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Re[2]: Modulus of subgrade reaction

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The values you quote, which appear in Table 2-7 of that particular edition, 
are not modulus of subgrade reaction, which incidentally has units of force 
per length cubed. For example, "pci" would mean psi of bearing pressure per 
inch of displacement. Therefore, 1 pci means that 1 psi of bearing produces 1 
inch displacement of the soil surface.  Be careful when using "k" values out 
of a text and applying them to something large like say mat foundation, for 
example. Many times the values given are for a 1'x1' plate they have to be 
reduced for a mat foundation using formulas originally reported by Terzagi in 
his 1955 paper on the subject.  Soil texts such as Bowles usually give these 
equations, which are different for clays and sands.  The reductions are 
significant !  For example, for a soil with a 1x1 plate value of say 300 pci, 
when applied to a 60 ft. sq. mat, the "k" value for use in the analysis is   
5 pci according to the Terzaghi equations. Rather than use the 1'x1' values 
as a base, I personally rather do (or use from a Geotech report) a settlement 
calc for the particular soil using standard mentods, and to then divided the 
bearing pressure by the predicted displacement to obtain a "k" value.  The 
resulting "k" value will be different over the loaded area, and then one has 
a choice of using non-uniform k's in a program such as PCA MATS, which allows 
that, or using a single value of k in an analysis. Obviously, judgement is 
needed when doing this because you would like to minimize the number of 
different k's input into the analysis, and some mat analysis programs such as 
ELAMAT can accept only one K value, which is then applied uniformly over the 
mat in the analysis.     

______________________________ Reply Separator _________________________________
Subject: Re: Modulus of subgrade reaction
Author:  seaoc(--nospam--at) at Internet
Date:    2/12/98 8:27 AM

>From Bowles, "Foundation Analysis and Design", 4th ed:

      Silty: 150-450 ksf
      Loose:  200-500 ksf
      Dense:  1000-1700 ksf

Lew Midlam, PE

Shafat Qazi wrote:

> Question:
> Can anyone tell me what is a standard value for the modulus of subgrade 
> reaction for soil type S2?

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