Lateral Pressure from Frost? 1

[LUGuy]

Is there a prescribed method for designing against lateral frost pressure? How does the magnitude of pressure exerted by frost compare with hydrostatic pressure?

I am considering various causes for cracking in CMU basement walls. I have heard that there has been an above-average occurrence of CMU basement cracks (particularly those pesky horizontal cracks) this year in the northeast, perhaps due in part to an extremely cold winter with record frost depths.

It would be interesting to know if the cracking could in fact be connected to the excessive frost depth(/force?).

Jim

 

[Focht3]

Is there a prescribed method for designing against lateral frost pressure?

Not that I've seen - most of the design/construction effort is geared to avoiding it.

How does the magnitude of pressure exerted by frost compare with hydrostatic pressure?

It can be quite large, perhaps in the 1 tsf+ range. Or greater, depending on the circumstances.

It would be interesting to know if the cracking could in fact be connected to the excessive frost depth(/force?).

It's certainly possible. But each building and foundation problem is unique - I wouldn't paint with too broad a brush...





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[cap4000]

First, I will assume you that your masonry walls are properly designed and meet the NCMA/ANSI LATERAL BRACING requirements. But, if the house is not heavy in terms of axial load per foot on the masonry wall and not laterally braced properly, flexural cracks can easily occur at mid-height of the wall. As far as frost depth in NJ its 3 feet where I live. I think your problem is associated with my guess: "CLAY Backfill" and thus swelling pressures below your frost line can be extremely large. With a clay liquid limit of 30 to 40, an SPT N value of 10 to 20, expansion can be up to 5%. The "Swelling presuure" could be 3 to 5 kips per sq. foot on the foundation wall. I seen all these conditions before. Good Luck!!

 

[Focht3]

[blue]cap4000[/blue] makes a good point: 'fat' clays are much more likely to be the source of the observed problem. And lateral swell pressures can be very high - more than 2 tsf (i.e. roughly in the same range as [blue]cap4000[/blue] suggested.)

My frost heave experience primarily has to do with footings beneath freezers that weren't properly insulated. The dead loads were a bit above 1 tsf; the footings had heaved 6 to 7 inches over a period of 2 or 3 years. I fully expect that the lateral ice pressure could be as high as the vertical pressure under some circumstances.

What leads you to think the cracks are due to frost action? Keep in mind that:[ol][li]The cracks may have been developing for some time and were only noticed recently;[/li][li]The clay has swelled in spite of the very cold winter; and/or[/li][li]The wall was under-designed for the imposed loads - which may have nothing to do with ice lenses or swelling clays.[/li][/ol]We'd like to hear more -



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[PEinc]

MiscMetals,

I believe that cracking of foundation walls, especially CMU walls, can be connected to frost pressures.

About 1997, in the northwestern Philadelphia suburbs, a prolonged cold spell left snow on the ground from early January to early March. Soon afterward, I received calls from a handfull of home owners who had newly damaged block foundation walls. Most of the homes were old with no previous damages. Each of the homes was damaged on its north side, the side with little sun exposure and very little thawing. In addition, the north side usually had downspouts with blocked or restricted outlet pipes. Therefore, any snow that melted off the roof puddled near the north foundation wall because the snow-covered ground hindered drainage away from the house.

Last winter I saw a few more homes with damaged block foundations walls. Again, the weather was extra cold and the damages were on the north side of the homes.

So, I say that cracking can be connceted to frost pressures. Home owners need to keep their downspouts draining away from the house and make sure that the ground surface slopes away also - especially along the north side of the house.

 

[PEinc]

P.S. Fat clays were not the problem with the homes I saw. If fat clays were the cause of the cracking, there should have been damage on the other 3 sides of the home also, not just the north side.

 

[LUGuy]

CAP4000 - I appreciate the assumption, and the wishful thinking. Actually, the foundation was designed in accordance with the builder's best opinion and experience. Hmmm. The makeshift pilasters on the 9' wall with 8' of backfill might have stopped the wall from moving, had the cores been grouted. Yup, nice shiny rebar exposed through the 1/4" horizontal crack.

Focht3 - I have no idea if frost is really to blame, but the builder seemed to be sure of it and sometimes it helps to give the builder something beyond his own inabilities to blame. And since I had never thought about frost loading, it seemed worth asking the question.

PEinc - this house is in the woods at the south edge of the Poconos, and it is shaded from most winter light by a mountain ridge directly to the south, so all sides are similar, except where the garage is attached.

The house is on a slope and the sump pit is bone dry, even after all of the rains we've had in northeast PA this winter. But when I look at my backyard and see puddles on top of ground that is so solidly packed from one freezing event after another, it makes me wonder if all of those rains aren't locked in ice near the ground surface.

Regarding the cracks, the big problem wall has 1/4" horizontal cracks down 3' from the top. Two other walls have one or two 1/16" horizontal cracks running the length of the wall.

I am considering fixing the walls with the 1/16" cracks thus:

- Locate the rebar somehow (by luck, intuition, drilling, etc.)
- Grout the cores with rebar in them by knocking holes in say the bottom, middle and top of the wall.
- Grind out and repoint the cracked joints.
- Verify design capacity once we determine what rebar is in the wall.

There is no discernible bulge to these walls, and I am working under the premise that they are otherwise in good enough condition to stay.

The wall with a 1/4" crack is bulged somewhere around 3/4". The owner does not care if the bulge remains, as long as the strength is restored. Also, the (ineffective)pilasters are about 11 feet apart, 9 feet tall. I am considering repairing this wall as follows:

- Remove the top 3-4 courses of the pilaster down to the crack level
- Grouth these cores completely solid with existing rebar
- Cast the remainder of the pilaster to full height, anchoring into the adjacent wall as necessary.
- Repair all horizontal cracks the same as the other walls.

I am figuring on waiting until everything thaws before any work should be done. If it is frost heave, perhaps the wall might move back enough to close the cracks a bit. Would that possibly prove the frost theory if the cracks actually closed?

TIA,
Jim

 

[jheidt2543]

MiscMetals,

Here are a few comments to consider:

1. My first reaction to your problem was, where is the maximum deflection in the wall? If the maximum deflection is in the upper third of the wall height, I would suspect frost action. If the maximum deflection is midheight or lower, I would suspect the cause was soil related.

2. Between the heat loss from the basement wall and the relatively constant temperture (around 50-55 degress) of the soil below the frost line, the frost pressure will show its effects in the upper portions of the wall.

3. A 9' wall block wall with 8' of backfill against it is way overloaded for an unreinforced wall. Check the tables in the Masonry Society's Design Guide. You say you have found rebar, but it sounds like it is in ungrouted cores. That sounds pretty strange to me. Why put the rebar in if you're not going to grout it?

4. 3/4" deflection is not a lot when compared to the full height of the wall. I looked at a 25 year old house with an unreinforced 8" block wall 8' tall, with 7'-6" of backfill against it. The wall had a mid-height deflection of 1-1/2". While it was located in frost suceptable soils, I concluded the overload was caused by saturated backfill soils due to poor drainage around the foundation. Roof drainage was not carried far enough away from the foundation and the grading around the house was virtually flat. The wall was reinforced with vertical steel columns anchored to the floor slab at the bottom and the floor joist above.

5. I really doubt that the wall will "rebound" after the frost thaws, there is no force to push the wall back. It will just maintain its present position. Year after year it will deflect a little more with each successive "frost load", unless the situation is corrected. I would suggest correcting the drainage around the foundation to a distance of at least 10'.

 

[LUGuy]

The cracks were about 24" below grade, and the grade differed on each of the three exposed sides.

Also, it was a house under construction, so the gutters had not been installed yet.

 

[PEinc]

No gutters installed is just as bad as clogged gutters. There is nowhere for the roof runoff to go except directly to the ground at the foundation wall. Is the ground surface around the house properly graded away for drainage? If the house was or is not yet finished, it is possible that there were or are depressions next to the wall where water collected, penetrated, and froze?

jheidt2543 made a few good comments.

Also, sometimes new basement walls are backfilled too soon or with too much effort (force). If the first floor slab is in place during backfilling, it could restrain the top of the wall while the lower portion pushes inward, causing horizontal cracks - especially if the block wall is high and ungrouted.

 

[Focht3]

[blue]PEinc[/blue] hit some very key points to consider. And based on your posts of March 1 and 2, 2004 I strongly suspect the primary problem is construction quality, not frost heave. But that's a hunch, not an opinion...




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[Focht3]

And [blue]jheidt2543[/blue] did a good job of discussing the issue - and hitting other key points to consider.

[I hit the 'Submit' button instead of the 'Preview' button -]



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[CSEllc]

The CMU foundation wall has failed or is failing.....that has been my experience that CMU foundation walls with horizontal cracks.

The most likely cause is that the tension stress exceed what the mortar joint could handle....the cause is all of what has been discussed increased lateral pressure due to no gutters, poor workmanship, and the list can go on and on.....

Since the cracks are relatively small at this point (1/16 to 1/4"), it indicates that they occured recently.....I have inspected several homes with CMU foundations most of the cracks were about 1/2" wide or more. At that point, my recommendation has been to replace the foundation wall or reinforce it.

 

[zendog]

Different Quist
I have a house in West Chester PA I am under contract to sell.
The buyer's inspector and structural engineer found a horizontal crack and concluded that it was due to lateral load. Of eleven courses it is on the bottom of the third course. That is 1 foot below actual grade. The thing I disagree with is that it is due to lateral loads. I say this because the horizontal crack (8ft long) “steps” to the top of the course on one side and steps to a window sill on the other side. The crack is maybe mostly 1/16" (1/8" max). The bottom CSM is in tact with no other crack on side, so it has not moved. How did top piece move if it is only 1 foot below the surface?

 

[LUGuy]

Try searching google for basement cracks. You will find several companies that do inspection and/or repair. Along with that, you will find some excellent pictures of damaged foundations due to lateral pressures.

As far as the stepping-that is common at the ends or near windows. And you should see that on some of the web site photos too.

Suggestion - if you have a new question, try starting a new thread.

Good luck selling.

 

[agsgeophysics]

MiscMetals:
Locating rebar in structural cement/concrete is relatively simple if you have the right tools. A ground penetrating radar (GPR) equipped with a 1GHz antenna can very reliably map rebar, post-tensioned cable, pipes, etc. A local geophysical subcontractor that also performs "materials testing" can do this type of work for a reasonable price.

 

[Duane45]

Sorry I haven't been on for a while, but I want to relate some of my experience here in Alaska with frost action on retaining walls.

Yes walls do fail from frost action, but the freezing front has to be moving horizontally through the wall, so we only see the problem on exterior retaining walls. Unless the wall is backfilled with clean granular fill or insulated enough so that frost action can't reach the frost susceptible soils, very large forces are developed. Each year the wall will progressively move a little until quite a large tilt occurs.

I can't remember ever seeing frost action moving a wall in an occupied building with a heated basement or crawl space. The freezing of the soils ourside the building is not expected to have any lateral efffect, and generally, heat loss through the wall reduces the freezing next to the building.

If the wall has a heated interior space, it sounds like soil loadings are the problem.