Soil Supporting Garage Slab

[ANE91]

I am reviewing an engineered design for a new residential structure having two stories above grade and a basement. A garage on a slab is built into the first story.

For the garage slab, the drawings indicate use of fill from the basement excavation. No compaction or moisture conditioning is specified. The slab is non-structural, as detailed, and utilizes interior grade beams. (I traditionally call these stiffening beams.)

My understanding of the code is that the slab should be founded on native soil or engineered fill. Excavated soil placed without compaction/conditioning is neither, in my opinion. The engineer states that the grade beams take care of any soil voids or other “looseness.” The contractor states that it is always done this way.

What is your design approach with respect to soil-supported slabs? I see the argument that the slab itself is not technically part of the foundation, but I maintain that the slab will tend to dish (settle) and crack under the weight of parked vehicles.

 

[Eng16080]

If the soil isn’t properly compacted and settles, resulting in the slab not being supported by the soil but by grade beams instead, then I would consider the slab structural since it will need to be self-supporting over some distance.

I perhaps don’t understand what the common construction is in whatever geographic region this happens to be, but I would want a non-structural slab to only be supported directly by well compacted structural fill or existing undisturbed soil. I wouldn’t even want it to be supported by grade beams (even with proper compaction) because then any soil settlement will result in voids below the slab. If supported by grade beams, I would only design the slab to be structural, spanning between grade beams.

 

[jhnblgr]

What is the reason for your review?
I’ve done a few but typically they are contractor submittals for temporary work or substitutions for retaining walls. One engineer when I pointed out an error corrected it, went a bit over board with a new software, since they didn’t have a clue what they were doing they designed a massive wall, but I determined it was safe on review at least. Other times I had wood shoring and they didn’t want to embed to bottom sheeting into the soil so the soil could act as a strut but instead wanted it to just rest on top of the soil which means the bottom is unrestrained and the max moment is no longer a simple beam but a cantilever beam. I informed them of the error they came back with we have always done it this way and refused to check the member in bending. With pressure to approve and the contractor being the engineer in the case with direct connection to the owner. I did the calculations needed and marked up and approved as noted. The calculations they had done didn’t match the members they had selected so on further review as a result of their upsizing the member they lucked out having the required capacity. But this engineer refused to listen to reason and also violating ethics and state laws. If allowed in your capacity I would write a letter summarizing your suggestions to the owner and let them decide. I assume your task is to verify that standard of care is followed so that the owner is protected since they don’t know any better. Engineers aren’t infallible and if a reviewing engineer is required by the laws I would say it is in the best to inform by reasoning the owner the risk. I have seen whole slabs settle 3-4” in cases just like you have and always recommend clean stone in excavation to simplify the backfill and to help prevent settlement. Clients often don’t follow that part of the plan since all they see is money but when there’s an issue they will spend a lot more to fix it.

Question how are they transitioning from wall foundation to grade beams? The typical thing I argue against is switching foundation systems in buildings. I typically show frost walls with steps from basement to garage. Differential settlement otherwise would be a concern where the two meet.

 

[Mark_J]

Should be fine. What do they have specified, 2 grade beams going into the assumed over excavated garage/basement location (18' wide)? I put a 2x4 key and thickened slab in anticipation of the builder not compacting the soil properly.

 

[rlmengineer]

I am reviewing an engineered design for a new residential structure having two stories above grade and a basement. A garage on a slab is built into the first story.

For the garage slab, the drawings indicate use of fill from the basement excavation. No compaction or moisture conditioning is specified. The slab is non-structural, as detailed, and utilizes interior grade beams. (I traditionally call these stiffening beams.)

My understanding of the code is that the slab should be founded on native soil or engineered fill. Excavated soil placed without compaction/conditioning is neither, in my opinion. The engineer states that the grade beams take care of any soil voids or other “looseness.” The contractor states that it is always done this way.

What is your design approach with respect to soil-supported slabs? I see the argument that the slab itself is not technically part of the foundation, but I maintain that the slab will tend to dish (settle) and crack under the weight of parked vehicles.

How deep is the fill material?