-Low Strain Integrity Testing of Piles
- Scope
This test method covers the procedure for determining the integrity of individual vertical or inclined piles by measuring and analyzing the velocity response of the pile induced by an (hand held hammer or other similar type ) impact device applied axially to the pile normally at the pile head . This method is applicable to long structure elements that function in a manner similar to foundation pile , regardless of their method of installation provided that they are receptive to low strain impact testing.
- Significance and use
Low strain integrity testing provides velocity and force data on structure elements ( that is , structural columns , driven concrete piles , cast in place concrete piles , concrete filled steel pipe piles , timber piles , etc). This data assists evaluation of pile integrity and pile physical dimensions ( that is, cross-sectional area, length) , continuity , and consistency of the pile material. This method will not give information regarding the pile bearing capacity.
- Method of testing
* Pulse Echo Method ( PEM )
The pile head measured as a function of time . The time domain record is then evaluated for pile integrity
* Transient Response Method ( TRM)
The pile head motion and force ( measured with an instrumented hammer ) are measured as a function of time . The data are evaluated usually in the frequency domain .
We will explain here the PEM test .
- Apparatus
* Apparatus for applying impact
The impact may be delivered by any device ( for example , hand held hammer ) that will produce an impact force pulse of generally less than 1 ms duration and should not cause any local pile damage due to the impact . A hammer with a very hard plastic tip can induce a short impact force pulse without causing local pile damage . The impact should be applied axially to the pile ( normally on the pile head ).
* Apparatus for obtaining measurements
Obtain velocity data with one accelerometer , provided the accelerometer signal can be integrated to velocity in the apparatus for reducing data. The accelerometer should be placed at ( or near) he pile head and shall have its sensitive axis parallel with the pile axis .
* Signal transmission
The signal from the sensor shall be converted to the apparatus for recording , reducing and displaying the data as a function of time. The apparatus included graphic display of velocity and force and a data storage capability for retrieving records for further analysis. This apparatus is capable of averaging data of several blows to reinforce the repetitive information from soil and pipe effects while reducing noise effects . Apparatus is also able to apply increasing intensity amplification of the motion signal with time after the impact to enhance the interpretation of the measured motions that are reduced by soil and pile material damping . The apparatus have filtering capability with variable frequency limits for eliminating high frequency , or low frequency signal components , or both. The apparatus allow for a permanent graphical output of the record.
- Test procedure
* Preparation of the test pile
Integrity test shall be carried out once piles have been chipped to their final level of working or at the top of clean concrete.
The test shall be carried out only after a minimum of 7 days of the cast of concrete .
The pile head shall be clean , free from water , loose concrete , laitance and readily accessible for the propose of testing.
Protruding helical reinforcement is normally cut or bent away to provide access to the center of the pile. However, uncontrolled bending may weaken the bars and the designer should always be consulted before bars are bent.
The presence of an attachment to the pile head , such as thick blinding concrete ( more than 75 mm thick ) or cast grout beams should be avoided as these will complicate the signal.
Presence of reinforcement for grout beams or slabs will cause complications in either the undertaking or interpretation of the test, and should be avoided .
* Attaching the sensor
Attach the accelerometer with a small amount of Pietoplast to the top of pile , ensuring that the sensor is vertical. Choose a relatively flat , off-center spot and press with hand weight . The Pietoplast helps transfer the signal and damp the resonance from pile to sensor.
If the pile top is not easily accessible , The accelerometer can also be attached to the pile shaft
* Application of blow
A sharp blow from a height of about 150 mm is applied to the top of the pile in an axial direction and a distance no longer than 300 mm from the accelerometer , The hammer should be held lightly and allowed to rebound freely from the pile top.
* Taking measurements
Apply several impacts and record each individual impact or the average , if required, or both.
If only the individual impacts are recorded , ensure that the apparatus for recording , reducing and displaying data is averaging individual records .
Record the number of impacts for a specific averaged record . Take record and display a series of velocity and force measurement .
* Data quality check
For confirmations of data quality, the operator shall monitor the velocity and force from several impact events for consistency .
Consistent records are the result of uniform impacts on sound concrete , motion sensors that are firmly attached and the apparatus for recording , reducing and displaying data properly functioning.
If records are not repeatable , don't use the data.
- Interpretation of the results
The interpretation of the result shall be carried out by experienced engineer.
With Pulse Echo Technique measured pile head velocity is analyzed as a function of time.
The displayed data should include the velocity records with time, an indication of the pile length and assumed wave speed and a representation of the shape and magnitude of the amplification function when used .
pile integrity is based on relative increase and decrease of velocity after the initial impact pulse.
A clearly indicated signal together with a fairly velocity trace between the impact and toe signal are signs of sound pile.
Strong velocity variations may be the result of changes in pile top velocity may be the result of either a cross section, concrete quality or soil resistance. For example, relative increase in pile top velocity may be the result of either a cross sectional decrease or soft soil layer.
In the absence of soil resistance changes , pile top variations are caused by pile impedance changes . the pile impedance (z) is define as
Z = C*P*A =A √ E*P
Where
A= being the pile's cross sectional area.
C= the velocity of propagation of the stress wave through the pile. Thus ,an impedance reduction can cause by a decrease either area (A) , or in the concrete's modulus of elasticity (E) , or in density (P).
Since both modulus and density are related to concrete strength , it is fair to day that impedance depends on cross sectional area and concrete quality.
The calculation of the acoustic length of the pile ( L) on the depth to other intermediate features on the signal response curve is directly depended upon the value of propagation velocity (C) as per following equation:
L = C*T/ 2
Where
T = the time lapse between the first hammer impulse from the remote end of the pile.
C = is sound concrete is typically quoted as varying between 3700 and 4300 M/S . Often value of 4000 m/s is used for analysis.
Variation of the actual propagation velocity from that used in calculated pile length from the signal- response curve could generally be expected to be within 10% of the actual value.
- Test requirements
The report shall include the following information
* Identification of site and locations , date of testing.
* Identification of the piles testes and their type, pile reference number should be related to the drawings.
* Integrity test result related to individual pile under test including signal response curve.
* A statement mentioning whether the testes pile exhibit any structural problem / the approximate location of defects , if any.
* The method of test adopt and equipment used.
* All measured test results with graphical output as appropriate.
* Measurement of dynamic stiffness with derivation value, as appropriate.
*Detection of abrupt changes in pile cross section and in concrete length .
In this article, we reviewed one type of concrete pile tests, which is Low Strain Integrity Testing of Piles and in next article we will explain another type of pile concrete test.
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