## Subgrade modulus of soil

## Subgrade modulus of soil

(OP)

Hi!

I read few articles and books that describe this soil property.

I dont know if this could be even called soil property since its an mathematical model wich tries to imitate soil behaviour under pressure.

Anyway the formula as we know it is k=pressure/settlement (kN/m2) / m.

As I understood "k" only represents elastic behaviour of soil wich only happens for small deformation (settlements).

Now this is where I get confused.

Small deformations (settlements). How small?

Many authors describe "k" as an tagent to pressure/settlement curve. Is this related to my previous question?

I think it could be couse for small settlements the pressure/settlement curve is prety much linear. But I could be wrong...

Wich field or laboratory test could most accuractly predict "k" value?

Ive seen many graphs from Plate Load Test wich are basicly pressure/settlement graphs. Are they reliable?

I have many doubts here.

First of all, the pressure bulb from small plate and foundation are VERY much different. If the soil properties dont change with depth then its ok.

Second, PLT is only for soils that dont undergo consolidation! So only reliable for soils that only undergo instant settlement. No long term settlements!

Ive found few formulas from "Bowels" that try to coolerate bearing capacity and "k" value.

Can these be safely used while designing?

Ive read aslo few forum discussions where experts say that "k" value should vary under a raft foundation and that one value shouldnt be used under the whole plate. I dont understand this? Why?

If "k" is used based on the elastic behaviour I dont see any problem of using one value for design process.

Please help.

Ana

I read few articles and books that describe this soil property.

I dont know if this could be even called soil property since its an mathematical model wich tries to imitate soil behaviour under pressure.

Anyway the formula as we know it is k=pressure/settlement (kN/m2) / m.

As I understood "k" only represents elastic behaviour of soil wich only happens for small deformation (settlements).

Now this is where I get confused.

Small deformations (settlements). How small?

Many authors describe "k" as an tagent to pressure/settlement curve. Is this related to my previous question?

I think it could be couse for small settlements the pressure/settlement curve is prety much linear. But I could be wrong...

Wich field or laboratory test could most accuractly predict "k" value?

Ive seen many graphs from Plate Load Test wich are basicly pressure/settlement graphs. Are they reliable?

I have many doubts here.

First of all, the pressure bulb from small plate and foundation are VERY much different. If the soil properties dont change with depth then its ok.

Second, PLT is only for soils that dont undergo consolidation! So only reliable for soils that only undergo instant settlement. No long term settlements!

Ive found few formulas from "Bowels" that try to coolerate bearing capacity and "k" value.

Can these be safely used while designing?

Ive read aslo few forum discussions where experts say that "k" value should vary under a raft foundation and that one value shouldnt be used under the whole plate. I dont understand this? Why?

If "k" is used based on the elastic behaviour I dont see any problem of using one value for design process.

Please help.

Ana

## RE: Subgrade modulus of soil

Richard A. Cornelius, P.E.

WWW.amlinereast.com

## RE: Subgrade modulus of soil

Im interested in understanding the philosopy behind this matter.

## RE: Subgrade modulus of soil

Rey Villa, MS, PE

http://geotech-apps.com

## RE: Subgrade modulus of soil

you can always go to more detalied analysis considering the facts you mentioned.

## RE: Subgrade modulus of soil

futurepressure that the structure will engage on the soil under the foundation?This could also be the explenation of the fact that k value shouldnt be the same under the whole foundation, wich some authors are saying.

More k values should be defined wich basicly imitate different pressures under the foundations.

What do you think?

## RE: Subgrade modulus of soil

## RE: Subgrade modulus of soil

What you mean by this?

Are you saying that slab thicknes wont have impact on bending moment diagrams of the slab and necesray reinfrocement?

## RE: Subgrade modulus of soil

## RE: Subgrade modulus of soil

Will read this.

I found usefull link:

http://www.civil.iitb.ac.in/~kvkrao/CE%20742%20Pav...

It clearly says that if you are using Plate Load test , K coefficient is to be determined for 1.25mm settlement.

## RE: Subgrade modulus of soil

Try it- get the Hetenyi equations and plot two cases - A mat foundation (or even a grade beam) and then laterally loaded pier. In both cases, foundation geometry and compressive strength of concrete has more influence than the subgrade soil modulus value.

## RE: Subgrade modulus of soil

What other method can be used for determining modulus of subgrade reaction besides plate load test and CBR test?

For plate load test "k" is determined s described in previous link and also using modification equations provided in the link geobdg gave.

What aboth CBR and maybe triaxial test?

## RE: Subgrade modulus of soil

Very interesting text.

That Oasys software seems very powerfull.

Can you tell me whats the difference between soil analysis that Oasys does and SAFE or STAAD?

Seems like a new approach, I might be wrong...

Thank you.

## RE: Subgrade modulus of soil

k value is not a proxy for bearing pressure.

k is a surrogate to evaluate the elastic response of soil to applied loads. Problem is you are dealing with an elastic response in the very near surface of the soil profile (i.e., within the first couple of feet).

I've always used correlations betwseen CBR and k. I typically use the ACI guidance for k atop the subbase, which allows some increase to the soil's k-value depending on the thickness of stone base atop the soil subgrade.

Yes, there are crazy folks in structural engineering that love "k" so much they want to find such a value for deep seated soil behavoir. Some even refer to "long-term" k-values. This is poppycock!

An industrial floor slab is designed as a beam on an elastic founation (well that was prior to computer modeling). When a load is applied to this beam, there will be load spread within the beam and load transfer from the beam to the elastic soil. What happens in the soil? What happens in the beam, what reinforcing is needed? These are structrurally controlled by the materials and the k-value.

Foundation design includes some component of material strength and reinforcing - for sure! It's just the load transfer from the foundation to the soil is not a point load, is uniformly distributed immediatly below the foundation and the soil strength is mobilized to much greater depths. These depths may influence more than one soil material and these soil responses will vary. A simple "k" value cannot address these details - and shouldn't be used!

Just a few of my thoughts.

f-d

¡papá gordo ain’t no madre flaca!

## RE: Subgrade modulus of soil

Basicly user would need to model raft geometry, load it and then analyze stresses under the raft with similiar setllement wich can then be grouped and then used for modulus estimation (i said grouped since K value is never the same under a raft)

## RE: Subgrade modulus of soil

Can anyone responde maybe regarding PLAXIS usage?

Questions to geotehnical experts.

When testing soil samples is there any test that could be used to determine K value?

An pressure-displacement diagram like in plate load test should be needed or this also isnt a good indication...

## RE: Subgrade modulus of soil

(Note: In my statements below, K is related to overall modulus such as for mat design......not K

_{1}such as from plate load test used for pavement design).Regarding Plaxis, you could use load/deformation results like you discuss to help define a K value. But this would be going backwards......the point of a Plaxis analysis would be to get away from the over simplified modulus based analysis.

Basically any test on soils that can correlate load to deflection can help with determining K. Most of these would be field tests (Pressuremeter, dilatometer, SPT, etc) although various lab tests and empirical correlations could help.

Not sure this will help but what I frequently do when thinking about what modulus to use for a mat, etc is: make a spreadsheet that has different square footing sizes going down the left column (ex. 2' square, 5', 10', 25', 50'). Then across the top, label different columns with different bearing pressures (100 psf, 200 psf, 500, 1000, 2000, etc). Run a bunch of settlement analyses (such as schmertmann using SPT, cone, dilatometer, etc) and fill in the cells of the excel table with the settlement that corresponds to each pressure/footing size combination. Then I create another duplicate set of columns for each bearing pressure and in each cell of the table calculate the associated modulus (i.e. pressure/settlement). This helps give me a good overall feel for the scatter in K value based on different pressures and different sized loading areas. Then I roll this around in my head to consider what value is appropriate and also provide the excel file to the structural for a discussion on what we should use.

## RE: Subgrade modulus of soil

"But this would be going backwards......the point of a Plaxis analysis would be to get away from the over simplified modulus based analysis."

Yes argee. This would basicly be like an simulation of an loaded raft in Plaxis.

One would first define structure geometry, material properties and then define soil properties ( I think PLAXIS soil models use E modulus, poisson ratio, consolidation parametars...).

Second step would be to load the raft geometry with loads determined from analysis and design software (softwares like ETABS, RISA, STAAD...)

Third is running the analysis in PLAXIS and analysing results (settlements, pressures...). WIth pressure-setllement values from PLAXIS users could define overall K modulus wich varies under the raft (somthing that I recentry found out reading some research papers).

With these values user would go back to his analysis/design program and define K value under the raft, run the analysis and design raft together with its superstructure.

You said:

"But this would be going backwards......the point of a Plaxis analysis would be to get away from the over simplified modulus based analysis."

I completly agree with you, but then how to design raft (required reinforcement) in PLAXIS?

Another understanding problem that I have is that loads determined of foundation structure from programs like ETABS, STAAD all depend on the foundation behaviour.

## RE: Subgrade modulus of soil

f-d

¡papá gordo ain’t no madre flaca!

## RE: Subgrade modulus of soil

## RE: Subgrade modulus of soil