Shallow Foundation Suggestion

[ets1]

We are in the early structural design stages for a 100 home housing development in Florida. The houses are 2500 to 3500sqft and two storey constructed from 8" CMU. The site is a reclaimed swamp area, a few test pits indicates visually the top 3 feet is loosely compacted limestone sand fill on top of another 3 feet on medimum dense limsetone fill. No organics or clay and both layers were placed 20 to 30 years ago. The limestone fill is on top of 2 feet of loose sand, followed by 3 feet of "muck". Below the muck is limestone bedrook at 10 to 12 feet below the existing grade. Water is at 6 feet. We are going to request some SPT test on the fill material but from the visual inspection a raft foundation should be okay, however, the developer wants to raise the houses by an additional two feet. Our options are now a strip footing with 2ft stem walls or end bearing auger piles to bedrock with grade beams (costly).
How will the for additional fill under the building influence the spread strip footings' potential bearing pressure and settlement potential? Any suggestions?
What test should we have carried out now and what should we require from the geo-engineer

 

[Ron]

ets1...you are describing some fairly adverse subsurface conditions. Assuming you are in South Florida from your description, find a good Geotechnical Engineer and let him decide the appropriate course of investigation. Don't dictate that to him....he should know the subsurface stratigraphy in general and coupled with some information you already have, he should be able to guide you in an appropriate investigation and subsequent foundation system.

 

[ets1]

The geotechnical engineer is scheduled but I want to continue working on the design. Just want creative alternatives before I get the usual safe solution...piles at 12 feet on center. I don't think the SPT test will be done because the site is so remote. Any suggestions on portable test that we can ask about?

 

[jgailla]

Organic "muck" is likely to experience consolidation settlement over time in addition to settlement caused by organic decay. Since the fill was placed 20 to 30 years ago, a significant portion of the primary consolidation settlement has likely already occured. The addition of 2 feet of fill will cause some additional (likely minor) settlement. Perhaps you could run tests on the muck to determine how much organic material remains. If the water table has dropped much of the settlement due to the organic decay of the muck may have already occured. Unfortunately this is unlikely because of the muck depth below groundwater. You may want to consider post tensioned slabs and beefed up grade beams. This will minimize the effect of differential settlement of the underlying soil. Piles can be dangerous when bearing on limestone because of possible sinkhole or punch through considerations.

 

[fndn]

If you compact the initial layer before the new fill, then you could do exploratory borings after the new fill is complete. This subsurface info is needed before embarking on footing or raft option in the foundation design.

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[BigH]

First off - this is a major development of houses that will cost a fair amount of the green stuff if I read it correctly. A geotechnical engineer of record is a must - he should decide on the course of action. He may only wish to do test pits to obtain some samples of the "muck" to see how bad it really is; how compressible. Remember that the muck has been overlain by 8 ft of rather old fill so secondary consolidation should be well underway. The way I look at it is as follows:
1. As it is a major development, you will need to bring in compactors and likely need to do some grading. Then compact the piss out of the upper fill layer - this will give you a better upper couple of feet of fill.
2. As you have 8 ft of decent fill above the muck, this might/should be enough to consider house raft foundations. In that the developer wants to raise the houses, an older style grillage foundation might be perfect - just think of this as the "divider" cardboard in a case of whiskey bottles. The large D/W ratio will give you stiffness, the height will get you the extra two feet and then your floor slabs (concrete or wooden) could span the grillages.
3. I take it that in the two foot raise, this will not be done by fill.
4. With the stiffened grillage foundation, it should be stiff enough that if there is any differential movement it should be of uniform tilt.
Anyway - this is my early take on the matter - Geotech is imperative even at this early stage.

 

[Ron]

I agree with BigH...again, let the geotech decide the course of action.

ets1...you make two statements that bother me.....
"Just want creative alternatives before I get the usual safe solution...piles at 12 feet on center." and "I don't think the SPT test will be done because the site is so remote."

If you think you'll only get one predicted recommendation from the geotech, then you need to be a bit more discriminating in your selection. A good geotech will look at several alternatives, not just the overly conservative one. This development is large enough that you might consider paying for a 2nd opinion based on the first geotech's data.

In your second troublesome statement you state that SPT's won't be done because of remote site. Bull$#!^. Pick a geotech who has the capability to do remote sites. It won't be remote after development!

 

[ets1]

Thanks for all the opinions, esp. BigH and jgailla, this is the kind of information that I was looking for. Will be able to be more comprehensive with the engineer. Thanks again.

 

[BigH]

ets1 - you would be surprised at some of the remote locations at which I have carried out penetration testing! On one site, we sent up hammer, spoons and rods and I, along with a couple of other strapping lads (I was one - once!) did a manual job! Drove 6 continuous samples to 12ft (we used 2ft spoon). Did a couple of holes; got tired out and, then worked on our elbows that night! But it can work - you need to want to make it happen - but if the site is that remote, why would you be putting in 100 homes!

 

[ets1]

BigH, The site is a reclaimed area off the Cays, for stopover yacht owners who want a place to rest and party. I was thinking the SPT could be done manually and try to get it done. Right now, considering cost, construction and materials we are looking at two alternatives...1. Grillage foundation with 2ft exterior cmu stem walls on reworked and compacted existing fill, 2. End bearing auger cast piles with 2 ft elevated timber framed floors and walls. BTY, the houses will probably be timber framed prefab panels and not CMU for construction reasons. Either footing should work its just a matter of cost and ease of construction. Thanks for everyones help.....

 

[Focht3]

Put a drill rig on a barge and go. You can also drill for the docks, bulkheads and marina at the same time. Just be sure the rig has diesel engine(s).

Hire an experienced geotechnical engineer. Particularly one with marine experience; the project you have described is not that unusual - I'm currently working on a similar one in South Padre Island. (Minus the organic muck and limestone, of course.)

Sounds like a fun job - where do I sign up?!



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"