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Modeling a subgrade
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- Subject: Modeling a subgrade
- From: GSKWY(--nospam--at)aol.com
- Date: Wed, 5 May 2004 10:34:05 EDT
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
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