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Structural design of test piles vs working piles

 
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durgaprasad
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PostPosted: Wed May 09, 2018 6:29 am    Post subject: Structural design of test piles vs working piles Reply with quote

Dear Sefian,

I wish to discuss the structural design philosophy for initial test piles and working piles. In my understating reinforcement for initial test piles will be higher then working piles.  
Request you all to go through the case study below and provide the valuable feedback/suggestions.

1.0 CASE : As per soil data : 500 dia 32M long pile : Safe load 245MT and as per project spec test load is 2.5 times of safe load.

2.0 GENERAL :  As per cl. 6.1 of "IS 2911 (Part 1/Sec 1) : 2010" The pile shaft should have adequate structural capacity to withstand all loads (vertical, axial or otherwise) and moments which are to be transmitted to the subsoil and shall be designed according to IS 456.

3.0 Initial Test pile : Test pile has to transfer the 2.5 times the safe load. Hence test pile's structural design has to be done for vertical load of 612.5MT (2.5*245*1). Here partial safety factor for load is taken as 1 because its a test load and probability of increasing is zero.

4.0 Working pile : Pile has to transfer the safe load. Hence working pile's structural design has to be done for vertical load of 367.5MT (245*1.5). Here partial safety factor for load is taken as 1.5 based on Table 18 of IS 456:2000.

5.0 Structural design of piles : Based on above para 3 & 4 it is noted that reinforcement requirements for initial test pile will be higher then working pile. Piles may be designed as per cl 39.3 of IS 456 considering short axially loaded members in compression because buckling is prevented by soil.

I thank you very much for reading and valuable feedback/suggestions.

Thanks & regards,
Durga Prasad
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sakumar79
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PostPosted: Thu May 10, 2018 4:27 am    Post subject: Reply with quote

Dear Er Durga Prasad,
     IMHO, the test pile is not to be designed to transfer 2.5 times the design load, but to be checked that under 2.5 times the design load it does not fail abruptly. If the test pile and actual pile have different reinforcement, the test pile results cannot be directly correlated with actual piles' capacity.

Hope that helps
Arunkumar
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durgaprasad
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PostPosted: Thu May 10, 2018 5:39 am    Post subject: Reply with quote

Dear Er Arunkumar,

Thanks for your reply.
But still I believe that our aim is to asses the ultimate frictional and end bearing resistance of the pile. Now during test load of 2.5 times of safe load if pile fails structurally before achieving the ultimate frictional and end bearing resistance we will not get the correct assessment of these.  

Regards,
Durga Prasad Kulhari

sakumar79 wrote:
Dear Er Durga Prasad,
     IMHO, the test pile is not to be designed to transfer 2.5 times the design load, but to be checked that under 2.5 times the design load it does not fail abruptly. If the test pile and actual pile have different reinforcement, the test pile results cannot be directly correlated with actual piles' capacity.

Hope that helps
Arunkumar
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u.mukesh
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PostPosted: Thu May 10, 2018 11:30 am    Post subject: Structural design of test piles vs working piles Reply with quote

The working pile has to be designed for axial load & moment because of horizontal loads.Its can be assumed as cantilever up-to the depth of fixity. So amount of reinforcement may be
more in this case too; because the test pile (for vertical capacity) has to be designed for vertical load only.


Regards


Mukesh Upadhyay






On Wed, May 9, 2018 at 11:59 AM, durgaprasad <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Sefian,

I wish to discuss the structural design philosophy for initial test piles and working piles. In my understating reinforcement for initial test piles will be higher then working piles.
Request you all to go through the case study below and provide the valuable feedback/suggestions.

1.0 CASE : As per soil data : 500 dia 32M long pile : Safe load 245MT and as per project spec test load is 2.5 times of safe load.

2.0 GENERAL : As per cl. 6.1 of "IS 2911 (Part 1/Sec 1) : 2010" The pile shaft should have adequate structural capacity to withstand all loads (vertical, axial or otherwise) and moments which are to be transmitted to the subsoil and shall be designed according to IS 456.

3.0 Initial Test pile : Test pile has to transfer the 2.5 times the safe load. Hence test pile's structural design has to be done for vertical load of 612.5MT (2.5*245*1). Here partial safety factor for load is taken as 1 because its a test load and probability of increasing is zero.

4.0 Working pile : Pile has to transfer the safe load. Hence working pile's structural design has to be done for vertical load of 367.5MT (245*1.5). Here partial safety factor for load is taken as 1.5 based on Table 18 of IS 456:2000.

5.0 Structural design of piles : Based on above para 3 & 4 it is noted that reinforcement requirements for initial test pile will be higher then working pile. Piles may be designed as per cl 39.3 of IS 456 considering short axially loaded members in compression because buckling is prevented by soil.

I thank you very much for reading and valuable feedback/suggestions.

Thanks & regards,
Durga Prasad
     



     


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C.J. Ragunathan
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PostPosted: Sat May 12, 2018 4:30 am    Post subject: Structural design of test piles vs working piles Reply with quote

Thank You, Mr. Durga Prasad for the valuable inputs.


Regards,


Ragunathan C. J.


On Fri, May 11, 2018 at 6:16 PM, u.mukesh <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]            The working pile has to be designed for axial load & moment because of horizontal loads.Its can be assumed as cantilever up-to the depth of fixity. So amount of reinforcement may be
more in this case too; because the test pile (for vertical capacity) has to be designed for vertical load only.


Regards


Mukesh Upadhyay






On Wed, May 9, 2018 at 11:59 AM, durgaprasad forum@sefindia.org (forum@sefindia.org))> wrote:
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skhalid
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PostPosted: Mon May 14, 2018 4:26 am    Post subject: Reply with quote

Dear Sefian,

Please find my reply below point wise :

(1) Working Pile -- Vertical Load = 245 T
What about the Working Uplift/Tension AND Lateral Load. They should always be considered. Mostly Lateral Load governs the no. of piles.

(2) If you are taking a FOS of 2.5 for the Test Pile i.e. checking the Geo-technical Capacity of Pile then Initial Test Loads should be done for all the three i.e. (2.5 x Working Vertical Pile Capacity) , (2.5 x Working Uplift Pile Capacity) and (2.5 x Working Lateral Pile Capacity). And each test on a new pile. Same pile should not be used for any two tests.

(3) Yes u r right, when u r designing the test pile then the reinforcement shall be calculated based on the increased loads i.e. ( 2.5 x .........). Provide the most critical reinforcement in all the three test piles (i.e. for envelope of forces). Reinforcement should be same to avoid confusion at site. Moreover while designing u need to take load =
2.5 x capacity x 1.5.

(4) Yes u r right working pile structural design has to be done based on factor of 1.5 as per IS 456.

(5) Test Pile & Working Pile Design - Piles may be designed as short axially loaded columns with vertical loads & moments. Moments may be calculated from IS 2911 Appendix. Additional moment may be taken for Vertical load eccentricity i.e. 20 mm or as per clause 25.4 of IS 456. Hence pile will be designed for combined vertical & lateral load. NOW COMING TO uplift load , when uplift load will act , no vertical compression load & lateral  load effect will be null & void as the pile is in tension. Therefore calculate the reinforcement based on the respective uplift load. PROVIDE REINFORCEMENT FOR ENVELOPE FOR ALL THE FORCES.

Regards
Khalid
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durgaprasad
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PostPosted: Mon May 14, 2018 7:36 am    Post subject: Reply with quote

Dear Mr. SKHALID.

Thanks for your reply.
Yes, your observations are correct. However I have doubt on comment no 2 & 3. If you can further elaborate would be great.

No-2 : "Same pile should not be used for any two tests." Here if compression test is successful can we use that pile for lateral or uplift check?. If not, what are the concerns?.

No-3 : "Moreover while designing u need to take load = 2.5 x capacity x 1.5."  -- "2.5 x capacity x 1.0" should be ok as partial safety factor of 1 is acceptable for test loads.

Thanks and regards,
Durga Prasad

skhalid wrote:
Dear Sefian,

Please find my reply below point wise :

(1) Working Pile -- Vertical Load = 245 T
What about the Working Uplift/Tension AND Lateral Load. They should always be considered. Mostly Lateral Load governs the no. of piles.

(2) If you are taking a FOS of 2.5 for the Test Pile i.e. checking the Geo-technical Capacity of Pile then Initial Test Loads should be done for all the three i.e. (2.5 x Working Vertical Pile Capacity) , (2.5 x Working Uplift Pile Capacity) and (2.5 x Working Lateral Pile Capacity). And each test on a new pile. Same pile should not be used for any two tests.

(3) Yes u r right, when u r designing the test pile then the reinforcement shall be calculated based on the increased loads i.e. ( 2.5 x .........). Provide the most critical reinforcement in all the three test piles (i.e. for envelope of forces). Reinforcement should be same to avoid confusion at site. Moreover while designing u need to take load =
2.5 x capacity x 1.5.

(4) Yes u r right working pile structural design has to be done based on factor of 1.5 as per IS 456.

(5) Test Pile & Working Pile Design - Piles may be designed as short axially loaded columns with vertical loads & moments. Moments may be calculated from IS 2911 Appendix. Additional moment may be taken for Vertical load eccentricity i.e. 20 mm or as per clause 25.4 of IS 456. Hence pile will be designed for combined vertical & lateral load. NOW COMING TO uplift load , when uplift load will act , no vertical compression load & lateral  load effect will be null & void as the pile is in tension. Therefore calculate the reinforcement based on the respective uplift load. PROVIDE REINFORCEMENT FOR ENVELOPE FOR ALL THE FORCES.

Regards
Khalid
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skhalid
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PostPosted: Tue May 15, 2018 5:42 am    Post subject: Reply with quote

Dear RESPECTED Sefian,


Please find my replies as follows :



No-2 : "Same pile should not be used for any two tests." Here if compression test is successful can we use that pile for lateral or uplift check?. If not, what are the concerns?.



I don't remember the exact clauses of codes as I have done 8-10 years back for NTPC, but the concern is SETTLEMENT. When u test a pile for vertical load , it means u have tested the pile to its extreme load or failure. Broadly speaking u have mobilized the soil-pile interaction, therefore the results will be much lower of the NEXT TEST on the SAME PILE.



No-3 : "Moreover while designing u need to take load = 2.5 x capacity x 1.5."  -- "2.5 x capacity x 1.0" should be ok as partial safety factor of 1 is acceptable for test loads.


Take this as , when u r designing a beam or column or any RCC element for increased load then also we have to multiply the basic load (i.e. increased load by 1.5)
In crude words, (2.5 x capacity) x 1.5 = (basic load) x 1.5
and remember pile will experience this basic load in actual.
Moreover, sometimes this load will be increased at site up till the Pile failure.


Regards
Khalid
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spsvasan
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PostPosted: Tue May 22, 2018 1:10 pm    Post subject: Re: Structural design of test piles vs working piles Reply with quote

Dear Er.Durga Prasad

Yours is a peculiar case. The safe load on the pile is quite high, probably because of the considerable frictional forces developed for a 32m long pile

The very high pile capacity makes it difficult to design for 2.5 times safe load on pile

The pile diameter cannot be changed for the initial test pile. We can modify only the pile concrete strength and reinforcement. It is impossible to design the pile for a load factor of 1.5 for the load for initial test. In your case, you will have to reduce the load factor.

I am worried about adopting a loadfactor of 1, since the pile will be close to its structural failure stage. I think inputs from any Professor who has carried out several  tests upto failure on rcc columns should be obtained before taking a call on a load factor of 1.

If concrete strength is taken as M50 and 6% reinforcement is provided, a load factor of 1.25 can be achieved.

Concrete grade M50 is to be achieved with a slump of about 150mm. This will require a good mix designer.

Bar congestion will have to be avoided by avoiding laps in the top portion of the pile. This may be ok since the required pile capacity will go on reducing as we go down, due to side friction. hence in the top portion there may be only curtailment of rods and not any lapping

Very careful planning of the initial pile load test will be required

If this is a realtime problem, I request you to kindly come back to the forum after the test is over with the test results and the problems faced by you. We will all learn.

Regards
S.P.Srinivasan





durgaprasad wrote:
Dear Sefian,

I wish to discuss the structural design philosophy for initial test piles and working piles. In my understating reinforcement for initial test piles will be higher then working piles.  
Request you all to go through the case study below and provide the valuable feedback/suggestions.

1.0 CASE : As per soil data : 500 dia 32M long pile : Safe load 245MT and as per project spec test load is 2.5 times of safe load.

2.0 GENERAL :  As per cl. 6.1 of "IS 2911 (Part 1/Sec 1) : 2010" The pile shaft should have adequate structural capacity to withstand all loads (vertical, axial or otherwise) and moments which are to be transmitted to the subsoil and shall be designed according to IS 456.

3.0 Initial Test pile : Test pile has to transfer the 2.5 times the safe load. Hence test pile's structural design has to be done for vertical load of 612.5MT (2.5*245*1). Here partial safety factor for load is taken as 1 because its a test load and probability of increasing is zero.

4.0 Working pile : Pile has to transfer the safe load. Hence working pile's structural design has to be done for vertical load of 367.5MT (245*1.5). Here partial safety factor for load is taken as 1.5 based on Table 18 of IS 456:2000.

5.0 Structural design of piles : Based on above para 3 & 4 it is noted that reinforcement requirements for initial test pile will be higher then working pile. Piles may be designed as per cl 39.3 of IS 456 considering short axially loaded members in compression because buckling is prevented by soil.

I thank you very much for reading and valuable feedback/suggestions.

Thanks & regards,
Durga Prasad
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durgaprasad
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PostPosted: Wed May 23, 2018 5:25 am    Post subject: Re: Structural design of test piles vs working piles Reply with quote

Dear Er. S.P.Srinivasan ,


Thanks a lot for your reply.

Yes your observations are correct and this is a practical problem from one of our ongoing project at ENNORE-CHENNAI.

We have started installation of test pile considering load factor of 1 on test load. Our aim will be achieved even if structural failure occurs at test load. However this should not be the case because at that point geotechnical capacity of pile shall be developed to its ultimate capacity and still we will be having the partial safety factors of materials (concrete and RF) to allow for the additional load on pile before it reaches to its ultimate structural capacity.

With this we are keen to know the test results and surely will come back to forum with all details.

Thanks and regards,
Durga Prasad Kulhari



spsvasan wrote:
Dear Er.Durga Prasad

Yours is a peculiar case. The safe load on the pile is quite high, probably because of the considerable frictional forces developed for a 32m long pile

The very high pile capacity makes it difficult to design for 2.5 times safe load on pile

The pile diameter cannot be changed for the initial test pile. We can modify only the pile concrete strength and reinforcement. It is impossible to design the pile for a load factor of 1.5 for the load for initial test. In your case, you will have to reduce the load factor.

I am worried about adopting a loadfactor of 1, since the pile will be close to its structural failure stage. I think inputs from any Professor who has carried out several  tests upto failure on rcc columns should be obtained before taking a call on a load factor of 1.

If concrete strength is taken as M50 and 6% reinforcement is provided, a load factor of 1.25 can be achieved.

Concrete grade M50 is to be achieved with a slump of about 150mm. This will require a good mix designer.

Bar congestion will have to be avoided by avoiding laps in the top portion of the pile. This may be ok since the required pile capacity will go on reducing as we go down, due to side friction. hence in the top portion there may be only curtailment of rods and not any lapping

Very careful planning of the initial pile load test will be required

If this is a realtime problem, I request you to kindly come back to the forum after the test is over with the test results and the problems faced by you. We will all learn.

Regards
S.P.Srinivasan





durgaprasad wrote:
Dear Sefian,

I wish to discuss the structural design philosophy for initial test piles and working piles. In my understating reinforcement for initial test piles will be higher then working piles.  
Request you all to go through the case study below and provide the valuable feedback/suggestions.

1.0 CASE : As per soil data : 500 dia 32M long pile : Safe load 245MT and as per project spec test load is 2.5 times of safe load.

2.0 GENERAL :  As per cl. 6.1 of "IS 2911 (Part 1/Sec 1) : 2010" The pile shaft should have adequate structural capacity to withstand all loads (vertical, axial or otherwise) and moments which are to be transmitted to the subsoil and shall be designed according to IS 456.

3.0 Initial Test pile : Test pile has to transfer the 2.5 times the safe load. Hence test pile's structural design has to be done for vertical load of 612.5MT (2.5*245*1). Here partial safety factor for load is taken as 1 because its a test load and probability of increasing is zero.

4.0 Working pile : Pile has to transfer the safe load. Hence working pile's structural design has to be done for vertical load of 367.5MT (245*1.5). Here partial safety factor for load is taken as 1.5 based on Table 18 of IS 456:2000.

5.0 Structural design of piles : Based on above para 3 & 4 it is noted that reinforcement requirements for initial test pile will be higher then working pile. Piles may be designed as per cl 39.3 of IS 456 considering short axially loaded members in compression because buckling is prevented by soil.

I thank you very much for reading and valuable feedback/suggestions.

Thanks & regards,
Durga Prasad
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