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Pile Foundation Slenderness Limits
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saikiran gone
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PostPosted: Wed Sep 21, 2016 4:52 am    Post subject: Pile Foundation Slenderness Limits Reply with quote

Dear Engineers,

Is there is any limitation  on slenderness of pile foundation (i.e (DIA or width)/Length) . If yes please give me the suggestions and references .

Thank you,

Regards,
Saikiran.
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rahul.leslie
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PostPosted: Wed Sep 21, 2016 2:44 pm    Post subject: Reply with quote

50, I think... not sure

Rahul Leslie
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mallan
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PostPosted: Thu Sep 22, 2016 8:23 am    Post subject: Reply with quote

To the best of my knowledge, no Indian code recommends a minimum slenderness limit for piles. It is because, generally, piles are dseigned as skin-friction piles; and hence, it is assumed that the soil which is sufficiently stiff to provide skin friction will also prevent buckling.
This is highly debatable, especially in case of soils prone to liquifaction where the soil will not be able to give any lateral reaction to the pile. Hence, I was taught that international codes recommend certain limits.
From a practical aspect, generally certain limits ranging from 30 to 50 as mentioned by Er. Leslie  sir, can be adopted.

-Akshay Mallan
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es_jayakumar
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PostPosted: Thu Sep 22, 2016 9:57 am    Post subject: Reply with quote

What about the case in which the pile is predominantly end bearing ?

E S Jayakumar
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Dr. N. Subramanian
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PostPosted: Thu Sep 22, 2016 6:32 pm    Post subject: Reply with quote

Dear Er Jayakumar,

Older text books contained checking for L/r ratio of piles. Long back, we increased the section of a pile we designed based on the equation given in text books(same as for columns). But my friend Er Mohan Ramanathan of Build Aids Pvt Ltd educated us on this by sending a reprint of an old paper.

The essence of this is that piles, unlike columns are surrounded by some soil which imparts some radial pressure on the piles. Unless the soil has very low SBC, this pressure on the pile will prevent the pile from buckling, even if you design the pile as end-bearing pile. However, during earthquake, in loose sandy soils, there will be liquefaction and there will not be any such pressure offered by the soil. In such cases, the pile has to be designed based on L/r ratio.

More info on buckling of piles may be read from:
https://www.researchgate.net/publication/266896701_Buckling_considerations_in_pile_design
http://inpressco.com/wp-content/uploads/2014/02/Paper36201-205.pdf

I am also enclosing a paper for you to read further

Best wishes,
NS
es_jayakumar wrote:
What about the case in which the pile is predominantly end bearing ?

E S Jayakumar



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es_jayakumar
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PostPosted: Fri Sep 23, 2016 2:14 am    Post subject: Reply with quote

Thank you, Sir. Thus, wherever there is threat of liquefacion in loose sandy soils in earthquake prone areas, we have to design the pile as an unsupported column.  It may call for larger diameter for the deep piles.

E S Jayakumar
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sakumar79
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PostPosted: Fri Sep 23, 2016 11:07 am    Post subject: Reply with quote

Dear Dr Subramanian and Er Jayakumar,
    Even in the case of liquefiable soil, I feel we can restrict the unsupported length to depth of soil susceptible to liquefaction + Depth of fixity. Also, while calculating depth of fixity, we should consider the depth of soil subject to liquefaction as L1 and calculate Length of fixity from below that depth.

   Please correct me if I am wrong.

Yours sincerely
Arunkumar
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Abishek_Siingh
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PostPosted: Sun Sep 25, 2016 4:10 am    Post subject: Reply with quote

I think I agree with Mr. Arun Kumar's point.
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Manoharbs_eq
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PostPosted: Mon Oct 03, 2016 5:24 am    Post subject: Reply with quote

A good debate,

On thing the piles design is not simple, and cannot be explained in conformity. Totally pile behaviour is dependent on following aspects
1) Type of soil
2) Type of pile adopted (End bearing/friction/friction and end bearing)
3) Type of pile used (Steel H type circular/rectangular concrete)
4) Location of the project
5) Liquefaction potential
etc,.

Hence, the slenderness check cannot be confirmed so easily. Hence, this needs to be evaluated carefully base on the actual site conditions.

I am not aware of any Indian codes for slenderness check but Euro code 7 suggests slenderness check.
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knsheth123
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PostPosted: Mon Oct 03, 2016 2:26 pm    Post subject: Reply with quote

Dear Engineers and respected NS Sir,

1.
A more rational approach for the solution is possible based on stability analysis using horizontal linear side springs for soil in sides using Staad.

Divide the pile in elements and provide lateral springs (Hori-X & Hori -Z) at nodes based IS 2911 guidelines. If vertical load is primarily through base capacity only, provide fixed support at base.  This will work fairly good in socketted pile trough over burden.  Staad will calculate Buckling Load. compare it with Euler bucking load with both end pinned, will give effective length of pile.  

(Prerequisite for this is calculation of spring constant : for clay the code values are constant with depth and for sand- linearly varying with depth, an initial discussion/validation of spring constants thro' Geotechnical  Engineer will serve the purpose)


A Pile carrying significant load through skin friction and  fully embedded in ground, long column action will not be there. It is accepted fact that a lateral restraint carrying 2% vertical load is adequate to prevent buckling/long column action.

Pile with large unsupported length and/or a combination of friction and or base supported shall be analysed by a Geotechnical Engineer considering both vertical (t-z ) and horizontal (p-y) springs at a time.

2.
Please note : The depth of fixity given for the Lateral Load capacity as per IS 2911 codes is not for the calculation of effective length.

With Regards

K. N. Sheth
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