Estimating Lateral Stiffness 3

[JoshPlumSE]

I'm a engineer who works for a structural engineering sofware company.

There's a question that comes up from time to time that I don't have a good response for. Within the context of a linear elastic analysis program, we as structural (not geotechnical engineers) will use Modulus of Subgrade Reaction from our Geotech report as the basis for establishing vertical springs to resprsent the soil. I know that soil is not a linear elastic material. But, that's generally what is done.

The only problem is that we don't have any similar concept for lateral resistance of the foundation. I'm curious if anyone in this forum has any suggestions for what to use. Or, any references that might have suggestions.

My initial thoughts are the following, but I could be way off:

1) If I've got lateral friction resistance at the soil / foundation interface, I might use some percentage of the modulus of subgrade reaction. Something like 0.1 * the vertical soil spring used at this location.

2) For passive pressure resistance, I can make hand calculations to estimate the passive force resistance along the height of a retaining wall. But, to establish a spring constant I would need to estimate a expected deflection which would coincide with that passive pressure.

 

[FixedEarth]

It depends on the type of analysis. For mats, combined footings and grade beams, we often give vertical subgrade reaction in units of F/L^3 an it is just one fixed value. This is based on Hetenyi's book. Also for mats there are detailed equations if the soils are cohesive & for various footing shapes (Ref. 3).

For laterally loaded pile with single shear load & for embedded retaining structures like sheetpiles and soldier beams, we give a horizontal subgrade reaction per each soil layer. In general, lateral subgrade reaction is 1.5 to 2 times the vertical subgrade reaction. To obtain lateral subgrade modulus, multiply the lateral subgrade reaction with pile or pier diameter or width.

One detail to keep in mind is to follow one researcher's method. For example, I use Blum's method for cantilevered and tiedback soldeir beams. It is also possible to analyze cantilevered pole type structures using other methods like Teng, IBC, Czerniak, Broms and others. Just be careful on mixing analysis of two different methods under one calculation module.

There is no comparison between lateral subgrade modulus, LSG, and passive resistance, PR to my awareness. If you use LSG, then you need to check for maximum lateral deflection at the ground surface. If you use PR, you need to check that the induced soil pressure from the lateral load does not exceed the soils allowable passive resistance.

The following texts cover your topic:

1- "Single Piles and Pile Groups Under Lateral Loading", Reese & Van Impe, 2011
2- "Foundation Analysis & Design", J.E. Bowles, 1995
3- "SP-152: Design and Performance of Mat Foundations", ACI, 1995
4- "Foundation Analysis" R.F. Scott, 1981
5- "Analytical and Computer Methods in Foundation Engineering", J.E. Bowles, 1974

I also understand Budhu and Poulos have their own methods.

 

[RFreund]

Josh - I'm curious as to what sort of foundations you have in mind?

FixedEarth - What is a good reference for Blums Method?

EIT

http://www.HowToEngineer.com

 

[JoshPlumSE]

I'm thinking more shallow foundations. Spread Footings, Mat slab foundations. Those would be the primary interest.

Of secondary interest would be retaining walls and / or basement walls.

 

[FixedEarth]

Blum's Free Cantilever Method (good for sheetpiles and soldier beams with slight modifications) & his equivalent beam method (good for tieback walls) is covered in great detail in "Foundation Engineering", Alfreds R. Jumikis, 2nd edition, 1987, Krieger Publishing. This book is a jewel. It also has materials on ring and communication tower foundations, pile groups and many graphical methods.

 

[RFreund]

FixedEarth - I think I know have more books in the geotechnical field than all other materials combined.

Josh - My first instinct there would be to use passive pressure with some sort of safety factor, but I still think it may depend on the application.

Do you want to apply lateral pressure to the pier and face of say a spread footing that is subject to vertical, horizontal, moment, and axial load. Where the lateral pressure would help reduce bearing and increase overturning resistance?
I don't do a lot of mat-slab foundations so I'm not sure what the situation is there.

As for retaining walls / basement walls I usually see classical methods used- i.e. passive, active and at-rest pressures used.

I would actually be curious to see a design using LSG

EIT

http://www.HowToEngineer.com

 

[Mccoy]

Joshplum, values for horizontal subgrade reaction for shallow foundations of various configurations are extensively provided in the classic article from Gazetas: "Foundation vibrations", chapter 18 in Fang's foundation engineering handbook, 1991.
You can find it in googlebooks.
The static case, also provided in the cited article, is simpler because deals with real numbers.
There is some other useful material on the web, like the FEMA 273 publication, chapter 4, which I'm attaching here.
If you'd like to have a look at the gazeta's relationships (article excerpt) please PM me.
Another very useful reference for both static and dynamic subgrade moduli is the Deeks & Wolf book:

http://store.elsevier.com/Foundation-Vibration-Analysis/John-Wolf/isbn-9780750661645/

 

[Mccoy]

Sorry apparently the FEMA pub hasn't been attached, it's available here though:

http://www.wbdg.org/ccb/DHS/ARCHIVES/fema273.pdf

 

[RFreund]

How many books do you Geotechs own ?!?!
I think I have just as many soils text as I do for all other materials...and we do structural work.

EIT

http://www.HowToEngineer.com

 

[Mccoy]

Rfreund, I noticed that some structurals tend to stack up geotech books, the load of structure and soil maybe a concern when calculating bookshelves capacity