Modulus of Subgrade Reaction 5

[geotechgal]

Can anyone suggest ways of accurately estimating the Modulus of Subgrade Reaction for a soil. Available parameters:
SPT N values, Moisture, Unconfined Compressive Strength & Angle of Internal Friction.

 

[PEinc]

Search the forums for this topic. There are many posts on Modulus of Subgrade Reaction.

 

[UcfSE]

Text books usually have this information. Das's books for instance have tables throughout with coorelations using N values, Dr and so forth. I understand that "accurately estimating" the modulus of subgrade reaction is a bit of a loose term however.

 

[geotechgal]

UcfSE,

I agree with what you say.."accurately estimating" is a bit of a loose term so to speak and hence my posting. The SE I am working with insists that I do that and I am not sure how to do that!
I am referring to Bowles it has some equations and stuff but to estimate that for 50 Borings with 10 soil layers in each seems like its going to take me a week to do all that!

 

[swine]

Hi geotechgal,
It all of a sudden seems like I'm writing into these dirt forums a little too much--so this will be my last one.

I read a soils report in the last couple months. Where the soils engineer gave the modulus and stated it was for a 1 ft x 1 ft plate and that the value given should be adjusted for the actual footing size. No guidance as to how it should be adjusted was further given.

 

[geotechgal]

swine,
I am not sure about what you designed for but this foundation analysis is for a Bridge Abutment 150 feet wide passing over a six-lane highway. They are going to use micropiles and H-piles combination. I am not gonna get off as easy as your soils engr.

 

[eric1037]

Is this lateral subgrade modulus or modulus of subgrade reaction? You would need to know the exact elevation of the subgrade to determine the modulus of subgrade reaction.

Check out the NAVFAC manual for both.

 

[geotechgal]

eric1037,
Which NAVFAC manual are you talking about? I have four or five of them.

 

[tincan]

try Seelye's Design Data Book for Civil Engineers Vol 1
printed sometime around '50

best tincan

 

[VAD]

Hello geotechgal

Your SE is probably looking at lateral deflections of abutment piles. As Eric1037 sugests you may require lateral subgrade modulus which can be obtained by multiplying the values obtained for a 1ft square plate by 2. However, I do not want you to take values without understanding their limitations. As such you may wish to do some further reading. Foundation Analysis by RF Scott is a text which you should look at as he discusses this topic.

I presume that these modulus values are required for a computer program that Structural Engineers like to use for these problems as you can model utilizing a large number of springs. I cannot remember the name of the program. However there are other methods as well.

Just a suggestion before you go picking numbers why don't you discuss with the SE what he is aiming at, find out how deep the piles are to be embedded and after that examine the soil characteristics and use a few ounces of judgement in deciding the information to be provided. Discuss same with your SE and come to some agreement. This way you avoid giving numbers that may be inappropriate and giving numbers just for numbers sake. Too often we are controlled by computer programs without fully understanding the limitations. As well try out some simple approaches while he tries out his approach.

Regards

 

[eric1037]

geotechgal:

I use the NAVFAC Design Manual 7.01 and 7.02. See pg. 7.2-235 for lateral subgrade in 7.02. The modulus of subgrade reaction is in 7.01 chapter 5 pg. 7.1-219.

 

[jdmm]

The subgrade modulus is not a soil property since soil is generally elasto-plastic and may follow a strain hardening or softening relationship.

Structural engineers like it because it is simple. I try and keep it simple. The subgrade modulus is frequently specified in terms of pounds per cubic inch or kips per cubic inch. For a footing on the ground (say on sand) we frequently quote an allowable bearing pressure as pounds per square foot for one inch of settlement. This is essentially a subgrade modulus that considers the size and shape of the footing. A one foot square footing on sand will have a different allowable bearing pressure than a 10 foot square footing for the same 1 inch of settlement.

I would be interested in feedback on this simplistic interpretation.

 

[BigH]

To clarify what most know, the 1 ft by 1 ft value is from a plate load test which some engineers like to extrapolate to larger footing sizes as per methods outlines in Terzaghi and Peck, Bowles and others. But, to do this, you need to be sure that the soil strength (modulus) is either constant throughout the depth of influence of your footing or increasing (in which case you will be conservative). But, as Tschotarioff so elegantly discusses, the plate load test gets you into a situation known as FUBAR. Trouble with a capital T. This is why is it so important to understand, as many have pointed out, what the purpose of the value determination is for - and have the necessary judgment/experience to understand what to provide. But, in any event NEVER use a small plate load test for a bridge abutment foundation! Focht3 has given a number of very good discussions on this topic in other threads. I suggest to all that you find them, read them and understand his views.

 

[geonerd]

to get the lateral subgrade modulus, divide the vertical subgrade modulus by 1.5 times the pile diameter. To get the vertical subgrade modulus, refer to Terzhagi's 1955 paper.

 

[BigH]

By the way - the charts. You can find a chart in Fang's Foundation Handbook and can find a different one in Baker's Highway Handbook. Looks like JAE's is basically from the later although abbreviated.

 

[miecz]

JAE's Fig. 4 appeared in an Article by Boyd C. Ringo, "Design, Construction and Performance of Slabs-On-Grade for an Industry", which appeared in the November 1978 Edition of the ACI Journal, page 597. I believe the figure appeared complete with the handwritten notes.