FAQ » PDA Test

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Q001

How often do the Pile Driving Analyzer® (PDA) sensors need to be recalibrated?

In accordance with American Society for Testing and Materials (ASTM) D4965-08 recommendation, Pile Dynamics recommends that all sensors be recalibrated every two years, regardless of how much they have been used during this period. If any sensor gives agen bola questionable data prior to two years, Pile Dynamics also recommends recalibration.

It should be noted that Pile Dynamics has a policy of recalibrating any sensor that is returned to its Maintenance Department for check-up or repair.

Q002

How important is it to periodically have the Pile Driving Analyzer® (PDA) sensors recalibrated?

Pile Dynamics, Inc. (PDI) recognizes that generally both acceleration and strain transducers are stable and unlikely to change calibration within the specified tolerance of +/- 2 %. You may evaluate this by comparing the velocity (times impedance) with the force at the early impact times; if there is good proportionality between the two, then it is likely that both strain and acceleration are properly calibrated. However, the calibration process may identify a defective sensor. It is therefore important to have sensors periodically checked by PDI or one of its authorized agents, to ensure that they are performing according to specifications.

Q003

Does Pile Dynamics, Inc (PDI) provide calibration certificates of the Pile Driving Analyzer® (PDA) sensors? What do these certificates look like?

Yes, all sensors are supplied with calibration certificates, and new certificates are issued when a sensor is returned to its owner after calibration by PDI. You can download a sample certificate by clicking one of the links below:

Q004

For driven piles, is there any recommendation regarding the time between end of drive and restrike test?

The Pile Driving Contractors Association (PDCA) Installation Specification (intended for the private sector work but very similar to the American Association of State Highway and Transportation Officials (AASHTO) installation specification intended for transportation projects) recommends that the restrike time and frequency be based on the time-dependent strength changes that are characteristic of each type soil. The PDCA specification presents the following minimum often used waiting times:

Soil Type
Time Delay Until Restrike
Cleans Sands
1 Day
Silty Sands
2 Days
Sandy Silts
3-5 Days
Silts and Clays
7-14 Days*
Shales
7 Days

* - Longer times sometimes required. Specifying too short of a restrike time for friction piles in fine grained soils may result in pile length overruns.

Q005

Can battered (inclined, raked) piles be dynamically tested with the Pile Driving Analyzer® (PDA)? What is the difference in the procedure compared with vertical piles?

Battered (inclined, raked) piles can be dynamically tested with the PDA, and there is no practical difference in procedure. Just make sure that the gages are installed in an axial orientation, that is, that they are aligned with the pile.

Q006

Can the Pile Driving Analyzer® (PDA) be used to measure the energy transferred to an SPT rod?

The Pile Driving Analyzer may be used to measure the energy transferred to Standard Penetration Tests (SPT) rods by SPT hammers, as called for by the many project specifications that mandate compliance with the American Society of Testing and Materials Standards American Society for Testing and Materials (ASTM) D4633 and ASTM D6066, or with the European Standard EN ISO 22476-3. Both the European Standard and ASTM D4633 accept only one way of determining this energy - through Force and Velocity measurements on the rod. This is something that the PDA can do, provided that the sampling frequency requirements of the code applicable in your part of the world (ASTM D4633 or EN ISO 22476-3) is met.

The PDA model PAK fully complies with the analog system requirements of ASTM D4633-05 when samples are taken at 20 kHz. It should be noted, however, that in soils with low N values (easy driving), sampling at 20 kHz may result in too short a total sample time.

An add-on SPT Software Program is available for the PDA model PAX. This add-on makes it fully compliant with the minimum digital sampling frequency requirements of ASTM D4633-05 (50 kHz) and EN ISO 22476-3:2005 (100 kHz), as well as with the low pass filter cut-off requirements of ASTM D4633-05. The add-on also requires a hardware change to allow automatic switching of the input circuit cut-off frequency. All new PAX units are already shipped with the modified hardware, so all that is required is the additional SPT program. Older units may have to be sent back to Pile Dynamics, Inc. (PDI) for updating the hardware. Please contact our sales department to check if it is necessary to return your unit to PDI.

Measurements on SPT rods also require the use of an instrumented rod. It consists of a piece of rod about 0.6 m (2 ft) long, with foil strain gages glued to it and calibrated. Regular PR accelerometers are attached to the rod in the usual way. The strain transducers used for testing foundations cannot be used on SPT rods.

Q007

Above what foundation diameter is it preferred to test with 4 strain transducers?

Pile diameter is generally not a concern for driven piles. For most driven piles two strain transducers are sufficient since they can be placed diametrically opposite to cancel any bending. In relatively rare cases where bending is an issue, four strain transducers may be used, since with two strain transducers you can only assess the bending in one plane while with four strain transducers you can assess bending in two planes. Also for “spiral welded pipes” it is often beneficial to have four strain transducers.

When dynamically testing drilled shafts (also called bored piles) and augered cast-in-place piles or continuous flight auger (CFA), four strain transducers are always recommended for any pile diameter. In this case, in addition to assessing bending on an initial blow in order to better center the drop weight for subsequent blows, four strain transducers provide a benefit for assessing data quality. In these foundations the quality of the concrete of the top is sometimes an issue, therefore the attachment of the strain transducers may be marginal. Four strain transducers (attached as two pairs of diagonally opposite units) allow comparison of the averages of each pair - if the averages are consistent then there is confidence in the measurement. If the averages do not agree, then perhaps there is information about which strain transducer is not performing correctly. That strain transducer and its “mate” should be turned off and the analysis done on the remaining pair. Four strain transducers are also recommended to provide back-up - since a limited number of impacts is applied during a dynamic test on a drilled foundation, it is more crucial to have good data on each impact.

 

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