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# Soil Spring Constant5

## Soil Spring Constant

(OP)
Is the Soil Spring Constant the same as Modulus of Subgrade reacton? I argued with a collegue that it is not. Please tell me I am right! Can anyone give me references where I can get some info about Soil Spring Constant.

### RE: Soil Spring Constant

5
Modulus of subgrade reaction is a unit of pressure divided by a settlement - lbs/sq. in per inch of deflection.

So a modulus of 75 pci would mean that you have a soil that will deflect 1 inch for every 75 psi that's placed on it (I believe the standard is a 30 inch diameter test plate).

The "spring" that you use in a structural model supported by the soil would be a value of lbs/inch or kips/inch...where your spring would represent a particular area - say 1 square foot of a footing...

The spring value would then be:

75 pci x 1 sq. ft x (144 sq. in / sq. ft) = 10,800 lbs/inch

So if you had a 3' x 3' footing - you could use a grid of springs under the footing based on a presumed soil modulus.  The springs on the edges would have 1/2 the spring value, the springs on the corners would have 1/4, and the interiors would have full value.

(OP)

### RE: Soil Spring Constant

I don't have a specific book - I have a chart that lays out various modulus values for different soil types and the rest is in the gray matter of my head.  Perhaps someone out there can point to something.

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