Slab subbase spec
Slab subbase spec
(OP)
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.
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.






RE: Slab subbase spec
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?"
RE: Slab subbase spec
RE: Slab subbase spec
RE: Slab subbase spec
RE: Slab subbase spec
RE: Slab subbase spec
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...
RE: Slab subbase spec
- 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.