Pile Frost Heave

[302Hugo]

We are seeing some possible frost heave on some of our pipe rack. They are steel driven piles and the geo is sandy and water table is high. I was hoping someone can provide advice. My main question is can we expect the piles to return to normal elevation when the frost subsides? Or will the piles stay at this elevation? And would we expect to see the piles heave again next winter?

 

[jayrod12]

There are some people with much more experience than me with this but here's my take:

It may return slightly but the chances of it going back down to the original elevation are unlikely. I would also expect it to happen again next winter, in fact I would expect the frost cycles to have a net uplift throughout each year until failure of either the pipe rack above or the pile coming out so far that it starts to list sideways.

Do you know how deep they are embedded?

I've been told depending on the roughness of the steel and the frost susceptibility of the soil that the adfreezing pressure can be in the range of 200 kPa.

 

[KootK]

I agree with Jayrod. I attended a presentation yesterday where the topic of discussion was the use of cellular concrete around piles to eliminate frost heave. It might be an option to consider if you head in the direction of remediation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MotorCity]

I don't think this is a frost heave related issue. Generally, frost heave is a concern for foundations located at or above the frost line. The frost line varies by location, but here in the midwest it is about 3' to 4' below grade.

Since you have piles, I assume that they extend deeper than the frost line.

For shallow foundations that are above the frost line, one remedy is to provide rigid insulation, extending vertically against the face of the foundation and an additional horizontal layer that extends outward from the foundation. I came across an excellent publication that gives guidance on the required thickness and layout of the insulation, but I cannot recall it at the moment.

Some frost heave recovers, some does not. Every year around here, there are concrete stoops at entry doors that go up and down in the winter due to frost heave. For the most part they return to their original location, probably because of the porous granular base below. If they do not go back down, owners usually try to grind the top surface so that the doors can open.

 

[jayrod12]

I would bet these things are embedded no more than 8-10 feet below ground if they're pipe-rack supports. I would think they weren't driven to refusal but rather to a "ya that's deep enough" depth akin to a fence post. 3-4' of frost action uplift on a steel pipe can more than overcome the friction values over a 10' embedment. Just ask anyone with a chainlink fence where there was no concrete poured around their fence posts.

Or any of my clients that I fixed their decks because some bozo only provided 12" diameter x 12 foot deep piles below their elevated deck.

 

[302Hugo]

Thanks guys. Another note is that I'm in Alberta Canada. Our typical frost line is around 6-7ft deep.
Jayrod12 - I wasn't sure if there was a "rule of thumb" regarding if the pile will go back down or not. I would assume that if the soil is sandy it would stay because there wouldn't be enough skin friction to pull it back down, and if it was a clay the heave would pull it back down? I think that makes sense, but I need someone with experience/civil background to confirm that? I do know the depth but I don't, ha ha. The drawings I located have a schedule showing a depth of 9m (30'), however this area is known to have very sandy soils and I'm not sure what the actual driven depth was during this install, and if a geo tech report was done to identify that this depth was sufficient for this area (this is a typical depth we use in other areas, but this area is known to be different). This site was basically built on a swamp, ha ha. We now use helical piles at this site because of this.

KootK - That's the next process is to see how we can fix this. If we can determine it is frost heave, and assume it will happen again next year then we are going to have to look at remediation.

MotorCity - I'm no expert in pile design, but from what I understand the design of the length doesn't just take into account the depth of the frost line but you also need to ensure the depth goes past far enough that there is enough skin friction below the line, that any pressures from the frost heave won't have any effect on it. I'm thinking this is the issue here and the piles were not driven deep enough to account for that.

 

[MotorCity]

That's a lot of frozen ground. If your frost line is 6' to 7' and the piles are 8' to 10' deep, I revise my opinion....sounds like you have a frost problem. Insulation may still be a viable solution.

To answer your original location, I would bet that you will see it year after year. Is this new construction and this is the first time being observed?

 

[Brad805]

Many geotech reports have an section on uplift calculations. Pipe racks and other light structures are prone to this problem since the dead loads are small. This is one case where I prefer screw piles since they offer significantly more uplift resistance.

Is the area around these piles clear of snow and does it experience traffic?

 

[Splitrings]

How much vertical displacement are you seeing? You need three things for frost heaves to happen: a frost susceptible soil, sufficient available moisture and extended periods of below freezing temperatures. Sands are not typically frost susceptible. Typically you need more than 6% passing the #200 sieve for a soil to be frost susceptible. The soil has to have the ability to form ice lenses and it must be above the water table. The soil "wicks" the moisture up to the forming ice lenses that continue to grow. This is why a heave can have a much greater magnitude than just freezing water.

I have seen people try to reduce the skin friction between piers and soil with pvc and other materials. I question whether it would work as the lateral forces from the heaving soil are enormous. A screw or belled pier below the frost depth would seem to be a better choice.

ASCE 32 "Design and Construction of Frost Protected Shallow Foundations" may be a good reference to look at.

 

[GeoPaveTraffic]

My suggestion is to talk to a geotechnical engineer with experience in the far north.

Based on conversations over beer with our folks who work up there, it could be frost heave if the piles were not driven deep enough to overcome the uplift pressure from the frost. As I understand it the frost can jack shallow piles and fence posts right out of the ground in a few years.

Mike Lambert

 

[302Hugo]

Also, note that we are only seeing about 3"-6" movement from the design elevation, and we are only seeing this lift in one area and only about 1" lift about 50ft down the rack. This pipe rack is within a tank cell, so there is a lot of standing water.

MotorCity - The piles are 30' (9m) deep, not 6'-7'. This construction was completed in about 2001, so some time ago. From what I understand this is the first time it's been observed but it's not moving a signification amount so it could have been happening since install but no one noticed.

Brad805 - That's another issue why I assume it's frost heave. As mentioned it's a very wet area and it does have a lot of standing water when it rains heavily or after the snow melts (like right now). The traffic is limited.

Sliprings - There is about 15 piles (2 different areas about 50' apart) and most of them are only up about 1" but there is a couple that are 4-6" higher. In both cases you can see that the one in the middle is lifted up and the ones adjacent to them are a little lower, and the once adjacent to them are a little lower. Thanks for the info!

 

[Leftwow]

KootK, how does the cellular concrete work with frost-heave? Is it the porosity?

 

[KootK]

The porosity gives it a high insulation value. About half that of the equivalent polystyrene insulation thickness. Basically, it works very similarly to the shallow frost protection systems mentioned above. I can provide a contact for your (our) local supplier if you're interested. They'll actually run the thermal modelling for you too.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Brad805]

3 - 6" of movement on a 30'-0" deep pile suggest something other than some seasonal frost to me. We have seen 30" diameter x 26'-0" deep piles heave 4" as a result of traffic, combined with snow clearing and poor soils, but what you are seeing sounds like there might be something else afoot. Are there driving records or is that what was shown on the drawings? If you do not have driving records I would be tempted to rent a tool to check the depths. This would not be the first case where the contractor was short on material or tired at the end of the day.

 

[CarlB]

An engineer practicing in the northern latitudes should be familiar with the term "frost jacking" which is exactly what you are describing. A very common occurrence. In Alaska, at least a short course in "Arctic engineering" is required, where this is covered. The "rule of thumb" for deep foundations is that the embedment below the depth of maximum frost penetration be at at least twice the frost depth. So presumably your pipe supports shouldn't have jacked if frost was no more than 10 feet. Perhaps frost has been going deeper than you think, piles are shorter, or their are unusually high adhesion in the frozen soils or low resistance in the deeper unfrozen soils.

At a military base near Fairbanks, many pipes supports have jacked. They were embedded20 to 30 feet. Frost depth is up to 10 feet. They are being replaced as needed by redriving them, digging out upper few feet around piles ang wrapping them with layers of visgueen, backfilling with NFS soil, laying horizontal rigid insulation. Are also trying adjustable pipe supports. Their new standard is 50 foot deep piles.

And no, after seasonal jacking they do not drop down after spring thaw. That's because thaw is from the top, ice lenses melt, soil above drops but can't drag down the pile with the much greater underlying pile adhesion resistance.