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Author:
Liew Shaw Shong
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2001-05-18 |
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Recently, I've some High Strain Dynamic Pile Test (HSDPT) results for pile driving monitoring on a test pile. I notice that there is a obvious trend of tensile stress increase after jointing the first extention pile, where the starter pile is 11m into the ground.
At the end of driving, the pile is about 17m long with 12m starter pile and 12 extension pile. There is about 3m toe penetration into hard layer (SPT-N > 50). The general overburden has SPT-N of 5 for first top 4m and 15 for the subsequent 10m. The subsoil is mainly Silty Clay with some gravel at the lower stratum.
The tester's explaination on this phenomenon can be contributed by the following:
a. The welded joint tends to open up during the upward reflection wave passing back to the upper extention pile, which allows the upper extension pile to move more than the lower starter pile, therefore tensile stress is recorded.
b. The contrast in shaft friction allow the upper portion of the pile segments, which have lower friction, move a little bit more than the lower pile segments, which have more restraints for movement from the higher shaft friction, during the impact wave reflecting upward.
Is there any other reasons for the occurrence of high tensile stress on pile in this ground condition?
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Follow-up:
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Author:
Edmund
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2001-05-18 |
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Tensile stress will normally occur on a pile if low hammer weight with high drop height was used to drive a pile on soft ground. If PDA was used to monitor the driving, you will notice that tensile stresses will be reduced when the pile hit a hard layer. To minimise the tensile stresses, it is advisable to use a heavy hammer and low drop height.
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Follow-up:
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Author:
Liew Shaw Shong
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2001-05-18 |
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Edmund, Thanks.
This pile behaves differently as what you have described. The tensile stress builds up when penetrating into relatively harder soil stratum before termination.
Additional information:
Pile : 300mm Dia. x 60mm Thk Prestressed Concrete Pile
Driving Hammer : 7 Ton Hydraulic Hammer (single acting)
Hammer Drop Height : 300mm to 450mm
During starter pile driving, tensile stress ranges 0.4 to 1.3MPa. But after joining extension pile, the tensile stress gradually increases from 1 to 6.6MPa with depth. Suprisingly, the tensile stress reduces to 1.7MPa during last 50blows before end of drive. The RMX also increases gradually with depth and has a sudden increase near to the end of drive blows. The DFN (Computed Set per blow) is in the range of 30 to 40mm/blow during the starter pile penetration and reduces to below 20mm/blow after jointing the extension piles (probably some pile setup and also closer to hard bearing stratum). Pile cusion was changed before driving the extension pile.
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Follow-up:
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Author:
Liew Shaw Shong
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2001-05-18 |
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Edmund, Thanks.
This pile behaves differently as what you have described. The tensile stress builds up when penetrating into relatively harder soil stratum before termination.
Additional information:
Pile : 300mm Dia. x 60mm Thk Prestressed Concrete Pile
Driving Hammer : 7 Ton Hydraulic Hammer (single acting)
Hammer Drop Height : 300mm to 450mm
During starter pile driving, tensile stress ranges 0.4 to 1.3MPa. But after joining extension pile, the tensile stress gradually increases from 1 to 6.6MPa with depth. Suprisingly, the tensile stress reduces to 1.7MPa during last 50blows before end of drive. The RMX also increases gradually with depth and has a sudden increase near to the end of drive blows. The DFN (Computed Set per blow) is in the range of 30 to 40mm/blow during the starter pile penetration and reduces to below 20mm/blow after jointing the extension piles (probably some pile setup and also closer to hard bearing stratum). Pile cusion was changed before driving the extension pile.
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Follow-up:
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Author:
s.v.ekanayake
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2001-05-22 |
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I would like to have more details about stresses develop while driving a pile.please send me in details.
No.23,
GAJABAPURA(NHS),
KOLONNAWA,
SRI LANKA.
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Follow-up:
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Author:
Mun Kwai Peng
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2001-05-23 |
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Dear Ir. Liew S.S.. You posted an interesting subject of Dynamic Load Pile Monitoring where tensile stresses experienced by the pile is high and of concerned to you as a Geotechnical Engineer.
The explanation from your tester (definitely not from me even though I am from the same country as you) is not true.
Please take note that tensile stress in a pile from Dynamic Load Test is a computed parameter based on certain assumption and not a measured parameter.
Tensile stress in pile driving can happen in many ways, irrespective of type of hammer, pile or soil condition. It depends on the combination of all the above.
When there is easy driving with no toe resistance, tensile stress occurs during the downward traveling wave along the pile shaft. The pile is being pulled from the toe due to the hammer blow.
The other possibility of occurrence of tensile stress can happen when you have hard driving like your case. There is little skin friction at the time of pile driving to dissipate the hammer energy. The hammer blow travels through the pile shaft with little dissipation and reflected almost 100 % or sometimes double depend on end condition at the pile toe; back to the pile top. As the pile top is not restrained, the return wave would pull the pile upwards thus tensile stress would be experienced by the pile at end of drive.
What is your actual problem in the piling project? Is there broken piles problem? Capacity problem? What is your concerns in the high tensile stresses?
Pile monitoring should be able to solve your problem if an experienced operator provide the pile monitoring services. If you can't solve your problem, please contact me for further discussion.
To. Mr. s.v. Ekanayak, it will be difficult to provide you in detail as you know nothing about dynamic load test. I suggest you start with the Proceeding of the previous International Conferences on the subject of which all together, there are 6 of them being held so far.
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Follow-up:
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Author:
Bengt H. Fellenius
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2001-05-24 |
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Just a few quick comments. I understand that Mr. Shong found that the tensile forces appeared or increased after adding pile length (welded pile splice on a precast prestressed pile) as "measured" (observed) by the Pile Driving Analyzer (PDA).
First, the PDA will indicate from where in the pile the tension originated. This should answer your query about the cause of the tension being an opening of the splice (a progressive "gap")
Second, the longer a pile (everything else being equal) the larger the tension. For a relatively short pile (in relation to pile cross section and hammer cross section and mass), little or no tension can develop because the stress wave is much longer than the pile. As the pile gets longer, the upward traveling tension wave (reflected from the pile toe) will act where the downward traveling compression wave has already passed (the wave are like two trains traveling in opposite directions, meeting and driving passed each other; and, of course, on different tracks).
Third, that the tension reduces for the last blows should not be a surprise. The pile is now driven into more competent soils and the tension is reduced. Unless the toe quake values are high, there should be no tension toward the end of the driving.
Fourth, use GRL WEAP to make a number of wave equation analyses of the driving for different pile lengths and toe conditions. Then, the behavior of the pile and the PDA results will be much more clear.
Bengt H. Fellenius
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Follow-up:
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Author:
sv ekanayake
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2001-05-24 |
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no 23,
gajabapura(nhs),
kolonnawa,
sri lanka.
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Follow-up:
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Author:
Edmund
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2001-05-30 |
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I haven't access this web site for the past 10 days and thus had not been able to reply earlier. From your reply, I do not think that your pile behaves differently from my earlier answer. Probably, I did not explain in details.
You mentioned that the tensile stress increases after joining the extension pile. This is expected since longer pile with same drop height and hammer weight will result in higher tensile stresses (See Bengt H. Fellenius's reply). In addition, you mentioned that the tensile stresses reduces to 1.7MPa during the last 50 blows before the end of driving. Again this is expected as I had earlier mentioned that the tensile stress will reduce when the pile hit a hard layer.As for the CFN which reduces from 30 to 40mm / blow to 20mm / blow after jointing the extension pile,this happened because the pitch length of the pile had been increased. With the same hammer drop and drop height, the energy transfered to the bottom of the pile reduces. Further, the changing of the cushion means the the new cushion will absorb more energy i.e. less energy will be transfered to the bottom of the pile.
On the explanation by Mr Mun Kwai Peng, it should be noted that the reflected wave from the bottom of the pile (when the pile is in the hard layer i.e. hard driving) is most probably a compression wave. Thus,the possible doubling wave that Mr. Mun mentioned is a compression wave.
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Follow-up:
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Author:
Mun Kwai Peng
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2001-05-31 |
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Yes the return wave from the pile toe is a compression wave in this case. When a pile is just driven with very little skin friction and the pile toe rest on rock, the compression wave from the hammer blow travels to the toe and reflected as double the compression wave, when the wave returns back to the pile head, as little skin friction along the pile shaft reduces the energy of the wave and also no protection exist at the pile head and the compression wave(Double) reflects back to the pile as tension.
Pile monitoring should not be used just to measure what happen to the pile but to be used as a tool to solve and eliminate any piling problems. The tester should be able to advise the piling contractor during monitoring to limit the stresses experience in the pile by giving instruction to the piler to change the hammer drop(for the case of Hydraulic Hammer) but at the same time achieve the capacity required in the pile driving.
I have experience in a pile monitoring for a 500mm diameter 100mm thick prestressed concrete pile using a 7 ton Hydraulic Hammer facing the problem of broken pile during piling. Most other tester could not solve the problem.
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Subject:
Tensile Stress on Pile Driving 