Placing a slab on expansive soils

[petrosoft]

A client in Ukiah, CA wants to put a 20' by 30' patio slab next to his house. The soil has cracks. It looks and feels like CH soil with some rock fragments. In some places the pad for the slab is a cut and in other places it is on 4 ft of fill. The owner is worried about the slab cracking. I'm thinking of a structural slab supported on grade beams spaced 6'to 8'apart. The alternative would be to recompact the soil. Any thoughts?

Also, what would you place under the slab? Gravel or sand? And how thick?

Thanks---Peter

 

[GeoPaveTraffic]

Does the owner understand that to prevent a slab from cracking at all in a cut/fill situation, leave alone if the soils are expansive, would be very expensive? How badly does the owner want to avoid cracks? Maybe a brick/paver patio would be a better choice.

That said, a pre- or post-tensioned slab may be cheaper than grade beams. Another alternative may be to use a waffle slab with pea gravel. The concept is to design the waffle to bear on the pea gravel with a factor of safety just above one, then if the soil expanes, the pea gravel fails in bearing and "flows" into the voids to either side of the bearing area. The specifics require a considerable amount of thought, but it can and does work.

 

[petrosoft]

Thanks GeoPaveTraffic,

I appreciate the options you mention. What is a waffle slab? I can imagine it but I've never seen one.

Thanks---Peter

 

[GeoPaveTraffic]

Don't know exactly how to explain it...

Picture a grid with concrete strips about 3 inches wide spaced about 18 inches on center. These strips are about 12 inches tall. The grid is palced on about 12 inches of pea gravel and is topped with a 2 to 4 inch thick concrete slab. There is nothing between the "grids" of concrete, I've seen cardboard "fillers" used to help cast the whole thing.

That said, it all seams like over kill for a patio...

 

[oldestguy]

Can you explain that "all concrete cracks" and if you use closely spaced jints that is where it will crack?

Your grade beam thing sounds line a waffle slab, but with just differing scale of vertical section spacing.

Some joint reinforcing will help keep the sections from faulting.

The client needs some educating about concrete.