×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Placing a slab on expansive soils

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

RE: Placing a slab on expansive soils

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.

RE: Placing a slab on expansive soils

(OP)
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

RE: Placing a slab on expansive soils

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...

RE: Placing a slab on expansive soils

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources