Computing allowable load from an SPT N value 2

[nkorn]

To meet an inspection requirement for a deck footing, I need to convert an SPT N value to lbs/sqft to calculate the numebr and area of footings I need on some marginal soils. Is there a simple formula to compute SPT N values to allowable load /sq ft for the footings?

 

[Focht3]

Nope.

 

[VAD]

nkorn:

Check Chapter 9 Foundations in "Soil Mechanics in Engineering Practice", Third Edition by Terzaghi, Peck and Mesri and you will find the light. This should be sufficient for your purposes

 

[BigH]

If marginal soils - remember that above comments are for pressures from a "shear" point of view. You also need to account for anticipated settlements - also, if use spread footings, suggest you go a minimum footing width - say 600 or so. Deck loads are not that high.

 

[PEinc]

Bearing Capacity of Soils, by ASCE, as adapted from the USA Corps of Engineers No. 7, has formulae for allowable bearing capacity of soils for specific amounts of settlement based on past correlations. These formulae use SPT N-values. Refer to Chapter 4, Section 4-4.B, Standard Penetration Test (SPT).

This may be available on-line from the Corps.

 

[Ron]

I would bet that your footing loads are so small that the load influence is negligible at the first "N" value. I realize you're trying to satisfy an inane building inspection requirement with little relevance to the issue, but I would be more concerned about stability from washout and localized settlement. Unless you are in some bad soil conditions, the bearing capacity for a typical deck footing would not be an issue.

Focht3..yup. I'll remember to do an extra 12oz. curl to you.

 

[Focht3]

[red]Occasionally[/red] I'm a man of few words...

 

[DBNodurf]

the reference to Terzaghi & Peck fails to mention that the SPT vs allowable brg pressure is really only valid for sandy soils. Their allowable pressures for clay were developed using theoretical models and are not based on SPT values. Any soil in between (most are) has no broadly established correlation. As the soil gradation increases in fines, the correlation of SPT to allowable bearing pressure becomes less reliable. Unfortunately, every geotech firm I've ever worked for has used a SPT-Allowable Brg Pressure correlation for all soil types! Some knuckleheads are even using them in fills (sure thats ok if you have conducted a rigorous fill testing program on that site). The USACE manual doesn't add anything here. Lets face it, the SPT's only redeeming value is for lightly loaded structures where the strength of the correlation isn't significant wrt the quality of the resulting foundation, and for obtaining samples at depth. Otherwise, we should all be using CPT!!

Personally, I'd like to see everyone use CPT for all property determination and push some shelby tubes in auger holes for identification and lab testing.

 

[PEinc]

DBNodurf,

Please explain your comment that the "USACE manual doesn't add anything here."