N values of densified soils 2

[jgailla]

I use N-values to estimate settlement in shallow foundations in sand by correlating N with soil moduli. I use a linear relationship, higher N=higher Es. The approach is somewhat conservative, but it is as accurate as any cheap method to my knowledge. My problem is that often I am estimating an N-value of a densified fill subgrade. Structural fill in this area is usually SP sand, 95% modified Proctor, unit weight about 115 pcf. I have been using N=15 for densified sand subgrade. For most loads and bearing pressures this value is conservative but calculated settlement estimates are within tolerable parameters (i.e. 1"total 1/2"differential). I would like to use a higher N-value for estimating settlement on some projects but cannot find anything in the literature which justifies the higher value. Does anyone have any information concerning relationships of N-values to densified sands, silty sands, clayey sands, etc? I recently did some SPT borings on 5 feet of densified fill and averaged N-values in the 30's, although I think this number is not conservative enough to be used on a regular basis.

 

[BigH]

It may be - remember that the "N" value is giving you an estimate of the relative density. So, take your level of compaction and estimate the relative density from it - then go to the chart of N vs relative density to get an estimated "N" value. I would think that if you had a properly compacted sandy soil, done in lifts, that the N value would be in the compact range (medium dense for you Yanks!). If you show to be in the "acceptable" range, then why are you wanting to use higher N values (I understand the arguments about being "more precise"). Are there cases where the allowable settlement is to be less than 1/2 inch, say, or even a 1/4 inch? If this is the case, then you may wish to go to a different approach to get the estimate of settlement.
to one and all for a Merry Christmas.

 

[fndn]

There is a small publication booklet by ASCE called "Settlement of Shallow Foundations on Cohesionless Soils" Geotechnical Special Publication No.5, edited by William O. Martin paperback 1986. The first paper is called "procedures for predicting settlement in Sands" by J.k. Jeyapalan & Rolland Boehm. Basically, they take the settlement observed in 71 case histories, and then apply the 9 methods of predicting settlement in sand, and compare which method predicts the observed behavior the closest.

Looks like "D'Appolonia" method wins. Like your method, D'Appolonia uses E as a function N, and also adds footing width, influence factor etc. So try to get hold of D'Appolonia's 1970 paper, or ASCE's Geotechnical puiblication #5.

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[Ron]

jgailla....not sure what your lab capabilities are, but you might want to spend a few hundred bucks (even as an overhead expense, just for the info) and do a cyclic triaxial shear test or two to get some reasonably accurate modulus values. Since your fill is "remolded" and the compaction controlled, you can feel better about a "remolded" or set up triaxial specimen as being representative of the in-place conditions.

You can also do a consolidation test on these sands to develop the modulus as well.

 

[BigH]

jgailla - beyond what fndn indicated - there was paper some long time back (early 1970s) in Ground Engineering where some 17 methods or so of estimating settlement was made. The conclusion was to take three you are comfortable with, take the average and use it. Don't have the paper with me but you might be able to contact Ground Engineering and ask them. Ron gave some good advice - but, again, you should step back and review as to the real accuracy you need. If the settlement was 15mm or 20mm - would this a problem? Many, most (perhaps) geo reports never really "estimate" the settlement - only need to know if a problem or not.

 

[npthao121]

Normally, what kind of modulus do you use for settlement calculation? from the lab test (oedometer, consolidation or triaxial) or from the field test (plate load test, SPT, CPT). And which is better (lab or field) as your opinion? somebody told me that modulus from the field test is more reliable and exact than the one from the lab. I'd like to listen to your comments.

Appreciate all your comments. Thank

 

[jgailla]

I typically use the SPT to determine the soil's modulus of elasticity for foundation recommendations. This approach is not very accurate, so recommendations are conservative. It is my understanding that properly conducted field tests are more reliable than lab tests. Lab tests are performed when field tests are impractical or when lab tests are accurate enough for the particular job. I believe CPT is theoretically more accurate than SPT in determining soil density, but SPT correlations have been developed much further and so are well accepted. Plate load tests are more expensive than SPT or CPT. The limiting factor in bearing pressure calculations is typically settlement rather than an actual failure of the soil. Thanks to all for their help.

 

[jheidt2543]

Two other ASCE sponsored publications that might be of some help:

"Measured Performance of Shallow Foundations", ASCE Geotechnical Special Publication No. 15

"Predicted and Measured Behavior of Five Spread Footings on Sand", ASCE Geotechnical Special Publication No. 41