Piles and exterior slab on grade?

[SteelPE]

I am wrapping up my first pile foundation design. The drawings are currently for review with another engineer (to make sure I did the design correctly). A question was asked with regards to the exterior slabs on grade. Currently I do not have these slabs on piles however, I am being told, from the geotechnical engineer, that these slabs should be place on piles or else they would be prone to settlement. While I agree that they may be prone to settlement I am questioning whether or not it matters if they settle.

There are 3 locations in question. A dumpster pad located next to the building, an apron in front of a drive through door and a pad in front of a loading dock. I can somewhat understand the point about the pad in front of the loading but I’m still not sure if it is necessary.

What do other people do in this instance?

 

[vandede427]

struct usually doesn't show mech pads or dumpster pads. civil should show that.

 

[SteelPE]

It is currently being shown on the civil drawings. However, if it is required to be on piles wouldn't that then fall under the structural/foundation engineering?

 

[JAE]

If you support these slabs on piling, what about the slabs next to these slabs? There would be differential movement there as well. Where do you stop?

I've never put dumpster, drive-through paving on piling. That sounds ridiculous.

 

[ToadJones]

I basically agree with JAE
Whatever you do, DO NOT dowell the slab at the drive-thru to the slab inside the drive-thru = BAD. And, DO NOT dowell any pile supported slabs to non pile-supported slabs. This will cause cracking almost for sure.
I have, on occasion, while building homes, supported the driveway slab at the garage door slab off the wall or black footing. But, usually the problem there is more with heave rather than settlement.

 

[SteelPE]

Well, at least some people agree with me. I thought I was way off base with my thought process. My line of thinking was similar to JAE’s…. why isn’t the parking lot and road supported on piles.

I just find it a little hard to go against the geotechnical engineers recommendations. When I asked the question I never thought he would come back telling me that they should be pile supported otherwise they may settle.

 

[ToadJones]

Suggest to the GEO that you'd like to excavate the entire site down to bedrock and install a raft, that way nothing will move.

 

[mmillerpe]

If the slabs are not attached to the building foundation, I agree that they do not necessarily have to be supported on piles. In the past, I have provided for a decent base of structural fill below the slab.

Given the weight of the dumpster, and potential traffic, what type of settlement or differential settlement is expected. If these are settlements are tolerable, then let them float. It it settles too much, provide for a future topping slab to be applied once a majority of the settlement has occurred, likely this would be less expensive than driving additional piles.

 

[OHIOMatt]

If you have grade supported concrete next to the pile supported building concrete, you have the potential for extreme differential settlement. In the past, I have designed "hinge slabs". Basically the edge of slab adjacent to the building rest on, but was not doweled to the perimenter grade beam. I then designed the slab thickness and reinforcing for bending, assuming that there would be differential settlement. The span length was always a topic of debate so I typically went conservative.

 

[Bernied]

I agree with JAE regarding the dumpster and the drive through apron - those can both be built up at a later time if they settle excessively (as long as they are kept independent of the building). However, I am more nervous of the pad in front of the loading/leveling docks. This will get constant loading from delivery vehicles and repairs might become disruptive and settlement would render the levelers useless. If the soil is bad enough for the geotechnical engineer to recommend piling the pad I don't think I would do otherwise. Point out the problem to the future owner - he may choose to pay for the piled pad in this location.

 

[hokie66]

I think OHIOMatt and Bernied have addressed the areas of concern. Wherever a step between an exterior slab and the piled structure would cause an operational or safety problem, the exterior slab should be designed as a relieving slab to span a substantial distance, say 4 metres, from the building onto the ground. Ever drive over a bridge with a big bump at each end? That is where the relieving slab behind the abutment was forgotten.