Flowable Fill to Fill Voids in Stone

[MRM]

Would you please give me your opinion on the following; I received a message from a local structural engineer who is involved in the foundation design for a 2 to 3 story residence. The site of the proposed home is in an old limestone quarry area that I know quite well. I was told that approximately 2 to 3 feet of fill (wood, debris, sand, limestone fragments, etc.) was removed in the footing subgrade areas. The property is underlain by relatively intact or somewhat weathered, yet competent limestone bedrock.

I was told that the contractor added stone (rounded, poorly graded septic field stone) without thinking about it too much. The structural engineer is now wondering if that was acceptable. Of course, it isn't for several reasons.

I had an idea that flowable fill would be a good option to fill the voids, reduce the chance for mixing with fines, and reduce the settlement potential and would allow the stone to be utilized. Essentially, there would be an approximate 2 foot tall by 4 foot wide "footing" directly on

 

[MRM]

Here's the rest-sorry about that!

bedrock with a footing atop that. Provisions will be made for perched groundwater interception as that is usually a problem in the area. Have you got any opinions of this set-up? Thanks for your thoughts.

 

[Grouser]

MRM
How will you determine the void ratio of the gravel? How will you verify the saturation of the gravel by the flowable fill? What will be the strength of this conglomeration?

Since it is only 2 to 3 feet to competent rock, simply remove the gravel and increase the depth of footing or lower the entire structure the 2-3 feet or be the first on your block with cathedral ceilings in the basement. Seriously, why risk the integrity of the three story structure to save a few thousand dollars. If this foundation alternative is expensive, design the structure for the third floor but only build two now and add the third floor later.

Good luck! Cheers

 

[VAD]

Hello MRM:

Sounds to me that your idea is sound. On can also use Cement grout which should be able to promote grain to grain contact of the granular material. This should suffice as well.

 

[Focht3]

I'm generally with [blue]VAD[/blue] on this - noting [blue]Grouser[/blue]'s appropriate questions and concerns. Personally, I don't see an infilled gravel as a big issue if the mass is "locked" in place. The big concern for me with the use of the gravel "as is" would be a bearing capacity failure since the particles can roll past one another. An infilled gravel may be quite acceptable if the footing pressures are "low" and the gravel pocket will be confined. And I have a hard time seeing the footing pressures exceeding about 2,500 psf anyway.

In general, I prefer grouting under pressure to flowable fill using gravity placement. I'd be worried about how far the flowable fill will penetrate. But each application is unique - you're the guy in the hot seat! And you're only talking about three feet of gravel. Why not perform a field test outside the building footprint - or at least away from footings? Give it a day to set, then dig it up. Then you can see how well it works -

Good luck, and let us know what you do.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MRM]

Thank you for all your comments! I was pleased to see that some of my concerns were also your concerns when it comes to using flowable fill to "cement" the stone together. You also mentioned a few I hadn't thought of too.

I spoke to my client earlier this week. I gave him some options last week, each having varying levels of risk associated with it. He informed me that he has elected to establish the entire foundation directly on the limestone bedrock rather than worry about whether the flowable fill will work properly. He also wasn't interested in the added time and expense involved with properly checking the results of the flowable fill infiltration.

The thing that pleased me the most about the whole situation was that things actually happened the way they should...I was approached with a problem, I gave different possible solutions accompanied with all the problems and risks associated with each. The client then made his OWN decision on which option to pursue. Beautiful! I was actually the risk assessor, as it should be! As most geotechs know, we typically act as the risk assessor, decision maker, AND insurance company with unlimited liability!

 

[Focht3]

Great job - and case history! It's always nice to hear when things go the right way -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.