Placing a slab on expansive soils
Placing a slab on expansive soils
(OP)
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
Also, what would you place under the slab? Gravel or sand? And how thick?
Thanks---Peter





RE: Placing a slab on expansive soils
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.
RE: Placing a slab on expansive soils
I appreciate the options you mention. What is a waffle slab? I can imagine it but I've never seen one.
Thanks---Peter
RE: Placing a slab on expansive soils
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...
RE: Placing a slab on expansive soils
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.