Cracks in the wall of a house

[Robbiee]

Hello all,
My friend’s house has a basement that is smaller than the footprint of the house. Please see attached sketch. The portion of the house beyond the basement could have been an addition added at some time in the past. A crack starts from where the basement wall is and propagates diagonally towards the backyard then horizontally to the other end then diagonally down to the basement wall. The portion of the floor where the crack is has sloped down. I told my friend that it appears there is a differential settlement which could be due to different soil capacities at the two walls or due to the big tree which is about 15 ft. from one corner has sucked the moisture out and reduced the soil bearing capacity under the shallow footing. Well, My friend hired a soil engineer who dug a couple of pits and found that the shallow foundation is a trench filled with concrete. He told my friend that the crack is due to frost action resulting from the rough surfaces of the wall and the shape that widens at the top. The tree has no role in this: he said.
What do you think? is not the sloped floor an indicator of a settlement?

 

[SLTA]

Given that the tree is 15' away, I agree that it likely has little to do with the issue. Additions are often done by Bubba with a truck, flying below the inspection radar, and not designed by an engineer. A 4' deep concrete trench about as wide as the wall above would be more likely to settle than a wider actual footing under the wall. Without having seen it, I'd say differential settlement due to inadequate foundation under the addition.

 

[dhengr]

The sloping floor and the crack as you have shown it, would certainly seem to indicate the differential settlement you suggest. Exactly what’s causing that is another matter. And, we have some trouble seeing the foundation from here. But, the bigger question is how to fix the problem. I wouldn’t discount what the GeoTech. Engineer said, he said “frost action,” not just frost heaving. It might heave, and move (be pushed) inward at ground level, then upon thawing settle some more each year. These things are often tough to explain exactly. The freezing soil certainly can grab onto a found. wall like that, and move it around. You don’t know anything about the soil below that wall, maybe never well compacted, or poured on loose soil in the trench; and actually held up by skin friction at first. A tree might rob some soil moisture and cause drying and shrinkage of some soils, but I’m not sure that would be my first guess, as to the cause of the problem. I would ask the GeoTech. guy what repair ideas he has, he’s seen it and knows a little about the soils conditions.

 

[Robbiee]

Thanks slta and dhengr. The soil engineer has checked the soil bearing at the bottom of the shallow wall and found it stiff. He proposed to remove the wall and build a new proper one with a footing and smooth sides. The tree is a big one. Its drip line extended about 4 ft over the house, that is way I thought its roots would be all over the place.

 

[PEinc]

Although we have no information on the soil conditions, maybe you could re-level the footing and foundation wall by lifting the foundation with helical piers. Lifting could level the floor and close the 1/2 inch crack. In your location, is frost heave really a consideration, especially with a foundation 4' deep?

http://www.PeirceEngineering.com

 

[dhengr]

That’s about what I would have proposed. Obviously, you have to lift and support the existing wall to close the crack and level things up; build the new found.; remove the jacks; and fix the cracks, mostly joint tuck pointing, one hopes. Have the contractor make some investigation of and effort to cut back any tree roots in the new excavation. Then, depending on how brave you are, sprinkle a little copper sulfate in the bottom of the excavation before pouring the ftg., and again 2' below grade as he is backfilling. The copper sulfate should not be over done, but it acts as a root killer. And, in small quantities doesn’t seem to kill a large tree some distance away, and shouldn’t do excessive environmental harm. I’ve used it around drain pipes which are particularly susceptible to root intrusion.

 

[msquared48]

I would agree that the cracking is due to a vertical settlement generating a rotational failure seen in the crack pattern of the brick and foundation.

I would suggest either underpinning the existing foundation, or using 2" diameter pipe pile driven to refusal, then releveling and re-mortaring. Option C would be to cream the addition and rebuild it the right way. Chances are that the width of the foundation is woo narrow, generating high bearing stresses. Could be real crappy soil too.

As another thought, are the roof drains tightlined or are there slash blocks near the foundation here?

Interesting how brick veneer, a non-structural element, will bridge here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Archie264]

I wouldn't be too quick to rule out the tree as being the cause, fwiw. Even at 15' away it affect the moisture levels in the soil enough to cause that, especially if the soil is particularly susceptible to shrink/swell issues. I'm told that the root bulb of a tree is often roughly similar to the size and shape of it's foliage. If the trunk's 15' away the roots could still be much closer. That's not to say that it necessarily is the cause, just that it's possible for it to be.

I knew of a house that had a lot of trees in yard and it rocked and rolled - cracks often formed and/or closed up depending on rainy/dry cycles. The owner got rid of the trees for other reasons and the movement stopped.

 

[Robbiee]

Thanks for all contributors. My suggestion for repair was to stabilize the wall using 4 helical piles or underpin it and then repair the floor and the cracks. Frost line here is at 4'.

 

[hokie66]

Any of the factors mentioned, or all of them, could be contributors. It depends a lot on the location and soil types. Where I am, the tree would be the most likely culprit, as it doesn't frost here, and we have some very moisture sensitive soils. Second would be poorly compacted fill material that the footing bears on. I doubt that the width of the footing is the problem.

 

[moncalieri74]

hallo Ailmar,
it's easy: it's a differential subsidence.
the frost don't it's the cause.
the tree has nothing to do.
the solution i some micropiles along the continous fundation.
they block the subsidence

bye