How often does the Pile Integrity Tester (PIT) need to be recalibrated?

Pile Dynamics recommends that the PIT Main Unit be recalibrated every four years, regardless of how much it has been used during that period.

Posted: 12/1/2008

How important is it to periodically have the Pile Integrity Tester (PIT) recalibrated?

Even though American Society for Testing and Materials (ASTM) does not specify a period for recalibrating the PIT Main Unit, Pile Dynamics Inc. (PDI) recommends that it be recalibrated every four years. Besides checking the accuracy of the time base and of the internal circuits, during the calibration process the Main Unit is thoroughly checked for other problems that might affect its performance. Having the Main Unit recalibrated by PDI or by one of its authorized agents will help ensure that it is working properly.

Posted: 12/1/2008

How often do the Pile Integrity Tester (PIT) Accelerometer and Instrumented Hammer need to be recalibrated?

Pile Dynamics recommends that the PIT Accelerometer and the PIT Instrumented Hammer be recalibrated every two years, regardless of how much they have been used during that period. It should be noted, however, that these calibrations are only important in case of simultaneous force and velocity measurements. When tests are performed with the accelerometer only (velocity only) the data is normalized and the actual velocity amplitude is not relevant.

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

Posted: 12/1/2008

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

In accordance with American Society for Testing and Materials (ASTM) D4965 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 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.

Posted: 1/14/2009

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:

• Piezoresistive Accelerometer
• Piezoelectric Accelerometer
• Strain Transducer

Posted: 1/14/2009

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.

Posted: 1/14/2009

How often should the calibration of the Pile Driving Analyzer® (PDA) be checked?

American Society for Testing and Materials (ASTM) D4945 requires that an internal calibration check be performed at least once for each test day. Therefore, it is recommended that a Calibration Test (CT) signal be recorded at the beginning of each test (or at least once a day). This CT Signal can later be replayed with the calibration settings specified in Question Q1007, and the values of VMX and MEX checked as described in that Question. It will then be possible to confirm that the internal calibration of the PDA was correct when the test was done.

If frequent calibration checks are made as described above (or with a Calibration Box in case Piezoresistive (PR) accelerometers are used), there should be no need for further calibration of the PDA, unless the measured data is questionable (such as if there is consistently bad proportionality between force and velocity times impedance at early impact times).

Pile Dynamics recommends that the PDA be sent for recalibration at least once every two years. At that time the performance of the PDA will be thoroughly checked, upgrades can be made, etc. Please note that even though ASTM D4945 also recommends that the PDA be recalibrated at least once every two years, other codes or specifications may have more stringent requirements. Please be aware of the codes or specifications that apply to your specific situation.

Posted: 1/14/2009

During a CAPWAP analysis the following message was displayed 'Warning: Not fully activated resistance' and then 'Totally activated resistance: XXX'. What does this mean?

This message means that the maximum displacement of one or more soil elements was less than the quake. We therefore say that the resistance was not fully activated. When this happens the solution is unacceptable, as this means that either the ultimate resistance or the quake (or both) for that soil element has been specified too high. The pop-up window has a table showing the elements where the full resistance was not activated, with the value of the resistance specified for that element (Rult) and the activated resistance (Ract), that is, the resistance corresponding to the actual maximum displacement of that element. When this happens you should reduce the resistance and/or the quake, until the maximum displacement of the corresponding element becomes equal or greater than the quake. If you check the box saying “check for proportional Ru and quake reduction”, the program will reduce both the resistance and the quake by the same factor, therefore keeping the ratio between them, but making sure that the full resistance is now activated (note: this may result in different quakes for different shaft elements, possibly indicating the solution is a lower bound).

Posted: 1/14/2009

I’m trying to install PIT-S on a computer with Windows Vista (or Windows 7) and I am getting an error message. Does PIT-S work with Vista (or Windows 7)?

Yes, PIT-S works with Vista (and Windows 7) but in order to install it you have to follow these steps:

1. Copy the file “SetupForPIT-S.msi” to your C:\ (root) directory
2. Click on Start->All Programs->Accessories, and point to “Command Prompt”
3. Right click and select “Run as Administrator”
4. A Command Prompt (>) will appear. Type CD C:\ and then type Enter (to switch to the C: root directory)
5. Type SetupForPIT-S.msi and then type Enter
6. The installation program will run. Click “Next” and “Continue” as appropriate.
7. After the program is successfully installed, open Windows Explorer and go to \Program Files\PDI\PIT-S and find the file PIT-S.exe. Right click on it and select “Run as Administrator”
8. After the program runs for the first time you can open it normally using the icon created on the desktop.

Posted: 1/14/2009

Does PDA-W run on Vista (or Windows 7)?

Yes, but you have to make sure that the program is properly registered before you run CAPWAP, PDIPLOT and PDI-Curves. In order to do that, find the file PDA.exe (it will be on the \Pdawin folder, unless you changed that during installation). Right click on the file, then choose “Run as Administrator” and confirm the operation. Once the program opens it is already registered. You can then close the program, and after that it can be opened normally from the desktop icon.

Also check Question Q1017 to make sure you install the correct Sentinel Hardware Key driver.

Posted: 1/14/2009

Is there a reference to testing pile capacity by re-driving / re-striking after it has "set up" in an American Society for Testing and Materials (ASTM) or other standard?

ASTM D4945-08 is a standard for high strain dynamic testing, either during driving or on restrike. Even though the expression “set-up” is not used, it is implied in the reasons for restrike testing:

1. Section 4.2 mentions that one of the factors affecting the foundation capacity from a dynamic test is “elapsed time since initial installation” (for driven piles this implies a restrike).
2. Note 1 mentions “dynamic restrike tests that account for soil strength changes with time”.
3. Section 6.1 directs “Allow sufficient time (...) prior to testing” and continues “Determine the pile response to high strain dynamic test from a minimum of ten impact records during initial driving and, when used for soil resistance computations, normally from one or two representative blows at the beginning of a restrike.”
4. Section 6.8 states that “ If the test results are used for static capacity computations, then dynamic measurements should (also) be performed during restrikes of the deep foundation, after waiting a period of time following the initial installation sufficient to allow pore water pressure and soil strength changes to occur.
5. Section 7.6.2 prescribes that the Test Report contain Date of test(s), sequence of testing (e.g. “end of driving” or “beginning of restrike”), and elapsed time since end of initial driving for restrikes,

The Pile Driving Contractors Association (PDCA) Installation Specification (intended for private sector work but very similar to the American Association of State Highway and Transportation Officials (AASHTO) installation specification for transportation projects), refers to “restrike” in Item 4.4.3:

“Dynamic testing shall be conducted in accordance with ASTM D4945 (...) If the required ultimate pile capacity is not achieved at the end of driving, the Contractor shall restrike the dynamic test pile following a waiting period specified in the contract documents or as directed by the Engineer...” The Commentary to this section adds: “Because the ultimate pile capacity of a pile may change substantially during and after pile driving, waiting after driving for additional testing may be beneficial for a safe and economical pile foundation. If possible, the dynamic test should be performed as a restrike test if the Engineer anticipates significant time dependent increases in nominal strength called setup, or reductions called relaxation.”

A copy of American Society for Testing and Materials (ASTM) D4945 may be purchased from http://www.astm.org/Standards/D4945.htm

A copy of the PDCA Installation Specification can be downloaded from www.piledrivers.org

Posted: 5/11/2009

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:

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

Posted: 5/11/2009

Is there a relation between concrete strength and wave speed? How can I determine the minimum acceptable wave speed?

Based on information reported in American Concrete Institute (ACI) 228.1 R-95 and some other codes and specifications, the concrete compressive strength is approximately proportional to the compression wave velocity to the 4^th power. This means that, if σ is the concrete strength, W is the wave speed, and k₁ and k₂ are constants:

σ = k₁ × W⁴ or W = k₂× σ^1/4

Those relationships can be used to compare the strength and wave speed of concretes with similar mix. Suppose the minimum acceptable strength for the concrete of a drilled shaft project is σ_A. Also suppose that on a given shaft on that project you determine a wave speed W_M (by performing a Pile Integrity Test or by Cross Hole Sonic Logging) and measure a strength σ_M (by extracting a cylinder, for example). A relationship may be established for the minimum acceptable wave speed on other drilled shafts on the same project, W_A, as follows:

W_A = W_M ( σ_A / σ_M ) ^1/4

Posted: 5/12/2009

Is the Sentinel Hardware Key driver used with Pile Dynamics, Inc. (PDI) programs compatible with Windows Vista (or Windows 7)?

Version 7.5.0 of the Sentinel Hardware Key driver is guaranteed to work with Windows Vista (and Windows 7). Earlier versions might work or not.

All new software installation CDs shipped by Pile Dynamics after December of 2008 come with the installer for version 7.5.0 of the Sentinel Hardware Key.

Posted: 5/13/2009

What is the usual range of concrete wave speeds that should be used with the Acoustic Concrete Tester (ACT)?

The usual values of good concrete wave speeds are between 3500 m/s (11,500 ft/s) and 4500 m/s (14,500 ft/s).

Posted: 5/13/2009

Does CAPWAP run on Windows Vista (or Windows 7)?

CAPWAP version 2006 works with Vista (and Windows 7), but you have to make sure that the program is properly registered. In order to do that, find the file capwap.exe (it will be on the \Program Files\PDI\CAPWAP 2006 folder, unless you changed the file location during the installation). Right click on the file, then choose “Run as Administrator” and confirm the operation. Once the program opens it is already registered. You can then close the program, and after that it can be opened normally from the desktop icon.

Some earlier versions of CAPWAP may also work if installed using the procedure described above, but they have not been extensively tested under Vista (or Windows 7).

Posted: 5/15/2009

Does GRLWEAP run on Windows Vista (or Windows 7)?

GRLWEAP version 2005 works with Vista (and Windows 7), but you have to make sure that the program is properly registered. In order to do that, find the file grlweap.exe (it will be on the \Program Files\PDI\GRLWEAP 2005 folder, unless you changed the file location during the installation). Right click on the file, then choose “Run as Administrator” and confirm the operation. Once the program opens it is already registered. You can then close the program, and after that it can be opened normally from the desktop icon.

Some earlier versions of GRLWEAP may also work if installed using the procedure described above, but they have not been extensively tested under Vista (or Windows 7).

Posted: 5/15/2009

Does the PIT-W program run on Windows Vista (or Windows 7)?

PIT-W version 2009 works with Vista (and Windows 7). Just install the program following the instructions on the screen.

PIT-W version 2003 works with Vista (and Windows 7), but you have to make sure that the program is properly registered. In order to do that, find the file PitW.exe (it will be on the \Program Files\PDI\PIT-W 2003 folder, unless you changed the file location during the installation). Right click on the file, then choose “Run as Administrator” and confirm the operation. Once the program opens it is already registered. You can then close the program, and after that it can be opened normally from the desktop icon.

Some earlier versions of PIT-W may also work if installed using the procedure described above, but they have not been extensively tested under Vista (or Windows 7).

Posted: 5/15/2009

Does the TomoSonic program run on Windows Vista?

Yes, but you have to use a newer version of the Java Run Time Environment (the files on the “jre” folder of the TomoSonic program), otherwise the 3-D outputs will not show correctly. Please contact software@pile.com for a copy of the Java Run Time Environment compatible with Windows Vista. Once you download the files, just rename or delete the existing “jre” folder and replace it with the new one.

Posted: 5/15/2009

Does the Hammer Performance Analyzer (HPA) program run on Windows Vista (or Windows 7)?

The HPA program has not been tested on Windows Vista (or Windows 7) and is not guaranteed to work on computers with this OS.

Posted: 5/15/2009

Does the Cross-Hole Analyzer (CHA) program run on Windows Vista (or Windows 7)?

Yes, but it is recommended that you make sure that the program is properly registered. In order to do that, find the file CHA.exe, right click on it, then choose “Run as Administrator” and confirm the operation. Once the program opens it is already registered. You can then close the program, and after that it can be opened normally from the desktop icon.

Posted: 5/15/2009

Does PDI-Curves run on Windows Visa (or Windows 7)?

Yes, PDI-Curves is totally compatible with Windows Vista (and Windows 7) systems. Some users have reported that after installing PDI-Curves on 64-bit machines with Windows Vista (or Windows 7) the program shows a message saying that it has not been correctly installed, and will not run. If this happens please contact software@pile.com for instructions, which involve writing additional information to the system’s registry. Also make sure that you are using the latest version (2009.1.35.1), which resolves an issue with printing on 64-bit systems.

Posted: 5/15/2009

1. For Pile Driving Analyzer® (PDA) (EMX, EFV, ETR, {ETH - diesel hammers only}),
2. For SPT (EMX, EFV, ETR, E2E, E2F, EF0, EF2, EV2)
   
   

The E2E method does the same computation as EMX, but it stops the integration at (2 * LE)/c. It can be used to stop the integration at a time corresponding to a given LE setting. It should be noted that ending the integration before the end of the record is not the procedure recommended by ASTM 4633-05 and European Standard ISO 22476-3:2005 for SPT energy testing, and that changing LE will also change the capacity calculations on PDA testing.

EF2 was used exclusively for SPT energy measurements on previous versions of ASTM D4633, at a time when a reliable method for measuring the velocity on SPT rods was not available. It is the maximum of the integral of the square of the force divided by the impedance, over the whole record, and is based on the theoretical proportionality between force and velocity along most of the first 2L/c period on SPT energy tests. In real practice this proportionality is hard to achieve due to non-uniformities along the rod, imperfect joints, etc. Other factors also affect the accuracy of this method, so several correction multipliers had to be used. It was later recognized that these correction methods did not yield reliable results (please refer to the Appendix on ASTM D4633-05 for further information). EF2 is therefore generally inaccurate and obsolete, and should not be used . EF0 is the same as EF2. The E2F method does the same computation as EF2, but stops the integration at 2L/c. It has the same lack of accuracy as EF2. EV2 is the maximum of the integral of the square of the velocity multiplied by the impedance, over the whole record. It is based on the same proportionality principle as EF2, so it suffers from the same lack of accuracy.

We would like to stress that EF2, EF0, E2F and EV2 should not be used to determine the energy transferred to an SPT rod or to a foundation, and are provided for research purposes only.

Posted: 5/16/2009

Can Steel Piles be tested with the PIT?

There is limited applicability of PIT on steel piles, due to

1. their generally high L/D ratio: The difficulties of performing low strain dynamic tests based on the sonic pulse echo or transient response theory on pile with a very large L/D is discussed on Question 1031.
2. their generally high surface area to volume ratio: This characteristic causes the input wave to be quickly dampened by the surrounding soil.

The PIT test is therefore mostly applicable to determine the integrity of concrete foundation elements. However, successful detections of defects on short (less than 15 m or 50 ft) H-piles in soft soils have been reported by others. PIT is not recommended for open steel pipe piles, unless they are filled with concrete. It should be noted that the PIT equipment and software allows changing the wave-speed and specific weight to those of steel.

Posted: 7/7/2009

What is the minimum pile length that can be tested with the PIT?

The PIT performs a low strain dynamic test, based on the sonic pulse echo or transient response theory. It is mostly applicable to determine the integrity of relatively long foundation elements with buried, hard to visually inspect lower portions. In addition, the one dimensional stress propagation theory which is the base of this method applies to slender rods, i.e., foundations with a relatively large ratio of L/D (L = Pile Length and D = Diameter of Pile). Note, however, that a very large L/D may present difficulties for this type of test (see question Q1031).

The hammers supplied by Pile Dynamics, Inc. (PDI) for use with the PIT have been optimized and tested for relatively long foundation elements. PIT is not recommended for testing on short piles less than 2 m without modifications. For example, the frequency content of the wave generated by the hammer impact will determine the length limitations of PIT testing with that hammer. The 450 grams (1 lb) hammer may generate frequency contents up to 1400 Hz, with its most efficient frequency range up to about 1000 Hz. This corresponds to a foundation length of 2 m (assuming a wave speed of 4000 m/s), so this hammer should be used to test piles or shafts no shorter than this length. Pulses generated by heavier hammers, which generate lower frequencies, are better suited to test longer foundation elements, and not well suited for shorter piles. For a 1 m-long pile, the frequency content of the pulse generated by the hammer impact needs to exceed 2000 Hz, so a special hammer or other impact device is usually required, such as a hammer with less weight and harder tip, or a steel ball bearing. For such short piles, frequency analysis is often used to interpret the data.

It should be noted that to assess the integrity of pile caps or slabs of less than 1 m thickness, an Acoustic Concrete Tester (ACT) may be helpful. Also, for very short piles the integrity can be easily determined by visual examination (e.g. simple excavation).

Posted: 7/7/2009

What is the maximum size foundation that can be tested with the PIT?

The maximum size for successful PIT testing is usually specified in terms of a maximum Length (L) over Diameter (D) ratio. An L/D ratio of 30 has in the past been mentioned as being the upper limit for the PIT test, since for older electronics and for piles or shafts longer than 30 times their diameter the input is usually depleted by shaft soil resistance so that no clear toe reflection would be observable. The limit, however, can in actual practice be higher or lower than 30, depending on several factors, like:

1. Length of foundation effectively subjected to skin friction. The part of the pile or shaft above grade or in very soft soil has very little influence on the total length that can be tested.
2. Amount of skin friction. Very high soil resistance can reduce the maximum length, while very low resistance throughout most of the shaft may allow longer foundation elements to be tested.
3. Amount of end bearing. A clear toe reflection requires some elasticity of the soil at the toe, thus creating a tension reflection since the underlying soil stiffness is considerably weaker than the foundation. Very rigid soil or rock may result in a compression reflection or an unclear toe reflection.
4. Hollow piles with relatively thin walls. These piles are difficult to test even in the best of conditions since a plane wave is never attained. Maximum length will be limited.
5. Piles or shafts with variable cross-sections or material properties. Impedance variations reflect the wave and it may be difficult to assess the integrity of the foundation below the first major change.
6. Piles with splices. Splices can reflect the wave, and it may be impossible to evaluate the integrity of the pile below the first splice.
7. Foundations with cracks. Even cracks that would not be detrimental to the usefulness of the foundation may cause strong reflections so that a clear toe reflection would not be detected.
8. Very long large diameter piles or shafts. The attenuation of the wave due to soil resistance and to internal damping may make it difficult to detect a toe reflection on foundations longer than 50 m (160 ft).
9. Long timber piles. Because of the reflections caused by the tapered shape of those piles, a clear toe reflection may not be detected on timber piles longer than 15 m (50 ft).
10. Quality of the electronics used. Low noise circuits and the use of high resolution (=16 bits) A/D converter allow the PIT to go beyond the usual L/D limitation, so that values of 60 or more can be expected in many cases.
11. Piles in structure. Reflections from existing structures might limit the maximum L/D ratio that can be effectively tested.

It should also be noted that sometimes the test will be able to assess the integrity of the foundation up to a certain depth. If it can be confirmed that no major defect is present in the upper part of the foundation, or in the upper zone of lateral movement, this is often very valuable information and sufficient to eliminate any major concern.

Posted: 7/7/2009

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.

Posted: 7/7/2009

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.

Posted: 7/7/2009

What is the difference between the efficiency of Hydraulic Hammers and that of Diesel Hammers?

The term "efficiency" can have several interpretations. In wave equation analysis (GRLWEAP) it is the ratio of the potential energy to the kinetic energy of the ram just prior to impact. A wave equation efficiency of 80% best matches the normal performance of most diesel hammers, therefore that is the value recommended by GRLWEAP. The wave equation efficiency of hydraulic hammers has variations among the manufacturers; the 95% recommended by GRLWEAP is a good assumption for those with built in energy monitors. In PDA testing, the term "Energy Transfer Ratio" (ETR) is the ratio of the energy transferred into the pile to the maximum rated energy of the hammer. This value depends not only on the hammer but also on the pile type, since typical plywood cushions used with concrete piles absorb some energy.

Diesel hammers have a median energy transfer ratio of 37% on steel piles (less on concrete piles), if you compare the energy transferred to the pile to the maximum rated energy of the hammer, calculated at the rated stroke. The stroke of a diesel hammer, however, is also a function of the soil resistance and of the elasticity of the pile, so frequently the diesel hammer does not achieve the full rated stroke. Consequently, the actual potential energy of diesel hammers is often lower than its maximum rated energy, which results in lower values of ETR. The PDA also calculates ETH, which is the ratio of the energy transferred to the pile to the actual potential energy of diesel hammers, that is, weight of ram times measured stroke (STK).

For hydraulic hammers on steel piles, the ETR may be up to 95% of the reading of the “energy monitor” of the hammer. ETR will be lower for concrete piles due to the energy stored in the pile top cushion. In addition, it should be noted that hydraulic hammers can be, and usually are, operated at lesser strokes than their maximum (for example with concrete piles to avoid high tension stresses in easy driving). In those cases the ETR values calculated by the PDA based on the maximum rated energy of the hammer will result in lower values.

It should be noted that energy rating, or efficiency, or even energy transfer is not necessarily the best measure for selection of a hammer. The capacity of the pile in installation, together with the ability of the hammer to impart force (which also depends on the pile strength or stiffness), may be a better way to assess the suitability of one hammer for a given project site. A wave equation analysis is always recommended to help select the hammer type for any particular application.

Posted: 7/21/2009

How accurate is the Pile Integrity Tester (PIT) when used for determining foundation lengths?

The accuracy of foundation length is dependent on the accuracy of the wave speed. Since the wave speeds for good quality concrete vary between 3500 m/s (11,500 ft/s) and 4500 m/s (14,500 m/s) on low strain tests like those performed with the PIT (and the Acoustic Concrete Tester (ACT) - see Question 1018), if an average wave speed of 4000 m/s (13100 ft/s) is used then a maximum variation of plus or minus 12.5 % is possible.

Posted: 9/23/2009

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.

Posted: 9/23/2009

Can the results of PIT tests be confirmed by comparing them with the piling records?

Certainly comparing the PIT results with installation records is always advised. This is particularly true for augered cast-in-place or continuous flight auger (CFA) piles with installation records obtained with a magnetic flow-meter and with depth increments of 600 mm (2 ft) or less (such as Pile Dynamics’ Pile Installation Recorder would produce). In addition, one should always compare results of PIT with the soil profile. There may be systematic changes in the PIT records that correspond to soil layers. For example, if a loose sand overlays a stiff clay, the pile may have a bulge in the sand and then return to nominal dimension at the clay layer. In this case the relative reduction should not be interpreted as a defect since the nominal diameter is maintained.

It is also good to compare the PIT records with each other, to try to establish the normal result and then to determine those that substantially deviate, particularly if a reflection indicating a defect is observed (i.e. large reflection with the same sign as the input pulse).

In summary, all information available should be considered when determining final acceptance of foundations using the PIT method.

Posted: 10/28/2009

Can piezoelectric (PE) accelerometers be used on steel piles, and can piezoresistive (PR) accelerometers be used on concrete or timber piles?

PR accelerometers may be used on any kind of pile under any circumstance. PE accelerometers may be used in any case EXCEPT when there is steel-to-steel impact, for example with some hammers whose driving system does not have a hammer cushion or on SPT rods. In those cases the use of PE accelerometers may result in unacceptable velocity traces (lack of proportionality with the force trace and/or excessive high frequency components), which means that the accuracy of the measurement may not be within 3% as required by American Society for Testing and Materials (ASTM). The use of PE accelerometers on steel piles when there is direct impact between the ram and the helmet or the top of the pile should therefore be avoided; the use of those accelerometers in other cases with steel piles is possible and will result in sufficient accuracy, provided that the distance between the top of the pile and the sensors is kept within the recommended limit (preferably more than 2 times the diameter).

Posted: 1/29/2010

Does the TomoSonic program run on Windows 7?

TomoSonic does not directly run on Windows 7, because the HASP hardware key driver is incompatible with this OS (it will work on Windows Vista or earlier). However, the Professional, Ultimate and Enterprise editions of Windows 7 can create a Windows Virtual PC with Windows XP Mode, which will run TomoSonic. Click here for more detailed instructions.

Posted: 3/9/2010

Can spiral welded steel pipe piles be tested with the Pile Driving Analyzer? If yes, should 2 or 4 strain transducers be used?

In some cases the walls of spiral welded steel pipe piles do not line up well. This causes local bending in the wall solely due to the weld, regardless of how far from the top the gages are installed. Poor results will be obtained when this happens, unless 4 strain transducers are used.

Posted: 4/28/2010

My CAPWAP analysis is showing a total capacity (Ru) greater than the applied force (FMX). Is that possible?

Theoretically, for a fixed end pile (no skin friction and “infinite” end bearing) the mobilized capacity would be equal to two times the impact force. In practice a mobilized capacity higher than about 1.3 times the impact force is unusual, and it is very unlikely that the mobilized capacity could exceed 1.6 times the impact force (FMX).

Posted: 4/28/2010

What is the maximum possible distance between the access tubes required for crosshole sonic logging?

PDI's 45 kHz nominal frequency probes have been tested in tubes up to 3.0 m (10 ft) apart. Larger distances between tubes may be possible, but so far have not been tested nor reported to us. It should be kept in mind, however, that defects could be missed with large distances between the probes, since reflection and refraction in the aggregate may cause waves to bend around the defect. The larger the spacing the larger the defect that could be missed.

Posted: 4/28/2010

When preparing a shaft for CSL, may I use access tubes of different diameters in the same shaft?

Yes, provided they are all larger than 38 mm (1.5 inches) internal diameter. The Cross-Hole Analyzer and the CHA-W software both allow entering the individual tube diameters.

Posted: 4/28/2010

Can the Pile Integrity Tester be used to evaluate the length and defects of piles supporting existing structures?

Yes, under favorable conditions it is possible to test piles under existing structures with the PIT. In order for the test to succeed, the pile itself must have a relatively solid concrete surface to channel the impact (this can be a solid section concrete pile, or a concrete filled steel pipe).

If the pile is made out of concrete and the structure is very light relative to the pile or shaft (for example a cell tower where the tower legs are significantly smaller than the shaft), then the structure is almost inconsequential provided there is access to the concrete at the top of the pile or shaft. The PIT test may be performed as usual.

If the structure is relatively massive, then the shorter the vertical dimension of the cap, the better it is for the test. One accelerometer placed concentrically above the pile may yield good results. Most likely, however, a Two Velocity Test will provide better results. The Two Velocity Test involves placing two different accelerometers along the shaft with ideally about 0.90 m (3 ft) vertical distance between them, and applying hammer blows about 0.30 m (1 ft) above the top accelerometer. This method relies on the phase shift between the two signals to determine whether the details in the signal are travelling up the shaft (from reflections of defects or pile bottom) or down from top after reflection from the pile cap. Because of the recommended vertical distance between accelerometers, this method requires that a sufficient portion of the shaft be accessible. It also requires the use of a Pile Integrity Tester with 2 channels of data acquisition (PIT-FV) and the software PIT-W Professional.

Whether the Two Velocity Method is used or not, it is helpful to use the PIT-S software to model the pile and see what signals might be expected (with accelerometer and impact locations below the pile top). A demo version of the PIT-S is available as a free download.

H piles, steel sheet pile walls, or open end steel pipes are not well suited to PIT tests. For these steel pipes testing with the LITE might be an alternative. LITE testing consists of lowering a probe into a hole bored parallel and in close proximity to the pile to be tested. The probe detects the presence of metal. Visit the LITE page for more information.

Posted: 5/26/2010

How do I know if plugging is going to occur?

During driving a soil column (or plug) is formed inside open ended piles or between flanges of H piles. If sufficient friction is developed between the soil column and the inside surface of the pile, and if the plug has become sufficiently stiff to no longer undergo significant compression, then the pile is said to be fully plugged, and it will tend to behave as if it were close ended. If these conditions are not met the pile is said to be unplugged. Partial plugging is also possible if these conditions are partially met.

Whether or not plugging can occur for open pile profiles is a complex problem which depends on many factors relating to pile, soil and even hammer properties. Only very general suggestions can be made:

In very dense sands or during restrike testing after a long waiting time, plugging may be expected in relatively small pipe piles (around 900 mm (30 inches) or less) unless the penetration into the dense bearing layer is very shallow (less than 3 diameters or so). For large piles (say 1500 mm (60 inches) or more) plugging rarely will occur during driving. There is an intermediate range where the type of soil, the depth of penetration, the accelerations generated in the pile by the hammer, the presence of pile non-uniformities, etc. can influence the plug formation.

Note also that sometimes under static load the full end bearing may be present even though the pile does not plug during driving.

For further information, please refer to our white paper about plug formation, plug resistance and internal friction in open profiles.

Posted: 6/16/2010

Can Ductile Cast Iron Piles (DYWIDAG brand) be dynamically tested with the PDA?

Yes. However, it is recommended that a drop weight be used for the test, instead of the driving system normally used to install this type of pile (a hydraulic excavator using a rapid-stroke hydraulic hammer). The drop weight used in the test should correspond to 1.5% to 2% of the specified ultimate load of the pile.

Posted: 6/16/2010

What Specific Weight, Elastic Modulus and Wave Speed should be entered into the PDA / GRLWEAP when testing / analyzing Ductile Cast Iron Piles (DYWIDAG brand) ?

The recommended parameters for the material of these piles are:

Specific weight 69.2 kN/m3 or 442 lb/ft3
Elastic Modulus: between 140 and 150 GPa or 20,000 to 24,000 ksi
Wave speed: 2,440 to 2,680 m/s or 8,000 to 8,800 ft/s

Posted: 6/16/2010

How do I take the internal friction in consideration when analyzing unplugged open ended piles?

Plugging is a phenomenon that occurs during driving, when a soil column (or plug) is formed inside open ended piles or between flanges of H piles. If sufficient friction is developed between the soil column and the inside surface of the pile, and if the plug has become sufficiently stiff to no longer undergo significant compression, then the pile is said to be fully plugged, and it will tend to behave as if it were close ended. If these conditions are not met the pile is said to be unplugged. Partial plugging is also possible if these conditions are partially met.

The total capacity of the pile is unit shaft resistance times shaft surface area plus unit end bearing times toe area. The shaft surface area is calculated based on the perimeter values that the user inputs. The perimeter used in the calculations is different for a plugged or unplugged pile.

Normally for H-piles, the perimeter is computed using only 4 sides of the pile cross-section. An argument could be made for using all 6 sides, but that is rarely done. Of course, there are situations, such as driving through soft into hard soils, where plugging may not develop, and therefore an increased perimeter could be used along the length of the pile where internal skin friction is expected to develop.

For unplugged open ended pipes, the internal friction is difficult to predict. It would be expected that an unplugged pipe pile has some inside soil resistance (the soil remains at its location, i.e., it does fill the pile and does not move with the pile – the “cookie cutter” effect). However, unless the diameter to embedment ratio is relatively large, the effective stresses will be relatively low inside the pipe and the driving process will reduce the internal friction. Thus normally for most unplugged analyses only partial internal friction is considered; it can be modeled in GRLWEAP by increasing the perimeter value over that length of the pile where internal friction is expected (e.g. on an internal driving shoe which is an increased pipe wall thickness at constant outside diameter). A case could be made for internal friction acting over 10 pile diameters if the pipe wall thickness is uniform. For further information, please refer to our white paper about plug formation, plug resistance and internal friction in open profiles.

Posted: 6/16/2010

How do I model the end bearing for unplugged open ended piles?

During driving a soil column (or plug) is formed inside open ended piles or between flanges of H piles. If sufficient friction is developed between the soil column and the inside surface of the pile, and if the plug has become sufficiently stiff to no longer undergo significant compression, then the pile is said to be fully plugged, and it will tend to behave as if it were close ended. If these conditions are not met the pile is said to be unplugged. Partial plugging is also possible if these conditions are partially met.

The total capacity of the pile is unit shaft resistance times shaft surface area plus unit end bearing times toe area. In the case of unplugged open ended piles, the annulus area should be used instead of the full toe area.

Note also that the recommended toe quake for non-displacement (unplugged) piles is 2.54 mm (0.1 inch), instead of D/60 or D/120 for displacement (close ended or plugged piles). Since the default values of GRLWEAP for the pile toe area and for the toe quake correspond to that of the closed end condition, for an unplugged open ended pile those parameters have to be corrected before running an analysis.

For further information, please refer to our white paper about plug formation, plug resistance and internal friction in open profiles.

Posted: 6/16/2010