Soil Bearing Capacity Based on Settlement (Bowles) 5

[ocgeo]

We have a clean sand site with average blow counts in the upper 2B (B = 12 ft. square = width of footing) of 30.

I calculate a bearing capacity of about 10,200 psf for a settlement of 1 inch per the analysis in Bowles Foundation Design (base don Terzhagi and Peck 1967).

We have column loads of about 1,500 kips so we need to recommend a bearing pressure for reasonable size of footing.

Traditional Terzhagi Bearing Equation with FS = 4 results in 15,000 psf.

Does 10,000 psf seem reasonable for a sand, some silty sand, with N average of 31 to a depth of 2B?

 

[MTNClimber]

I personally haven't recommended anything over 5,000 psf for dense sand and that's usually when push comes to shove. Typically its closer to 4 ksf. That's just based on how I was groomed by my mentors. I never questioned them on it.

I know that you can get a much higher value from bearing capacity and settlement analyses. But what I've read on elastic settlement analysis is that its not very accurate and mileage may vary which leads me to believe that its better to play it safe than be sorry.

Even though this is pretty basic, I'm interested in what others have to say on this topic.

 

[fattdad]

I think you are off too. Seems a 12x12 footing loaded to 5 tsf will return 2-1/2 inches of settlement for soil with a modulus of 300 tsf. Seems that you'd be able to limit the settlement to 1 inch if you were using 5,000 psf.

I literally used a napkin for this exercise; however.

f-d

ípapß gordo ainÆt no madre flaca!

 

[ocgeo]

I tend to agree with the Schmertmann analysis (Elasticity), but like to explore all relevant research data. For instance the Bowles analysis (modification to the Terzhagi and Peck), with N55 of 31, calculates to 10,200 psf for 1 inch settlement. In that case I would use an average bearing pressure, say 7,500 for 1", for the clean sand and 30 blows.

I am not disagreeing that 7,500 psf seems high, another option, maybe go to a mat at about 1,750 psf? hmmmm

Then give ks of 20 to 60 pci (min to max loads) with 1" settlement ?

 

[fattdad]

Can you distill the modulus value that's assumed by the N-value correlation?

(I never liked the Schmertmann method. It doesn't evoke the first principals in my mind. My approach is PL/AE. You are figuring out the P, the depth to 10% stress change is the L (sub i 'cause I'll subdivide), the A is a unit square and the E should return from N-bar for the layer.)

So, what's your modulus?

f-d

ípapß gordo ainÆt no madre flaca!

 

[ocgeo]

I am using the Terzaghi and Peck 1967 researched theory (Modified by Bowles), which is:

[N55/F}((B+F2)/B)^2(K) where F and F2 are 'factors' which were derived from the research and testing, and varies with B (width of square footing). The Es, I believe, is a function of the F factors. K is also a factor that was derived based upon the depth of embedment as a function of B. (Foundation Analysis and Design, Bowles 5th Edition.). N70 may also be used.

A lot of the early derivations were based on plate tests, or small scale testing, or lab testing, then extrapolated, which is why I prefer to solve with various methods, see what's reasonable, then take a reasonable average.

 

[BigH]

I won't get into the question per se, but I would suggest, given that you really don't have a good handle on E value to use, to be careful, even in discussions of suggesting too much "accuracy". 10,200 psf for 1" settlement? If your E value was off by 10% (and likely would be) the "200" seems a bit far fetched. Think significant digits and accuracy of input - both for the "capacity" and for settlement.

 

[ocgeo]

There is an existing 10 story building with 10'x10' footings (36 total on square grid) with pressures of 7 to 10 ksf, with the concrete building built 100 years ago, and showing some settlement/stress, but not much. Too bad we couldn't study that building/bearing soils to back calculate an E.

 

[BigH]

ocgeo - you would also, given the age, need to take into account secondary conosolidation (in clays) and creep effects (see Schmertmann) in sands.

 

[Okiryu]

I would try to estimate the friction angle and get the bearing capacity. Then use elastic theory to get the settlements based on the bearing capacity value that you got. If settlements are excessive, reduce your bearing capacity accordingly.

Also, as other mentioned, your estimation of your modulus is very important. If possible, I would try to get the modulus from pressuremeter tests. Here are also some correlations for E with N60. See page 336 (Table 5-16) of the below link:

https://vulcanhammernet.files.wordpress.com/2017/01/fhwa-nhi-06-088.pdf