Flowable Fill Under Foundations 5

[g_pat23]

Hi all,

I have a new residential job that consists of a 1-story slab on grade residence that is to be built on a moderately sloping hillside. My company did three test bores and found the following: Hole 1 consisted of low plasticity clay to approximately 13 feet where weathered but hard limestone bedrock was encountered. The second hole consisted of approximately 7 feet of low plasticity clay underlain with the same limestone bedrock. The third test hole consisted of approximately 2 feet of low plasticity clay underlain by limestone. In order to reach the designed finished floor elevations approximately 2.5 feet, 4.5 feet and, 4.5 feet of cut will have to be performed in the locations of test hole 1, 2, and 3, respectively. My recommendations to them are going to be to perform all their cut earthwork first then in areas where the limestone bedrock is not exposed (specifically along footing lines) dig until the limestone bedrock is reached and backfill with concrete flowable fill to their footing elevations. From there construct footings, etc etc.

Just looking to see what people have to say about this plan/what others experience you all have with flowable fill. Thanks in advance.

 

[Ron]

That seems a bit much for a single story residential structure with a slab on grade. Unless your clays have expansive capability, I would not excavate to bedrock. Your location 3 implies cutting into bedrock by 2-1/2 feet. Is this necessary? Seems overly conservative and expensive. Maybe I'm not seeing the whole picture, but this all seems excessive.

 

[g_pat23]

The area they are building in has height restrictions and requirements so they have to cut down and into the hillside that much just to meet them. Our concern is the footing transition from rock to soil, in general we do not like to see that sort of transition. That is why we would like to see all the footing bear on the limestone bedrock either directly on it or have the load transferred to it via the flowable fill.

 

[ltbayne]

With regard to Ron's comment, most clays have some expansive capability. As I was reading g pat23's first post, I was thinking that I would take the same approach. My flowable fill would probably be lean (3-sack mix ) concrete. What will the slab-on-grade rest on -- clay? In my opinion slabs-resting on clay are lawsuits waiting to be filed.

 

[jayrod12]

I'd likely be doing structural slab on piers down to bedrock instead of flowable fill for everywhere you aren't bearing directly on bedrock. But that's just me.

 

[Mad Mike]

So you've got some rock on one small portion of the platform and deep (but presumably firm) clay over the remainder of the platform.

Why let a small volume of rock dictate the founding? Rather than removing all the clay, rip out the nub of rock and replace it with a soil material that can settle somewhat. Consider replacing it with the lean clay, then you'll be founding into similar material on all sides.

Agree with Ron- your recommendation sounds tailored towards a bridge footing rather than a single-storey dwelling.

All the best,
Mike

 

[BigH]

Perhaps it is too early in the morning but am I wrong to understand that the slab on grade will be lower than the existing ground over the whole of the footprint since you have the height restriction? If so, for a one story residence, I wouldn't expect 'any' settlement of footings even if some were on rock and some on the low plasticity 'clayey' soil (presumably at least 'firm' in consistency) as the weight of soil removed would be more than the weight of the structure imposed. Certainly any differential settlements across the boundary would be nearly negible. If this is the case, you may need to ensure that you can handle precipitation.

 

[fattdad]

How's the undrained shear strength of the, "Low Plasticity Clay?" Then too, is there any sand?

I see clay on limestone in Virginia. Sometimes, that clay is very soft! Sometimes it has some OCR? Sometimes there's no sand content.

We just don't know enough!

f-d

ípapß gordo ainÆt no madre flaca!

 

[g_pat23]

Thanks everyone for their input! We have decided to give them a preliminary report with our findings just to get them start with excavation. Once their building site is cut we are going to go back out and have a better look at the subsurface conditions. Stay tuned...

 

[Tomfh]

We would document it as pier/piles in the soil zone, with suspended slab over, but in my opinion your solution is good and I think it should play a bigger role in resi projects. For whatever reason it’s generally not done. Too expensive I guess?

I’ve replaced a lot of crappy subgrade at our place (below pavements, driveways, garden walls etc). Many tonnes. I mix with at least 5% cement (and water) and pour it back in. Too bad I can’t do it to the house footings - the house is already build and difficult to excavate beneath. I’m stuck with a house on clays, like a lot of people.