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Soil-pile-structure interaction
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picajol
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Joined: 29 Mar 2012
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PostPosted: Thu Nov 29, 2012 7:35 am    Post subject: Reply with quote

Dear jshula, how is this different from STAAD AND SAFE programswhere the yuse K modulus (wich I still have problems understanding)
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gautam chattopadhyay
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PostPosted: Thu Nov 29, 2012 8:55 am    Post subject: Soil-pile-structure interaction Reply with quote

Is FEM applicable to soil? Is the assumption that material should be homogeneous and isotropic valid in case of soil? How to determine modulus of subgrade reaction in horizontal direction which is needed for piles subjected to horizontal forces due to earthquake and wind?

On Tue, Nov 27, 2012 at 8:40 AM, Econf_Moderator <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Forwarded message....


*********

My replies provided between ** ** below.

Thanks.
Jaydeep Wagh


Geotech Consultant




     
     Piled rafts are often used for tall buildings. I would request seniors to clarify some doubts that I have faced when proposing pile groups:

Today every large structure is analysed and designed using stiffness method programs with detailed study by FEA where appropriate. However, only specialised consultants are carrying out the analysis of piles/ pile groups in realistic geotechnical FE models. This leads to the following questions:


1. How is the pile-soil interface modelled in these geotechnical models?
**The pile soil interface strength can be reduced in FEM software Plaxis by specifying the interface strength reduction factor. **

2. Is it possible to use pile stiffness calculated from geotechnical models in analysis of the foundation raft and superstructure?
**The pile stiffness calculated from geotechnical models or analysis is used for further structural analysis of foundation raft by structural engineers FEM software (eg. Safe).**

3. Is it possible to model the interaction between different piles in a pile group and between the pile cap/ soil and piles?
**The interaction between piles can be manually calculated by charts available for pile group action in most geotechnical books. These charts are typically provided for uniformly or evenly spaced piles and uniform loading on piles. The interaction between piles and pile cap can be analyzed manually by methods provided in "Pile Design" by H. Poulos. Manual calculations may become highly tedious to use for multi layered soil profile and/or for complex loading conditions or for non-uniform pile layouts. As mentioned in "Pile Design" also, finite element softwares provide a more accurate tool for modeling the interaction between different piles in a pile group and between piles/pile caps. **
     



     



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jshukla
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Joined: 16 Jun 2008
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PostPosted: Thu Nov 29, 2012 3:23 pm    Post subject: Reply with quote

Dear Mr. chattopadhyay,

Is FEM applicable to soil?
Yes it is .. indeed it has reached to a confidence level by which it is possible to predict the displacement even in very complex geometry and complex soil structure interaction problems. PLAXIS, CRISP, ZSOIL, ABACUS, ANSYS are examples.


Is the assumption that material should be homogeneous and isotropic valid in case of soil?


Not necessary since those assumptions are required for elastic analysis but soil essentially involves plastic behavior. Classical example is Mohr-Coulomb model which is Elastic-perfectly plastic model for soil. Layered soil represents anisotropic and some material models are capable of handling those kind of problems.


How to determine modulus of subgrade reaction in horizontal direction which is needed for piles subjected to horizontal forces due to earthquake and wind?

If you are using FEM package no need to determine 'subgrade reaction modulus' since it is one of the way to idealize soil behavior to incorporate soil behavior in structural package. However, it is approximate method and sensitive to many parameters.

i hope it helps.

jay
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picajol
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PostPosted: Thu Nov 29, 2012 3:51 pm    Post subject: Reply with quote

One question on subgrade modulus of soil.
Please answer it as YES or NO.

Two foundation are of different sizes but with the same soil under them.

K factor will be different for these two foundations.

Is this correct statement?
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jshukla
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PostPosted: Thu Nov 29, 2012 4:20 pm    Post subject: Reply with quote

I acknowledge your anxiety for the question...

Yes it should be... since the size of foundation is different hence extent of soil straining is different. However if you say 2m wide and 2.5 m wide it wont have much different... but yes 10m wide raft and 2m wide footing.. I would recommend to analyze with different k-value. Please remember k-value is not fundamental property of soil. i.e. one foundation sits at 2m depth and your raft sits at 4m below ground.

I hope it helps.

jay
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picajol
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PostPosted: Thu Nov 29, 2012 5:37 pm    Post subject: Reply with quote

Inded, becasue of this K cannot be looked as a soil property.
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jshukla
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PostPosted: Mon Dec 03, 2012 6:22 am    Post subject: Reply with quote

Dear Colleagues,

Please attached article regarding pile group settlement estimation. Pile group interaction factors are also included in the paper.

I hope it helps.

jay



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Pile Group Settlement Estimation Harry Poulos.pdf
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Pile Group Settlement Estimation Research To Practice - Harry Poulos

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Sorabh.Gupta
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PostPosted: Tue Dec 04, 2012 5:23 am    Post subject: Reply with quote

Agreed. The k-value is not an intrinsic soil property at all, since it depends on the loading conditions (size of footing, depth of footing, etc.).  In fact, it is more of a mathematical ploy / parameter, used for simplified analysis of a complicated material (i.e. soil).  

The modulus of elasticity of soil (E), however, is a soil property, and directly proportionate to the k-value.  

picajol wrote:
Inded, becasue of this K cannot be looked as a soil property.
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picajol
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PostPosted: Tue Dec 04, 2012 12:00 pm    Post subject: Reply with quote

Dear experts,
could you please join in debate on K modulus in this topic:
http://www.sefindia.org/forum/viewtopic.php?t=5428&start=110
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