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Modeling a subgrade

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I guess one of my questions about modeling subgrades was what people were using for the k values.  Are they using values from the soils report?  If so,  how is the geotech coming up with the values?  Are they just be taken from the soils classification (if it's a silty sand,  k =  250?).

Nominally, values for k are determined by plate load testing using procedures given in ASTM D 1196, Standard Method for Nonrepetitive Static Plate Load Test of Soils and Flexible Pavement.  Load is applied incrementally and the deformation is recorded; the results are plotted to obtain a load versus deformation curve.  Results of plate-load testing should be reviewed with care, however. The modulus of subgrade reaction is defined as a spring constant and assumes a linear response between load and deformation of the subgrade. In reality, the relationship between load and deformation of a soil is nonlinear and is not a fundamental soil property.   Because the load-deformation results are nonlinear, either an arbitrary deformation or arbitrary load limit must be assumed. ASTM D 1196 does not specify a deformation limit but the final deformation is typically 0.05 in. or less.  Sometimes a load limit of 10 psi is used.

There is no single k value for a subgrade; the load-deformation relationship is a function of the soil's moisture content and prior loading.  The relationship is also strong dependant on the location of the load with respect to a slab edge and the size and shape of the loaded area.  

ASTM 1196 specifies a 30-in-dia. plate and the PCA, WRI, and COE design aids are based on the k value determined with a 30-in. dia. plate.  Testing an in-place subgrade with a 30-inch dia. plate is time- consuming and expensive, however.  Large loads may be needed to obtain even small settlement of the plates and adjustments need to be made for any plate deflections or non-recoverable deformation. Testing with a 12-in.-dia. plate test is much less expensive; however, since the load only stresses the soil to a depth of about twice the plate diameter, the results are not the same as with a 30-in. plate.  The value obtained from a 12-in. test is approximately twice that of a 30-in. test, but this can vary with soil type and compaction.

Even testing with a 12-in. plate is expensive,  which is why a value is typically taken from the soils classification.  I don't know where these values came from,  maybe from testing a long time ago ?  I'm not sure anyone things they are particularly accurate.

But the question still remains - what do the results of a 12- or even a 30-in. plate test have to do with your slab behavior?

How do people interpret their results?  

Gail Kelley