Voids under existing slab-on-grade which is to support temporary shores 1

[ajk1]

Where suspended floor is to be temporarily supported by shoring (post shores) which is supported on old existing slab-on-grade, is it the practice to check that the slab-on-grade has no voids under it?

If so, how is this checked?

If it is not the practice to check for voids under slab-on-grade, how can such practice be justified?

We have found significant voids using GPR, and confirmed by drilled cores, that there are areas where there is up to 4" void under slab-on-grade.

There may be more questions to follow, depending on what answers I get for the above.

 

[JedClampett]

I guess if I had a suspicion that there were voids under the slab, I'd check it out. But I've never had that issue come up.
But there's lot of things we should check, but don't. We could check concrete strength, reinforcing size and spacing and slab thickness, but generally don't. Some things you need to trust, unless you have reasons not to.

 

[ajk1]

Anyone have any other opinions?

 

[BUGGAR]

When I worked for a Contractor (Guy F.), we insisted that the owner or whoever provide for a full investigation of any slab or structure by others that we were about to erect falsework on. I hate body bags on construction sites.

See Engineering Failures and Disasters.

 

[oldestguy]

One way to approximately check for voids is by listening to the noise when you whack a hammer on the slab. This has been a convenient way for me to find lack of subgrade contact due to slab curling at joints or losing base course due to "pumping" of fine grained saturated material at joints when truck traffic goes by. There is a much lower tone when the slab does not have support. It does not take a heavy blow.

 

[ajk1]

Thanks Buggar. I think that is the responsible thing to do where life safety is at stake. I wonder in what percentage of the cases that is done though. Significant voids under slab on grade are not always visually apparent, so I am not comfortable with JedClampett's concept that it is ok to trust things are ok unless there is reason to suspect otherwise, nor do I understand his analogy of concrete strength being particular.

Thanks Oldestguy - that is useful information and we were just about to try that. Have you ever done it by chain drag, since to hammer tap the entire 15000 square feet of floor would be a daunting task? We have found GPR is effective, and can also tell the depth of void which hammer tapping cannot, but I am not sure which is faster, hammer tap or GPR.

 

[BUGGAR]

Chain dragging is generally to find delaminations in the concrete. I haven't tried the hammer whack (at least not on a slab), but someone mentioned a rotohammer which sounds good (no pun).

 

[ajk1]

hmmm

 

[thaidavid40]

If the void areas are small enough in their individual extents, then you can probably just crib across them with timbers, or used steel beams, and simply span the voids. This carries your reshore post loads to competent bearing areas of the slab.
Dave

Thaidavid

 

[ajk1]

yes of course. The reason that I am asking the question is because the extent of the voids is about 20 feet where we have investigated and found voids; where we have not investigated yet, we do not yet know the extent of the voids. Irrespective of the solution, the first step is to determine the existing conditions...diagnose the problem before proposing a solution.

 

[dhengr]

Ajk1:
You are being kinda secretive with any details about what you are actually going to do, and the kind of building involved, what you are temporarily supporting above, etc. Are the shoring loads 1kip per post or 1000kips, what post spacing, what is the shoring arrangement in more detail? How is the existing slab performing otherwise? These details might influence the conversation a bit. Why and how have you found the voids you’ve already found? Where are they in the building layout? The chain dragging thing won’t work for this, but any thumping means (hammer blows, etc.) should cause a hollow sound. Look first in areas of the original building where there may have been additional (secondary?) excavation, such as trenches for utilities, along and around foundations, etc. These are often poorly backfilled and compacted to bring them back up to grade, and then settle out from under the slab over time. Any water/moisture movement in the soil can exacerbate this settlement. I liken this unmanaged backfilling to putting bowling balls in a bushel basket, lots of voids. Over time the soil clods break down, fill the lower voids and the whole mass settles in the process.

 

[ajk1]

I can now report for everyone's interest what I have learned since I first posted this:

- hammer tapping does not work for this case. None of us, all very experienced in sounding slabs to detect delaminated concrete and the like, could identify any noticeable difference between where we know from coring that there is no void under the 5" thick slab-on-grade, and where we know from coring that there is a void. Perhaps if we had the hearing of A Beethoven or Mozart,we could distinguish...

- voids were accurately detected by GPR(at least so far; we are currently extending the area investigated. Where it said there was no void, the coring confirmed that; where it said there is a void, the coring confirmed that too.

- GPR cannot distinguish between a 1/2" void and a 4" void, at least not with the GPR equipment being used; I don't know if there may be more advanced equipment available

- in this site, the voids are caused by a weak layer of silty clay that underlies a dense upper clay crust, according to the geotechnical engineer who is very familiar with this area of the city. We are about to undertake soil borings early next week, but due to headroom restrictions

So thanks again to everyone for your comments. To dhengr, you seem to be several steps ahead of me, as your questions are mainly directed to how to solve the issue of voids under the slab on grade. A wise engineer mentor told me almost 50 years ago, not to try to solve a problem until the cause of the problem is understood. The post shoring to which I refer is the standard post shoring used to shore around around the perimeter of a suspended parking structure slab when breaking out the slab to augment or replace rebar with loss of cross sectional area due to corrosion.

 

[mjr6550]

I find voids below slabs on grade quite often. This is typically is residential construction, especially garage or family room slabs adjacent to basement. There is no control over compaction. With a 4" slab I can often feel the voids just by by standing on my toes and dropping my weight onto my heels. Sounding with a rod or hammer also works.

You could sample areas to get an idea as to whether this is a widespread condition or not. If the voids are small and the loads not too high, then probably not much concern. If the voids are large in area they should be easy to find.

To determine void size or get a general ideal as to low loose the soil is below the slab I often drill small holes and probe with 1/4" to 1/2" diameter rods. Small holes can be drilled in seconds with a rotary hammer and are easy to patch (assuming the client does not mind). For residential work GPR is generally not practical.

 

[BUGGAR]

Thanks for reporting back, akjr, that was some good info.

 

[andriver]

"Perhaps if we had the hearing of A Beethoven"

You wouldn't want Beethoven's hearing..

 

[ajk1]

Yes Andriver, you are right about Beethoven!!! Bad on me. I forgot that!!

On a more serious note, we did a further 8 core holes in various widely distributed areas of the floor where the GPR operator said there were voids in a follow-up survey of the remainder of the floor, and there was no void at any of them! The GPR operator this time was from the same company as the earlier operator who accurately found the voids last time (in a different area of the floor) but was a different operator.

So now I don't know whether GPR is a reliable way to find voids or not, or it depends on the skill of the operator, or perhaps he had a different GPR device, although one would think that being from the same company all their operators would be using the same equipment.

 

[CANPRO]

Do you need to shore the entire slab above or just specific areas? If it's only a few spots can you core a few critical spots to check for voids? I don't think dhengr was getting at a solution, I think he was trying to narrow down your search area and trying to get a feel for how critical it is you find all of the voids.