This query is reallocated from stru

[npthao121]

This query is reallocated from structural discussion group.


Dear All!
I am considering to soil improvement by using prefabricated vertical drain (PVD) method. The methods to calculate the final consolidation settlement, consolidation rate, PVD spacing ... are introduced quite clearly in most of geotechnical & foundation engineering textbook. But I could not find any lines or instruction regarding to how to calculate the shear strength of soil after a certain settlement is reached (let says 90% of final consolidation settlement reached) or pore water pressure is dissipated.

I understand that we have to make some field test & lab test to determine that parameter exactly, but how about the theory method?

Does anybody show me the way to calculate that parameter? Which book or which site contains necessary information? Also, by your experience with practical approach, how many times will the soil shear strength increase from the initial value? In my case the initial value of shear strength is 4 ton/m2 approximately, Will it increase to 8 or 10 ... ton/m2?

Thank in advance! All opinion will be highly appreciated!
Npthao121


From BigH

There was a technical note written back in 1975 I believe by N. Som regarding how to determine the increase in the undrained shear strength after consolidation. I have the paper, send your e-mail address to bohica@indiatimes.com and I will forward it to you - maybe, after a little procrastination (couple of days). I, too, am currently involved in similar exercise (Dr. Som is a consultant for the contractors on this one). I have very week soils and 9m high retaining walls to go on (RE Walls). Some data I found interesting from a soils boring programme here in the Calcutta area - undrained shear strength after long term (say plus 15 years) under 5m high embankment doubled the natural undrained shear strength where there was no embankment (and far enough removed, but adjacent, to not have been affected by the embankment.

 

[1967pradyot]

In all practical purposes the increase in shear strength due to consolidation can be calculated by 0.2 of increase in effective vertical stress (in your case either the full embankment height for one go construction or the height of embankment corresponding to a particular stage construction)

 

[npthao121]

Thank 1967pradyot!
Could you explain your idea more detail? Any background experiences & calculations to support for the factor of 0.2?

 

[1967pradyot]

The relation which I have mentioned is a conservative estimate. Of course, remember this is valid for only normally consolidated clay. Actually this is based on the relation that the undrained shear strength of normally consolidated clay increase almost linearly with depth. You can refer any book (say, Foundation Analysis and Design by Bowles)to get the relationship as proposed by Skempton & Henkel. Their equation relates undrained shear strength, effective overburden (or vertical) pressure and plasticity index.
Hope, this will be helpful to you.

Thanks

 

[BigH]

npthao121 - the relationship that Praydot is talking about is the slope of the line of undrained shear strength with depth. As the overburden pressure is more, the clay is "stronger". Plot your undrained shear strengths vs depth and you can see your own "site" curve. Of course, the 0.2 that was given is a "typical" value - Terzaghi, Peck and Mesri suggest 0.23. I discussed this in a thread a while back - about how to estimate settlements in clay from SPT samples - the thread escapes me at present. You could do a word search.

 

[BigH]

I did the search for you - it is thread 256-40097.

 

[npthao121]

Thank a lot BigH & pradyot