaps several small culverts in parallel, side by side will suffice. There should be no ponding along the flow path apparently.

As to concrete paving with joints, yes providing some joint load transfer will tend to keep the slabs sections together. I note the joint form has holes for re-rods to provide some added joint load transfer. Usually a joint spacing of not over 20 feet is needed, or added cracks will occur at no joint areas. To minimize slab shrinkage and resultant cracking, make sure the mix is strong, say 5 bag mix and a slump no more than 5 inches. To add to the belt and suspenders plan, place #4 bars in each direction at 18" spacing, in the mid height of the slab. For light traffic, 5" minimum thickness and for any heavy traffic, at least 6 inches, preferably 8 inches. Cure it for at least a week (keeping it moist)by spraying or covering with plastic).

If you want to embed stone paving in the concrete, do so, but not within the 5" section depth. This has to all be done in one "pass", no doing the slab first and then adding stone above days later or the bonds are likely to come loose. No wetting of the stone first, since dry surfaces take on the "glue" better than wet surfaces. Ain't no grass to grow in that setup.

This idea is for a paving over the culvert and no encasing of it. Differential final heaving probably won't be noticed.

That still won't solve the XYL situation, but might minimize it.

 

[LLigetfa]

I had planned to use 4 inches of concrete with rebar under the stone. The stone averages 1 - 1/2 inches thick. The path starts out 7 feet wide at the house and narrows to 5 feet at the culvert. I planned to put keyed crack control joints every 12 feet.

Right now there is 12 inches from the bottom of the culvert to the top of the path. There is not enough fall on the swale to make it any deeper. A couple of inches of water will pool inside the culvert when the swale downstream is not frost heaved. Any obstruction such as snow, ice, and frost heave, in the downstream swale will increase the standing water in the culvert to about 5 inches.

The swale was deeper at one time until a neighbor made comment about it being my wife's moat. It can never be made deeper again. The swale and culvert will move a lot of water in a heavy storm. I have seen the 10 inch culvert go full bore and water backup in front of it. For that reason the wife will not allow the 6 inch culvert and have storm runoff flow over top of the path even if it is a once in five years event.

As for a cost comparison, the amount of concrete to anchor and encase the culvert plus the path over it would be close to the amount of concrete needed to make the arched bridge. The amount of work to reset and anchor the culvert versus removing it and extending the swale plus forming and pouring the bridge is close to the same. It's pretty much down to what the wife decides and she is struggling to decide.

My plan for the arch would be to have a 4 inch rise over 6 feet (12 feet long overall) of the travelled surface. The arch of the underside would be shorter and steeper, not parallel to the top. If I made it 6 or 7 inches thick in the middle, and if I made the swale as deep as the culvert is I would have 9 or 10 inches of clearance under the bridge. I would not go as deep so the clearance would be 6 or 7 inches. Given the eye shape of the opening, it would have more capacity than the culvert if ice doesn't build up under the bridge.