SAFE: Values for Subgrade Modulus
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SAFE: Values for Subgrade Modulus
02-24-2009, 09:28 PM
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SAFE: Values for Subgrade Modulus
Hi folks!!! :cool:

Which value use for Subgrade Modulus on CSi SAFE ???

In the next table you can see values from Winkler's Modulus on kg/cm3, depending of the admissible stress in the soil in use (Esf Adm) on kg/cm2.
Easy to use it.

This table has been extracted from the master of degree work “Interacción Suelo-Estructuras: Semi-espacio de Winkler”, Universidad Politécnica de Cataluña, Barcelona- España. 1993 (Nelson Morrison).

Click in the image to see it. :idea:

   


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03-06-2009, 11:24 AM (This post was last modified: 03-06-2009 12:15 PM by Administrator. Edit Reason: )
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RE: SAFE: Values for Subgrade Modulus
There is a lot to be said about this issue. You might want to read this
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I'm still searching for an answer. Over time I found few tables with numbers but there were some differences (sometimes big) between them. I guess there is no clearcut answer and a load test should be carried out (which can be probably costly and time consuming task).
For some typical values have a look at pag 14 in this pdf file
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04-22-2009, 08:14 PM
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RE: SAFE: Values for Subgrade Modulus
I have this one...

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I read a textbook before that is keeping a factor of 2.5 to 3 to that formula... I can't find it now but I think it was from Bowles. I will find it and post it here...
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04-22-2009, 08:41 PM
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RE: SAFE: Values for Subgrade Modulus
(02-24-2009 09:28 PM)karla Wrote:  Hi folks!!! :cool:

Which value use for Subgrade Modulus on CSi SAFE ???

In the next table you can see values from Winkler's Modulus on kg/cm3, depending of the admissible stress in the soil in use (Esf Adm) on kg/cm2.
Easy to use it.

This table has been extracted from the master of degree work “Interacción Suelo-Estructuras: Semi-espacio de Winkler”, Universidad Politécnica de Cataluña, Barcelona- España. 1993 (Nelson Morrison).

Click in the image to see it. :idea:

Hi karla, I dont use that table, I asked to Nelson Morrison about that table and dont give me a good explain....

Subgrade modulus depend of many factors (tipe soil, water level, etc).

Beside subgrade modulus is unique for one soil, and admissible stress depend of tipe soil, load, shape of foundation, etc. One type of soil have differents admissible stress for different type of foundations.

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04-22-2009, 09:53 PM
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RE: SAFE: Values for Subgrade Modulus
if u check the formula of subgrade modulus in foundation design by bowles it is expressed as stress / settlement. various soil types have unique characteristics and unless a soil investigation is conducted it is quite hard to estimate these values. in many cases raft foundations are usually used for medium rise buildings so the need for a soil report is a must for such projects. for shallow foundations it is usually conservative to take 12000 kN/m3...hope this helps


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04-23-2009, 08:47 AM (This post was last modified: 04-23-2009 09:19 AM by verongskey. Edit Reason: )
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RE: SAFE: Values for Subgrade Modulus
I aggree with everyone, but the thing is we cannot just input something to CSI SAFE without having a feeling of where we get those values considering that mostly we have our Geotech report indicating a 25mm estimated total settlement with the corresponding net allowable bearing capacity. In one instance, we had our project here in KSA, 150kPa in one site and in the other site is 300kPa allowable bearing capacity. There is a big difference between the two, and if I will only use one value which is 12000kN/m^3 for both of them, then it makes no sense.

I have images here, although experts are saying these are also questionable and undependable. These are excerpt from Bowles' book and USceArmy Technical Data. I have seen several more books that mentioned the same formula except for the factor of safety of Bowles.


US army
[Image: us2.jpg]

Bowles' book
[Image: bowles.jpg]

Bowles' equation
[Image: ref2.jpg]
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04-23-2009, 05:30 PM
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RE: SAFE: Values for Subgrade Modulus
Dear Kar it & Staad:
It is evident that is not real to use a single value of Subgrade Modulus for lands with such differences in the allowable bearing capacity.
Yucapareja is right in its explanation, because the factors that influence in the value of Subgrade Modulus go from the type of ground, passing through the geomechanical characteristics of the soil until the conditions of formation of the lithography, because the structure of the subsoil and such parameters has to do with its formation process.
There are two texts that can be used like complementary reference to the Bowles: Handbook of Geotechnical Investigation and Design Tables, of Burt Look, and Tropical Residual Soils Engineering, of Bujang B.K. Huat, both texts in this one Forum.
But, there are anothers books with some differences. As we will be able to see in that documentation, the values of Subgrade Modulus usually are neither uniform nor unique, and in many cases it depends much on the criterion of the Geotechnical Engineer.
What if must be clear is the following thing: The Security Factor to use going to in inverse relationship to the information that we have from the soil and the confidence or reliability on the soil data that get ready.
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04-23-2009, 09:34 PM
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RE: SAFE: Values for Subgrade Modulus
Hi Guys and many thanks for the commentaries!!

I've used the table from Nelson Morrison for one year ago, before I've used the next table (from the Arthur W. Nilson's book: Design of Structures of Concret) depending the kind of soil.

[Image: Balasto_CBR.jpg]

I've calculated the values using the formula from Bowles and compared with the Morrison's table, and the differences are so great, for example:

For qult = 1.50 kg/cm2 :
=> Ks = 1.80 kg/cm3 (Bowles)
=> Ks = 3.10 kg/cm3 (Morrison) :s

I don't know why the diffence, I only know one thing, I gonna use the Bowles's formula, will be the Factor Security?


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04-23-2009, 10:46 PM
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RE: SAFE: Values for Subgrade Modulus
the value for subgrade reaction is evidently of great importance specially raft design. many references offer numerical values but a greater understanding of these should be rendered before jumping into assumptions in inputting these values in SAFE. the recommendations of the soil report is still the reliable source of obtaining these values...


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04-23-2009, 11:53 PM
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RE: SAFE: Values for Subgrade Modulus
Dear Karla:
The difference is the assumption of the Security Factor and the soil condition considerations. Each author have a different point of view depends on own criteria factors. Fredlung, for example, had a stress parameters table from plasticity patterns
Sometimes the main selection's factor is the security: If your structure is very important or for critical use you must to select the lower factor of Subgrade Modulus to simulate the most unfavorable scene allowable
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