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.