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Fill under slab on grade

Fill under slab on grade

Fill under slab on grade

(OP)
Looking for advice. We have a slab that spans 8 feet from a new shop to an existing building. It requires about 10 feet of fill below the slab. We recommended a well graded, free-draining crushed gravel with a maximum size of 3/4" compacted to 100% standard proctor under the slab. Contractor came back and asked if he could fill all the way down with 40mm, running equipment over every lift to compact. I think hes trying to get away from the cost to test compaction. This slab has the potential for forklifts to run over it. Are we being unreasonable in sticking with our original spec?

RE: Fill under slab on grade

Why take on liability because the contractor wants to cut corners?

RE: Fill under slab on grade

(OP)
100% agree MTNClimber. He's basically claiming that his proposed solution wouldn't be cutting corners, I guess that was more my question.

RE: Fill under slab on grade

Proposed solutions should meet or beat a (reasonable) requirement. If they don’t want to test, tell them to get lean concrete. If you allow them to cut corners now, they will only continue to do so for the rest of the project and for all future projects.

Edit: Yes, your request is reasonable for 10' of fill supporting a slab which will have large loads driving around on it.

RE: Fill under slab on grade

(OP)
Thanks for the information. I agree that they need some way to show that they meet a spec, otherwise how can we say it is fine. They often state they are using gravel that is essentially self-compacting, but everything I've read on here indicates that self-compacting gravel doesn't exist.

I'm wondering if anyone has any resources or books that would be helpful in creating specifications on things like compaction, material types, etc. for projects?

RE: Fill under slab on grade

(OP)
Further to this discussion, the Canadian National Building code (9.16.3.1.3) states "Except as provided in Sentence (4), fill beneath floors-on-ground shall be compacted." Section 9.16.3.1.4 states "Fill beneath floors-on ground need not be compacted where material is clean coarse aggregate not more than 10% of material that will pass through a 4mm sieve"

These statements seem very vague to me. What does the would "compacted" mean in sentence 9.16.3.1.3? Is there a specific level of compaction required by code? Also, in sentence 9.16.3.1.4, where it says it need not be compacted, would that be true for any depth of the material they stated?

RE: Fill under slab on grade

It is fairly common for a clean gravel, like No 57 Stone, to not be tested for compaction. It is only observed during placement and compaction to ensure it is compacted. It is difficult to drive a density probe through it.

RE: Fill under slab on grade

NAVFAC DM7.02 Table 4 on page 7.2-46 has some guidance on the degree of compaction for what the fill is being utilized for. It's a good start but some of those values differ from what I use.

I'm not familiar with the Canadian National Building Code but I would assume its similar to the US and that the degree of compaction is up to the discretion of the engineer. Based on your quote, it looks like its also stating that you don't need to compact the material if its 90% or more gravel. As you said, several of us, including me, do not agree with installing 10' of uncompacted gravel. You don't need to go crazy with it but installing it in lifts and compacting it with a few passes will help prevent issues. Further I remember someone stating that they used river gravel under a slab and had poor results. I would recommend always using angular crushed gravel when supporting anything so the particles interlock with each other instead of stacking them like marbles.

If you are not the geotechnical engineer, then you should contact the one on the project for their recommendations.

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