Interior Slab Pull Away From Wall

[BDE Engineering]

I ran into a odd situation in a roller skating rink this week. The structure is 216x100 feet with the walls constructed of CMU. The long faces are the N and S faces of the building. The SE corner of the building has approx 4" of slab settlement (the area of settlement is approx 20 feet ea way from the corner). That same corner has approx 2" of settlement of the CMU walls, which had piers installed several years ago and show no signs of further settlement. Measurements taken with a laser level that does a 360 degree laser in the vertical plane show the wall has not moved horizontally. Also the slab is about 1-2" low along the eastern 3/4 of the building along the south wall and its only settled in the 15 feet perpendicular to the wall ( the exception is, of course, the SE corner where it's settling approx 4"). For context the owner has arcade games such as skiball machines that he says are quite heavy ( I doubt they are heavy enough to cause any issue, but I mention it for context).

Now I have all that laid out, the main problem that I'm struggling with is the interior slab at the SE corner has pulled away from the CMU wall leaving about a 3 inch gap. Interior slabs moving up and down a small amount from moisture variables in the soil are not uncommon with the clay soil here in the Kansas City area; however, this is something I've never seen in almost 20 years of doing this. A interior slab is at a pretty constant temprature, so shrinkage can't be a factor. This structure was built 60 years ago, and this is a recent problem according to the owner who's owned it for 20 years.

The only thing I can think of is that when the slab is sinking its causing itself to pull away from the wall, although it doesn't seem rational to me to say that. I'm thinking that if 3/4 of that slab is settling per my first paragraph perhaps maybe that's causing the pull away from the wall?

What in the world is going on with this slab? I cannot think of a explanation I can truly rationalize.

Thanks.

Brian Hansen, PE

 

[dold]

What's the grade around the perimeter of the building like? I.e., is it on a hill / mound of dirt? How deep are the footings below slab elev?

What I'm getting at is that if the footing of the entire wall shifted outward even a small amount that could cause a fairly large volume of soil under the slab to be displaced. But you report that the wall has not moved outward. Did you investigate this with just the laser? Did you verify it is indeed still plumb with a level or stringline? Those are my thoughts. I have had large retail stores with similar slab settlement issues and it was due to the entire site being built up on a 30' uncontrolled fill on the side of a hill. Not saying that's what you're up against, just looking for clues.

 

[BDE Engineering]

Dold, thanks for the response. The grading is appropriate around all sides. Mostly flat, but with enough slope for water to run off. That was my initial thought that the wall had moved outward, especially since it is on piers. I figured the eccentric loading on the footing from the piers was causing the bottom of the wall to move outward. My dewalt 3x360 laser will do a 360 laser plane in the horizontal and both vertical directions. I used the vertical plane laser to verify the wall was plumb. I do not know the depth of the footings below the slab, but assuming it was constructed correctly it would be 36" here in Kansas City.

 

[bones206]

I’ve found that getting a 3D scan is a useful tool for evaluating building movement and deformation. With more companies doing it, it’s getting more affordable.

I put the scanned pointcloud into Revit and start cutting sections. It can be quite revealing once you see the deformed shape from that vantage point.