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(OP)
We are trying to determine the modulus of a soil subgrade underneath a concrete slab in an existing structure.

We are cutting a 3' x  3' square hole in the slab to access subgrade.

What would be the most cost-effective way to determine this?

The most cost effective way is to "guess" the modulus of subgrade reaction.

Given a reasonable soil classification, you should be able to estimate the modulus with enough accuracy that it won't make a big difference in your slab thickness calcs.

If you want to test for it, use a plate load test and cycle the load.

Can also refer to several charts that are available to give rough correlations of various soil properties to one another.  SlideRuleEra's web site (haven't heard from him recently) has the charts - can also find in Fintel's handbook on Concrete in the Concrete Pavement Design Chapter (2nd edition).  A CBR of 10 is approximately 200 psi/inch; 5.5 is 150 psi/inch; 3 is 100 psi/in and 20 is 250 psi/in.

(OP)

I think you'll find it on " www.slideruleera.net "

O.K. let's reach behind our back and scratch our elbow. . .

You can take a field density test of the existing subgrade.
You can take a bulk sample and pound a proctor.
You can remold to the percent compaction and do a soaked CBR.
You can correlate soaked CBR to subgrade modulus.

Other than the 10 days for the proctor/cbr it's pretty straightforward, but somewhat empirical.

f-d

¡papá gordo ain't no madre flaca!

first thread i've clicked on in months and all the regulars  here haven't missed a step. glad to be back!

hey, do any of you gents have the 1955 Geotechnique paper by Terzaghi on the topic? I've been looking around and can't find a copy. I looked for it a while back and got sidetracked...would like to read it since it's referenced in so many places.

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