Settlement vs. shrinkage cracking

[jay156]

I have a client with a cracked basement wall. I went to look at it and the attached picture is what I saw. It was about the same width, 1/8"-1/4" or so, all the way down to the second course from the bottom, where it tapered into nothing. As you can see, the guy she's buying the house from tried to put some crack filler on there to cover it up. There's also a concrete patio in front of this wall that has a crack in approximately the same location.

Now since it's a vertical crack, I think I can narrow down the causes to either settlement of the foundation, or shrinkage of the blocks. Is there any way I can be sure it's one versus the other?

Also, the lady told me her home inspector said this was an "active crack." How he determined that I don't understand. I couldn't see the crack moving, obviously, so unless he had some historical data, how could he know it's active?

Thanks.

 

[jay156]

Also, I didn't observe any movement inward of the wall. I should've put a plumb line next to it, but I sighted down the length of it and it wasn't bulging a bit and it didn't appear to lean either.

 

[msquared48]

Lsateral earth pressure could do this to the wall, but not the slab.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[FixedEarth]

It could be isolated heavy clay backfill or maybe a drain is clogged-who knows. It is also possible that active earth pressure suitable for sand was used in design instead of at rest earth pressure for clayey soil.

I was only involved in about 5 of these in the late 90's but it is more due to the lack of steel beam tying to that wall and the absence of any wall jogs to create a relief from the earth pressure.

Even in shoring design work, the soldier piles near the center of the wall length deflect more than the ones near the corner. Since there is no bowing and that the crack does not go through the footing, may be we can just replace some of the backfill in the center 1/3 of the wall length.

 

[msquared48]

I'm thinking that this is a two scenario situation.

The slab is probably a shrinkage situation as the concrete looks relatively new.

The wall damage is mostlikely lateral earth pressure damage for whatever reason - under-reinforced, plugged drain lines, clay, roots, etc.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

I didn't see an age of construction and if the CMU wall is reinforced, and if so, how often?

I am not sure why lateral earth pressure would cause a vertical crack in a CMU wall? Bending over-stress in a basement wall, if you model it pinned-pinned, may result in horizontal cracks near the center of the inside face of the wall, no?

And as far as lateral pressure caused by tree roots, very unlikely. I had a forensic job where I had to extensively research this topic due to a retaining wall failure, and tree roots will grow along walls and foundations but they will not press against them. They seek the path of least resistance. They make commercial root barriers out of thin plastic. Not to be confused with damage caused by sidewalks or foundations built on TOP of tree roots.

Based on the info we have, and the vertical pattern of the crack, I agree with whoever said this is a differential foundation movement issue. Settlement at the center or heave at the ends, or vice versa, could cause a vertical crack. Though it'd be nice if it was more textbook and wider at the top or the bottom... If you think it may be clay is their anything drainage related you can put your finger on- downspouts, sprinkler heads, new driveway or altered drainage pattern.

Bottom line is a solution. Since the porch slab is right over top its not an easy demo and excavate situation. Geotechnical testing may or may not give you data that helps you solve the problem, sometimes, it gives you more questions (not that I am against it). Do you have a local geotech friend that knows the area and can tell the types of soils, especially expansive clay, that may be in that area?

Without any other distress I would proceed with a repair after thoroughly explaining the options to the client. This may be a band-aid fix or it may last 10 years or the lifetime of the house. It will be the least expensive option for now and at least you will then have a clean slate moving forward. If those cells are hollow at the crack I would likely sawcut out vertical slots to each side, dowel vertical rebar into the foundation and bond beam, and solid grout, with some horizontal "stitch" U bars. Hopefully there is a window to stick the concrete pumping hose thru! If the wall is solid grouted, epoxy injection in the crack.