Bearing Capacity of 95% compacted fill 1

[SeanStructural]

We need to place a spread footing on a 95% compacted fill. The EOR requires 5000 psf. I am getting conflicted information. One of which that with 95% compacted you cannot reach 5000 psf; the other is that you cannot use DCP to verify that; and 95% automatically gives you the 5000 psf. HELP please?

 

[Ron]

The bearing capacity will depend on the type of material as well as its density. 5000 psf is a relatively high bearing capacity requirement, but certainly possible for some material types. What type of material is it?

 

[BigH]

95% of standard or modified Proctor? Ron is right in that a lot depends on the material. For a well graded crushed sand and gravel (road base) I've seen as much as 400 kPa used (assuming that there is no underlying softer material within the zone of influence). We've compacted residual clayey silt to silty clay at 95% standard Proctor and obtained an effective friction angle of 32deg. Could use that to estimate bearing capacity. But one thing to keep in mind - are you speaking bearing capacity (a shear based value) or allowable bearing pressure (based on the permissible settlement or differential settlement). The serviceability issue also needs to be addressed.

 

[SeanStructural]

I believe we are seeking allowable bearing pressure that is not based on the settlement. The soil is Light Brown Silty SAND w/trace Rock Fragments. The USCS Classification is SM, Nat. Moist. is 7.1, SP.G. 2.65, LL 35, PI NP, %>3/8 in. is 4.1 and % < No.200 is 12.6
What do you think?
And thanks a lot.

 

[cvg]

modified or standard?

 

[jgailla]

5000 psf seems high for a silty sand. How is settlement not an issue?
Without any other information I would guess 2000-2500 psf for a silty sand.
For a building of any size, the pressure bulb would be below any level where you could take samples and run Proctor tests.
Why is the EOR specifying the bearing capacity? This should be done by the geotech.
If you really need 5000 psf you are probably looking at ground improvement techniques such as stone columns or even piles.
How deep is the silty sand strata? Do you have underlying rock?

 

[SeanStructural]

I have to admit. I don't have all the information. We have decided to hold a meeting including the geotechnical and EOR. I will keep you posted.

 

[SeanStructural]

And thanks a lot!!

 

[oldestguy]

"Foundations, Retaining and Earth Structures" by Tschebotarioff, second edition quotes the NYC building code and for the SM material indicated allowable bearing value of 6 - 3 tons.SF. No info on density.

"The Design of foundations for Buildings" by Johnson and Kavenagh, 1968 on page 163 is quoted. This is for the SM, SW and SP sands, generally below the #4 sieve.
"N valuers not withstanding, experience seems to indicate that the bearing value of sands need not be taken as less than 2 tons per square foot,, unless some special consideration of impact or vibratory loading (earthquake , for example) exists." Although they say it depends on compaction and grading.

I'd look at where it sits as to relative density. 95% of Modified probably is way up there, near 100%

"Foundation Engineering" by Peck, Hanson and Thornburn, on page225 has a chart with N values, footing width and bearing showing a 5 foot wide footing with N about 22 (high medium density, settles 1 inch on sand. No indication of what kind of sand.

I'd want to be on site and do some probing and cone tests before saying OK, along with the 95% of Modified. By force of habit, I think my usual high for sand is 4,000 psf, but mainly because it works and no settlement problems result. Plate load tests have shown that as "safe" on fine to medium sand, extrapolated to typical footing sizes. That may be conservative however.

A final statement. N values are meaningless on uniform sands in my opinion. Reason: very small range of relative density, loose to dense.

 

[oldestguy]

For the chart by Peck, etc. that is for 5,000 psf.