Slab base depth on uneven grade


  #1  
Old 08-17-10, 07:03 AM
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Slab base depth on uneven grade

Hello and thanks in advance for responding.

I will be pouring a 10' wide x 14' long monolithic slab with 9" deep edges by 12" wide and a 4" slab. I removed top soil and vegetation and am now down to hard clay, however it is unlevel. If I were to fill with stone and compact I would have approx a 4" stone depth at the front and a 10" depth at the rear. I plan to use #57 stone, rebar aroung the edges and wire mesh.

Will I be ok going this route or do I need to dig down and make the earth level?

How deep is too deep for a base?

I will be grading around the sides to control water flow and here in Raleigh, NC frost heave is not a large concern.

I just do not want large cracking and worst case having the rear of the slab sink because of the thicker stone base.

A shed will be placed on the slab with no significant weight other then yard tools. I was also planning on hand tamping the stone but can rent a plate vabrator if absolutely needed.

Thanks again.
 
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Old 08-17-10, 08:17 AM
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You have a very light structure and the loads on the slab should not be too great.

Since the compacted base only varies between 4" & 10", if it is compacted, there should be no differential settlement from end to end if the edge is properly reinforced.

Just beating down on some rock does not really compact it. If it is too clean, it will not compact well - you just drive it into the clay, but clay moves slowly over time.

Dick
 
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Old 08-17-10, 11:12 AM
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Thanks.

When you say "if the edge is properly reinforced", what did you have in mind?

The back side, will have 10" compacted stone and 9" turned down slab. I will need to backfill about 5" of soil which will put me about 3" from top of slab. Front will already be at 3" below slab and side will slope down.

The entire perimeter will have 2 pcs of rebar and the entire slab will have wire mesh.

Should I add more rebar for proper reinforcement?

Thanks again
 
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Old 08-25-10, 06:43 PM
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Rebar is cheap, put it in the slab.
 
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Old 08-25-10, 07:43 PM
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How do you really compact "stone" or is that just a local term for something else? Without some appropriate fines, it really does not compact and provide good support.

Since you have minimal loading (projected for the short term) it should be adequate if the sub-base is decent soil.

Dick
 
  #6  
Old 08-26-10, 05:37 PM
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I hear that term used frequently with clean stone. Whether it's for the backfill behind my retaining wall, or for a driveway base.
 
  #7  
Old 08-26-10, 07:51 PM
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For a base that structurally supports a road (concrete pavment, pavers or a footing) it must be able to compact and support loads since it is a part of a sytems caarrying loads.

If used as a backfil it usually is required to be well-draining to reduce the load on a wall or transfer the moisture to a collection system. If it is too clean or coarse it might require a filter fabric to or some clean sand added before placement in order to prevent the existing soil from clogging the drainage. The fabric does not stop the flow and the proper sand prevents the clogging also.

Terms like "clean stone" do not mean much since some "clean stone" is fine enough to be compatible with the soil in place.

Cook book solutions and proper terms are what sepatrate are what separate fry cooks from chefs that need a good long-term reputation.

Dick
 
 

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