cracks in garage floor slab 1

[PSUengineer1]

I am trying to determine if the cracks in this garage floor slab (see attached PDF for pictures) are the result of upheaval by nearby tree roots.
1.) Garage faces north.
2.) The garage was built in 1950.
3.) Grade around garage is relatively flat.
4.) Approximate 9-inch diameter tree trunk near both northeast and southeast corners of garage.
5.) Roots exposed near northeast corner of garage (see picture #2).
6.) The trunk near the southeast corner of garage was cut about 3 years ago and was about 35 feet high (see picture #3).
7.) Top of perimeter footing about 15 inches below grade.
8.) About 1-inch vertical distance between floor slab on each side of crack.

Could the cracks have been caused by..... differential settlement due to topography (i.e. relatively flat grade directing water toward foundation)?.... lack of control joints?......inadequate compaction?

looking for thoughts...

Thanks,
jimjxs263

 

[JAE]

That doesn't look like a tree-root issue where the roots are lifting up the slab - the crack is dead center in the garage - parallel to the two side load-bearing walls.

To me, this looks like one of two possibilities:

1. There was frost heave inside the garage, pushing up on the slab such that the center cracked open and the sides were more "held down" by the load bearing walls.

2. There has been some settlement of the two side wall footings over time due to loss of bearing capacity in the soils - possibly due to moisture exposure to the footings or possibly from trees and roots diminishing the supporting soil densities.

 

[civeng80]

It doesn't look as though the slab has any reinforcement at all.
So any pushing up by front heave or expansive clays would have little resistance from the slab as it just cracks very badly as photos show.

 

[ztengguy]

any history on the age of the crack? Lack of control joints, reinforcing, and age. its 63 yo slab on grade, alot can happen in 63 years, flooding, drouts, all can cause subgrade issues.

 

[TDIengineer]

I see a couple of things..
Slab on grade with no reinforcing. There is a drain in the center of the floor. I would guess that drain pipe has probably now a leak. There is a good chance the subgrade may have washed away beneath the slab, although there would be minimal water entering this drain inside the building unless it is a low point for the site.
The wood post added to the center girder caused load reversal in the slab, I think this may have accelerated deterioration of the slab.
You have settlement from the 2 parallel building walls, and a point load from the post. Slab subgrade appears disturbed. And I agree, if you are in a frost zone, soil heave also played a part. I don't believe the tree was the issue.

 

[BAretired]

The two large trees were consuming large amounts of water until they were cut down. After that, rainwater remained on top of the grade or found its way around the garage footings and through the base course under the garage slab.

This would have the effect of softening the soil under the footings leading to potential settlement. If the soil is a swelling clay, it would expand, pushing the slab upward in the middle. If frost is present during winter months, frost heaving could also play a role.



BA

 

[TDIengineer]

BA's thoughts above make the most sense to me. Definitely water intrusion. Not sure of your soil type, but if clay, expansive makes sense and would lift that slab in the middle. If within a frost zone, heaving from soil freezing would also explain the slab failing in flexure near the middle.