Slab subbase spec

[haynewp]

Sometimes we get more detailed information in the soils report for the subbase under slabs-on-grade for buildings. Such as: "A 4 inch thick layer of free draining gravel should be placed beneath the floor slab. This material should consist of minus 12 inch aggregate with less than 60% passing the No.4 sieve, and less than 5% passing the No. 200 sieve". Other times it might say something like: "6 inch thick layer of quarry run" or "6 inch layer of crushed stone". Often, "#57 stone" gets specified in our area.

So who is responsible for determining the specification of the slab subbase? I have always thought it should come from the geotech but the reports vary in requiring something specific to just basically saying to just put a crushed gravel layer under the slab. The drainage capability needed for a site and the compact-ability of the material to used beneath the slab would seem to be something the geotech should be specifying the material needed to meet these requirements and not us. I have gotten the request on a project to give the spec for the subbase and the geotech is just saying that a quarry in the area should be able to provide their typical subbase run and that it would be adequate. So should the specification be: "a standard subbase run from a local quarry"?? It doesn't sound technically correct.

 

[JedClampett]

The Geotechs usually don't go into a lot of detail. For example, for a building in Florida, they pretty know what they're going to get locally. I've had very few reports that got into any details and the ones that did, pretty much align with the local materials.
To specify base, I go the local county or DOT specifications and copy their requirements (or just reference the specifications) in my specifications. Never had a problem. Plus, the contractor can't say, "...we can't get that here, what's the local equivalent?"

 

[haynewp]

Thanks, I was just hunting for that spec in the DOT website of the state the project is in when I read the response.

 

[cvg]

unless there is high groundwater, heavy loading or crappy subgrade, the need for a gravel layer under a slab is generally for moisture control, constructability and serviceability of the slab, not for structural support. so the geotech really doesn't care what you put there, he is way more concerned with the structural footings

 

[haynewp]

In this case it is a 1500 ft^2 mat slab with about 2000 kips of storage load.

 

[BUGGAR]

Geotechs I know run from slab subbase recommendations like rabbits because that gets them involved in water damage related issues when the architect specifies the wrong flooring or doesn't follow recommendations on slab moisture content before flooring.

 

[oldestguy]

For nominal floor loadings, such as residential, automobile parking, etc. the geotech probably doesn't need to spec anything. This assumes on site check by them will verify firm support conditions. for instance the geotech probably is there to check degree of compaction of wall backfill upon which the floor will sit.

For known heavy floor loadings, a common recommendation I am familiar with is furnishing a figure for modulus of subgrade reaction. This then assumes the designer uses some method for designing his slab, perhaps something like this:

https://www.wbdg.org/ccb/ARMYCOE/COETM/ARCHIVES/tm_5_809_12.pdf

 

[asixth]

I operate in Australia and I have seen a few different subbase's specified.

- piling mat (typically 400+mm thick gravel) when a piling rig needs to operate over the top.
- drainage layer (again 200+mm thick gravel) when the slab is below the water table.
- 50 to 100mm bedding sand for a typical slab on ground (lightly loaded).
- 150mm thick CBR15 (this is a road base material).
- 100mm DGB20 (densely graded base 20mm - another road base material).

For your example where the bearing pressures are over 50kPa I would specify that a geotech confirms the bearing capacity prior to placing concrete.