High Strain Dynamic Pile Testing. Test procedure, method statement template.
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High Strain Dynamic Pile Testing. Test procedure, method statement template.


1- Scope                                                                                                                                                                      
2- Significant and Use                                                                                                                            
3- Summary of the Test Method                                                                                                                           
4- Apparatus                                                                                                                                            
5- Test Personnel                                                                                                                                    
6- Test Procedure                                                                                                                       
7- Interpretation of Test Results                                                                                                                         
8- Test Report Requirements                                                                                                              
9- Reference 



1. SCOPE  


This test method covers the procedure for High Strain Dynamic testing of Piles to determine the  force and velocity response of the pile to an impact force applied axially by a pile driving hammer  that will cause a large strain impact to the top of the pile.    



2. SIGNIFICANCE AND USE  


This test method is used to provide data on strain or force and acceleration, velocity, or displacement of a pile under impact force. The data are used to estimate the bearing capacity and the integrity of the pile, as well as pile stresses and soil dynamics characteristics.  



3. SUMMARY OF TEST METHOD  



Dynamic testing involves attaching two strain transducers and two accelerometers to the pile approximately 1.5 pile diameters below the pile head during initial driving or at a convenient location during re-strike testing. An impact hammer or a heavy, guided block is dropped onto the specially prepared pile head. The generated compression wave travels down the pile and reflects from the pile toe upward. The waves, which are picked up by the sensors, are processed and automatically stored in the field by the computer. The data can be easily retrieved for further review, graphical presentations, or reporting.  



4. APPARATUS  


The testing firm must supply the following testing equipment and instrumentation:  

1. A Steel shaft top cushion to be applied on top of the pile to be tested. 

2. A driving system consisting of a drop weight suitable of generating a net measurable pile 
penetration or an estimated mobilized static resistance in the bearing strata which exceeds 
to a sufficient degree of the working load assigned to the pile as judged by the testing 
Engineer. 

3. Hammer dropping device provided generally by the Contractor. Usually a mechanical crane 
of sufficient capacity to carry the rammer system as per the testing Engineer instructions.  It 
will raise the hammer and drop freely and strike the pile. 

4. Pile Driving Analyzer (PDA manufactured by Pile Dynamics, Inc. Model PAL / PAK) 
installed with the necessary software for signal transmission, recording, displaying and 
recording of the measured data.  



5. Strain transducers capable of independently measuring strain versus time at a specific 

location along the pile axis during the impact event. 


6. Acceleration transducers capable of measuring acceleration versus time which can be 

integrated to velocity.  These accelerometers shall be handled carefully and calibrated.  





5. TEST PERSONNEL  


The field testing is performed by an experienced technician with at least 5-year experience in pile testing. 

The interpretation of records is performed by the Engineer having 10 years’ experience in pile testing interpretation.   




6. TEST PROCEDURE  


6.1 General 


Applicable project information shall be reviewed and recorded prior to test, including soil boring logs, pile installation records and concrete properties.   



6.2   Data Input   


1. Make a new page which creates a new file for the pile.  

2. Enter data for project, pile name, Area, Length (Length below sensors to pile bottom), 
Damping Parameter (JC), Sampling frequency, Specific Weight and Wave Speed. 

3. Enter transducer calibrations. 

4. Check the Offset for strain transducer balance (-5 v to +4 v acceptable) before and after 
installation of the strain gauges.  

5. To acquire data, put PDA in ACCEPT mode. After test, put PDA in STANDBY mode and 
remove transducers.  



6.3   Preparation of the Test Pile and Attaching the 


Transducers.  


Two sensors for strain and two accelerometers are connected to the side of the pile at 1.5 pile diameters below the top, opposite to each other by mounting blocks with threaded holes.   
                        
Prepare pile by drilling holes about 1.5 to 2 pile diameters below top (2 holes per strain transducer and 1 hole per accelerometer).
 
Gages shall be attached by the testing Engineer to the pile in a secure manner as to prevent slippage under impact. The transducers shall be placed, diametrically opposite and on equal radial distances, at the same axial distance from the bottom of the pile so that the measurements compensate for bending of the pile. The typical arrangements are shown below.  




6.4   Application of Blow.


Position the hammer in such a manner that the pile top and hammer bottom are parallel during  impact  so that the force is applied axially and concentrically with the pile and to make as uniform  contact stress as possible during the test. Set up the apparatus for recording, reducing, and displaying data so that it is operational, and the force and velocity signals are zeroed. 
 
First drop height should be minimal to allow the testing Engineer to assess the testing equipment, the driving system and pile stresses.  Subsequent impacts can then be applied by utilizing higher drop heights. 

 6.5   Analysis of Measurements.

6.5.1 The total travel time of the wave can be calculated by the formula 
          Time t = 2 L / C 
          Where L – Length of the Pile                       C – Wave speed 

For piles with variable cross section reflections occur at the interfaces of the segments with different properties i.e. reflections occur before 2 L / C. Also, resistance forces and the pile end cause reflection waves which travel upwards. In the analysis propagation of both upwards and downwards traveling waves is tracked.

6.5.2 Obtain force and velocity from the display apparatus. Record the impact force and velocity and the maximum and minimum forces for the selected representative blows. Obtain the maximum acceleration directly from the accelerometer signal or by differentiation of the velocity versus time record. Obtain the displacement from the pile driving record by integration of the velocity versus time record. Obtain the maximum energy transferred to the location of the transducers.

6.5.3 The recorded data is subjected to analysis in a computer. The results of the analysis include an assessment of integrity of the pile, and the maximum dynamic driving stresses. The results may also be used for evaluation of static soil resistance and its distribution on the pile at the time of the testing. Such further use of the data is a matter of proper engineering judgment.  
    

7. INTERPRETATION OF TEST RESULTS.

Dynamic Pile Monitoring with the Pile Driving Analyzer (PDA) and Dynamic Load Testing with the PDA and CAPWAP (Case Pile Wave Analysis Program) are both High Strain Dynamic Testing procedures. The pile driving hammers or drop weights used to perform these tests cause high strains in the piles.

The Pile Driving Analyzer testing analyzers use wave propagation theory to compute numerous variables that fully describe the condition of the hammer-pile-soil system in real time, following each hammer impact. This approach allows the verification pile capacity.  

  


  



The force is computed by multiplying the measured signals from a pair of strain transducers attached near the top of the pile by the pile area and modules. The velocity measurement is obtained by integrating signals from a pair of accelerometers also attached near the top of the pile. Strain transducers and accelerometers are connected to the Pile Driving Analyzer that internally performs all the necessary signal conditioning and processing to obtain output results.  


Soil resistance computed by the PDA includes both static and viscous components. The static component is a function of a soil parameter called the Damping Factor, which is related to soil grain size. The damping factor is an input to the PDA.  


Another technique that evolved from Smith’s approach of modeling the wave propagation theory of  pile driving is the Case Pile Wave Analysis Program (CAPWAP) which is a numerical analysis  procedure to solve for soil resistance parameters from the derived force and velocity. The CAPWAP results are based on the “best possible match” between the computed and the measured wave up curves. To compute forces (or velocities or wave values) at the pile top, pile and soil are modeled mathematically and an algorithm that allows for a step by step computation of all pile variables along the pile length is followed. The pile is divided into number of segments of uniform cross section with approximately 1m length and soil resistances are assigned to every embedded pile segment to model the shaft friction and the end bearing.  


In the CAPWAP the soil model consists of an elastic - plastic spring and a linear dashpot. The extended CAPWAP soil resistance model is shown in below figure.  



CAPWAP combines field measurements (obtained with the PDA) and wave-equation type  analytical procedures to predict soil behavior including static-load capacity, soil resistance  distribution, soil damping and quake values, pile load versus movement plots, and pile soil load  transfer characteristics. The employment of PDA field measurements in conjunction with CAPWAP analysis defines what is known as Dynamic Load Testing and estimates.  







8. TEST REPORT REQUIREMENTS.   





  • The report shall include the following information as a minimum will be submitted within 5 days of testing. 
  •  Project identification/location, and 
  • Identification (name and designation) of test pile(s), 
  • Working load and safety factor (or required ultimate capacity) of the pile(s), 
  • Type and dimensions of pile(s) including nominal or actual cross-sectional area, or both, 
  • length and diameter (as a function of pile length for timber of composite piles), 
  •  Description of all components of the apparatus for obtaining dynamic measurements  
  •  Date tested   
  •  Test pile identification, 
  • The length below sensors, cross sectional area, density, wave speed, and dynamic modulus 
  • of elasticity of the test pile, 
  •  Penetration resistance (number of blows per unit penetration) during the test, 
  •  Graphical presentation of velocity and force measurements in the time domain for 
  • representative blow of each pile tested. 
  • Method(s) and one-dimensional wave propagation theory used (give reference) to evaluate 
  • data (particularly for the capacity evaluation, if applicable), 
  • Comments on the capacity of the pile at the time of testing; mention shall be made as to if 
  • capacity is of remolded state as at end of driving or from a restrike with sufficient wait after 
  • driving. When applicable, summarize variables describing the soil model, including 
  • damping factors, quakes, and resistance distribution, 
  • Comments on the driving stresses in the pile, 
  •  Comments on the integrity of the pile  



9. REFERENCES.  





  • • ASTM D 4945-12 - Standard Test Method for High Strain Dynamic Testing of Piles.  • AASHTO T298-93 - High Strain Dynamic Testing of Piles 
  • • PDA User’s Manual.  









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