Foundation on Engineered Clay 2

[quillin]

I have a residential lot on which a 1.5 story brick 40000sf
house will be built. The lot is at the end lot adjacent to
10 lots which have all been excavated for basements and
clay soil bearing is good. The project in in Charlotte
NC.

My lot was built back up in 2' lifts approximately 5' above the adjacent lots and compacted to over 95% which I had tested at top and down 2' in a 2 spots.
I cannot afford to take the risk there might be areas
of poor compaction. I was not present when the compaction
was performed but I was able to see the machinery foot
print overlap at both the 2' and 5' level so I have
some confidence in the quality of the compaction.

I will be marking the footers next week. I feel I have two options. I will be bringing in a Geo Engineer for testing.

1. Excavate footings down to virgin soil and end
up with footings at an average depth of 4'.
cost about $4500 extra. (365 Lf of Footing)

2. Excavate footings and perform soil testing on
specific intervals and depths. (Intervals?)

Any opinions on the cost effectivenes verses the risk
or other options.

Thank you,

 

[BigH]

You say that you will be bringing in a Geotechnical Engineer for testing - good idea. Why not have him advise you of your concerns? For a proper answer to your question - we would need more information on the clay fill than you have given - i.e., plasticity index, liquid/plastic limit, etc. - what is the frost depth in NC - 2 ft? You'd have to go down there anyway . . .

This is a mammoth house at 40,000 sq. ft (3,800 m2) - wow? Or, is it 4,000? Still a big house - if so big, why aren't you going for a basement too? Just wondering - I would as it would make a nice "room" for ME!

 

[Focht3]

My lot was built back up in 2' lifts approximately 5' above the adjacent lots and compacted to over 95% which I had tested at top and down 2' in a 2 spots.

Two foot thick lifts are too much; they should have been in 6 inch lifts. I, for one, wouldn't count on it for any support because of the risk of differential settlement.

You say that you will be bringing in a Geotechnical Engineer for testing - good idea. ... For a proper answer to your question - we would need more information on the clay fill than you have given - i.e., plasticity index, liquid/plastic limit, etc. -

Agreed.






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?"

 

[BigH]

Mi amigo Focht3 - I've a good case history (oral only) about too thick of clay fill layers - agree with you wholeheartedly on this one!!

 

[quillin]

All,

Thanks very much. The cost of a little concrete
is nothing compared to the possible (liabilty) cost ramifications here.

I am going to dig down to virgin soil.

 

[BigH]

quillin - don't forget you can excavate down to the virgin soil and then "raise" grade to your desired level using select granular fill (e.g., subbase sand and gravel). This is cheaper than concrete.

 

[TYL]

Pile it mate!