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I was looking at buying a house and the concrete in the garage sunk about 2" on each front corner, leaving gaps under the garage door.

I then realized that at least 5 houses on the same street had the same problem. They all had double attached garages with an upper floor extending over the garage. Built in the 1980s, in Edmonton, Canada.

What I guess happened is that the garage beams, which are presumably on piles, sunk a couple inches and pulled the garage pad and driveway pad concrete (all connected by rebar) down with them.

My questions are...

  • What would cause this? Poor construction (too much weight, insufficient piles, etc)? Environmental conditions (flooding)?
  • Is there much risk it will keep sinking if one poured a new garage pad, or will it have settled after 40 years?
  • Is there anything else to be done to fix this (ie. jack things up and add new piles or something)

enter image description here

  • What concrete? The slab or the foundation walls? It's common for low-grade builders to skimp on compaction under garage slabs, allowing them to settle while the foundation is unaffected. That could be all it is. Photos would be just dandy. – isherwood Apr 5 at 0:39
  • Here's a photo from the outside. The concrete of both the driveway and the garage floor inside is sunken in the front on the sides. imgur.com/ss1himw – user35358 Apr 5 at 1:04
  • That looks like a lot of movement. If other houses on the street look the same way, the people laying the foundation weren't doing a good job or the ground in the area is particularly bad. I'd stay away from that, but I'm not familiar with Canada at all. – JPhi1618 Apr 5 at 2:05
  • That's not a photo. It's a watercolor painting. – isherwood Apr 5 at 12:52
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It has been ~40 years the settling is likely done.

The weight of the house on the posts or garage door jamb that support the beam over the garage door opening are putting point loads on the foundation but I can't see that portion of the foundation sinking only in those isolated spots.

If I had to guess, there is no foundation wall under the garage slab at the point where the garage slab meets the apron to the street. The soil here has just settled more at the point load locations or perimeter drain which runs along foundation footings has pulled soil away. Possibly the pdrain terminates or runs inside the garage instead of going straight across.

Do you have a basement? Do you have a powered sump pit?

Slabs don't typically get rebar unless you have a basement under your garage and it is serving as a suspended slab. They sometime get welded wire mesh to help with hairline cracking but I'd guess there is no metal in the garage slab.

I think you could redo the garage slab and not have any issues.

If you look at the interface of the garage apron that has sunk and the foundation wall that is supporting the garage jamb, you'll see some indication that the slab used to be higher on the foundation wall - showing that the foundation stayed in the same place while the slab sunk.

  • Thank you. Note that garage slabs around here definitely get rebar these days, which is dowelled into the foundation. I'm not sure about 40 years ago. We are going to check for indication that the slab used to be higher on the foundation wall as that will confirm only the slab sunk! – user35358 Apr 5 at 14:54
  • The slab appears to be bowed. How does that happen? – Sherwood Botsford Apr 5 at 19:36
  • @SherwoodBotsford, the most likely cause is the foundation was excavated close to the two edges for the stem foundation walls. The soil put back is not as compact as the inner relatively undisturbed soil. The slab above the disturbed soil sank as the soil compressed, it stayed in place where the soil was already compressed. – Fresh Codemonger Apr 6 at 0:11

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