Small existing structure with cracks in plaster exterior, owner wants to add a slab

[xp1]

Hello, I'm a structural engineering student working on a small project in Tanzania for a class. We're building a new structure and renovating an older one that has several cracks on the exterior plaster facade. The older structure is 1 story CIP foundation, columns, and ring beam with infill brick walls.

For this older structure, the local (Tanzanian) engineer suggested that a 6" slab be added on top of the existing slab to prevent further cracking. Now I haven't taken any foundation engineering classes yet (coming up this semester), so I consulted with a US engineer who said this wouldn't work. I believe the point is that the foundation was not built properly for the soil, and this is what is causing the structure to move excessively and therefore the walls are cracking. A slab would thus not help much if at all.

We're Skyping with the owner tomorrow and I wanted to explain so I was wondering if I could get y'alls take on this, e.g. did I understand correctly, is there anything else to be said etc. They're concerned about the soundness of the structure since it houses children but I think it's probably fine and just needs a new facade. I was thinking we could maybe examine the ring beams and columns just to make sure, and if everything's alright just remove the plaster and use some other material like wood siding.

Thanks for any suggestions!

Structural Engineering Intern

 

[Ron]

Those cracks are not likely foundation related and I agree with the US engineer. Adding a 6" slab on top of the existing will only add load to the existing structure that might be more detrimental and cause settlement.

The cracks are likely a result of shrinkage in the plaster or the substrate. What is the wall structure behind the plaster?

 

[xp1]

Thank you for your thoughts! The walls are these bricks they have instead of CMU, they're about 6"x9"x18" of solid concrete. I should ask them what exactly the facade is made of, or what kind of plaster it is - I hadn't thought it could simply be that.

I asked for more info on the foundation, and since they didn't have the old structural plans they dug at the side of the structure and it looks like there's a meter of concrete, probably a perimeter grade beam. The SOG is probably not very deep, maybe 6". From another set of plans for a similar new building they want to construct, I saw they had a 9"x48" perimeter grade beam and then a 27"x9" continuous footing. It might be something like that here.

We're not sure what the soils are, we're thinking clay though and they have rainy & dry seasons so that would cause lots of movement. They're asking $300 for a soil test but we have to see what that would include. I'm also trying to find a geotech engineer to discuss this with, hopefully this week I can find out more.