Footing Translation 1

[XR250]

Hi there.

Sorry to invade your forum 

I am a structural engineer and have seen an interesting phenomenon a handful of times in the last few years. On a residential foundation, I observed a vertical crack – say ½” in constant width. The sites are flat but located in a Triassic Basin with a lot of plastic clays (Durham area of NC). All things indicate the footing has broken and translated longitudinally (or laterally in some cases) without settlement or rotation. The differential backfill is minimal. My go-to Geotech says no way. I don’t believe him. Can the soils shrink (or swell) in a manner to cause footing translation? All the foundations were part of a crawlspace or garage stem walls.

What are y’all’s thoughts?
Thanks

 

[oldestguy]

Unless you have some permanent bench mark, such as a well casing founded way down, maybe on rock, you don't see differential settlement of a massive deposit. It is most unlikely for soil shrinkage or expansion to be only in one direction. I've had one case where sideways movement like this opened up splits in a house, yet no differential settlement noted. It was a deep layer of clay. Cure was to cable it together.

 

[r13]

Are you sure it is not shrinkage crack?

 

[XR250]

They were definitely not a shrinkage cracks.

 

[oldestguy]

retired13. May well be a concrete shrinkage cracks noticed on side of basement but concrete shrinkage also can be in 3 dimensions. Also that foundation top surface goes lower. Distance between cracks can be 10 feet or so. Due to weight the horizontals may not open visibly. If the wall is quite long and one crack, no horizontals will show up. Who says concrete also only shrinks in one direction? What proof? On another material, such as wood, yes one direction mainly due to cell make up. Not so concrete or most soil.

 

[r13]

What is the foundation - continuous pad footing, or grade beam between columns? How was it constructed, direct cast over the native soil? Do you have soil report? Is the crack through the depth, or on the surface? When you noticed the crack, before backfilling, before constructing the upper structure? You hypothesize the soil might have caused the footing to "translate" longitudinally, or laterally, how?

 

[XR250]

Typically 8" masonry crawlspace foundation on concrete strip footings 10-30 years old. The cracks are all the way thru thickness. The cracks are vertical. The walls are typically not long enough for these to be shrinkage cracks of this magnitude.

 

[r13]

Is this a existing structure? Structural cracks are usually occur in high stress areas, and the crack width rarely uniform/constant. If the footing has been pushed laterally, then there should have a few finer cracks by the sides of the 1/2" crack.

 

[r13]

OG,

Yes, concrete crack is 3 dimensional, random cracks do occur. Horizontal crack (splitting) is rare though, mainly due to confinement from gravity load. Yes, if it does occur, it will be very fine.

 

[XR250]

I've had one case where sideways movement like this opened up splits in a house, yet no differential settlement noted. It was a deep layer of clay. Cure was to cable it together.

What is the mechanism that causes this?

 

[Ron]

As OG noted, shrinkage is orthogonal....both in concrete and clays. If you have a long, narrow strip of concrete, the cracks will typically be transverse to the long direction. Shrinkage is a linear strain function...shrinkage occurs in three orthogonal lines, simultaneously. If you have a 10-inch thick section that is 10 feet wide and 10 feet long, shrinkage manifestation is exacerbated in the long directions but the strain rate is the same in all directions.

Clay has similar characteristics. While clay shrinkage has a different mechanism than concrete shrinkage, they act similarly. A long, wide but relatively thin layer of clay would move similarly. The big difference in clay is that it can expand and contract with moisture changes to a much greater extent than concrete. Concrete rarely expands beyond its placement volume. Not so with clays. When you have expansion and contraction of clays you can see non-recoverable movement in a structure. In short, the clay expands, pulls the structure with it and then it contracts but the structure "slides" across the contraction and doesn't return. Over the years this progressive movement can show in large cracks. Cabling is an appropriate repair for such conditions.

 

[XR250]

Thanks for the explanation, Ron