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Low Strain Pile Integrity Test
Pile Integrity Testing (PIT) is a Non-Destructive integrity test method for foundation piles. It is a “Low Strain” Method (since it requires the impact of only a small hand-held hammer). It is done on individual pile to evaluate the quality of pile shaft, its continuity, integrity and qualitative dimension of pile. This is the simple, quick and inexpensive method, which is the primary reason why it has been accepted worldwide. The test is also referred as low strain dynamic test generally, because a small hammer is used to generate a short wave of appreciable acceleration at low strain level. Because of its economy all piles on a site can be tested and defective piles are identified for corrective measures.The experienced test engineer can choose any of the methods from several modes of the basic low-strain test:
1. Pulse Echo Method PEM– the simplest method, merely requiring that pile top velocity records display as a function of time.
2. Transient Response Method TRM– displays the mobility, i.e., pile top velocity divided by force as a function of frequency.
3. A combination of test methods PEM and TRM which displays the velocity in both the time and frequency domain.
4. The profile analysis, determines an approximate shape of the test pile based on the pile top velocity curve.
various countries including Singapore & South East Asia, the U.K., the U.S.A., Canada, Germany, Sweden, Australia, China, etc., and recently Bangladesh use this system of testing to evaluate pile integrity widely.
The testing is standardized by ASTM under designation ASTM D 5882 with following terminology:1. Qualitative evaluation of the physical dimension.
2. Continuity of pile.
3. Consistency of pile material.
Background
When we stuck a pre-cast, driven, or drilled shaft or pile with a small hammer, a stress wave will generate which travels down the shaft to the pile bottom where it’ll reflect. So, when the reflected stress wave returns to the pile top, a measurable pile top motion occurs. However, If the reflection wave occurs at the correct time and if we don’t receive any other earlier reflection at the pile top, then the pile shaft is most probably free of major defects.
When a lightweight handheld hammer strikes the pile top, a small pile top motion acceleration is generated and measured by an accelerometer. So upon integration with respect to time velocity is computed. Furthermore, the associated pile strains are of such a low magnitude that they would be measured in the pile only with great difficulty.
This test is therefore also known as the low-strain integrity test. However, the force applied by the hammer can easily be measured by incrementing the hammer itself. Basically, the velocity record and to a lesser degree the force record contains information about the location and magnitude of pile non-uniformities.
So, under the assumption of proportional force and velocity records, and for short-duration impact pulses, the velocity record may be sufficient to judge the quality of a shaft.
Stress Wave Propagation for Low Strain Pile Integrity Test
Une-dimensional wave propagation applies to a linear elastic pile, which is long compared to its diameter. Moreover, an impact applied to the pile top generates a momentary compression and a particle motion of the pile top surface. So, the compression is related to the force, F: the motion causes a particle velocity, v. Basically, in concrete, the stress wave travels along the pile with a compressive stress wave speed, c, ranging from 3000 to 4500 MLS. The wave speed depends only on the pile material properties:
where,
c- is the wave speed
E- is the elastic modulus of pile material
p- is the mass density of pile material
Testing Instrument PEM for Low Strain Pile Integrity Test
A list of hardware and software used in the PEM method is given below:
1. Data Collector, Model-V, PDI USA
2. Accelerometer
3. Hammer
4. PCMCIA mass storage memory card
5. USB PCCIA memory card reader
6. Personal Computer Pentium III with operating system Windows XP
7. PIT-W (Basic) Software
Testing Instrument TRM for Low Strain Pile Integrity Test
A list of hardware and software used in the TRM method is given below:
1. Data Collector, TDR-2 of UK
2. Geophone
3. Instrumented Hammer
4. Personal Computer Pentium III with operating system Windows XP
5. PIT-W (Basic) Software.
The system measures both input force to a pile as well as the resulting velocity response and this allows the pile shaft to be examined in both x and y directions i.e. depth and cross-section. The technique also means that data can be viewed in the time domain and frequency domain, this is when interpreting the results. When a pile top is struck with a hammer a longitudinal wave travels down the shaft.
When the wave reaches the base of the pile it is reflected back up to the top. By assuming a wave velocity, the length of the pile is calculated. Reflections can also be obtained from acoustic anomalies within the pile shaft. The concrete density or conversely the cross-sectional area of the pile can be estimated from the mean height N of the resonating part of the curve using the following equation.
Mobility, M = 1/pCA
Where,
p – Density of concrete
C – Velocity of longitudinal weaves in concrete
A – Cross-sectional area of the pile
The mobility traces were then investigated for any reflection from increases or decreases in pile cross-section. However, in the majority of piles elastic modulus and wave speed are constant.
Pile Head Preparation for Low Strain Pile Integrity Test
For cast-in-place piles, integrity testing shall not be performed until the concrete has cured for a minimum of seven (7) days unless otherwise approved by the engineer. In order to obtain the very best data possible when testing a pile, it is essential that the pile head is prepared properly prior to testing. Without good data, any interpretation carried out will be meaningless. It is essential that the measurement transducers are mounted in the correct position and on sound concrete. The essentials of pile head preparation for integrity testing are given below:
-Piles should if possible be tested at the cut-off level and trimmed to sound concrete. Any weak, broken concrete that sounds hollow should be removed and the pile top left roughly horizontal over the complete cross-section.
-Reinforcing bars should be bent slightly away if practicable and the helical removed to allow for a good swing of the test hammer.
-Two areas should be prepared for the transducers, one for the hammer in the center of the pile and the other for the geophone close to the pile perimeter. The areas should be approximately 100 mm in diameter and prepared as flat and level as possible using a scabbier, cutch hammers, or even a hammer and chisel, then brushed free of debris. See the diagram below:
Testing Procedure
The testing procedure involves the attachment of a geophone/accelerometer onto the pile top with the help of bonding material like Petra wax. After attachment, the pile head is impacted with a hand-held device (a hand-held hammer). This generates a low-stress wave into the pile. The resultant strains are of extremely low magnitude, and the method is hence known as Low Strain Method.
The wave generated by the impact is measured by the geophone/accelerometer attached to the pile top and is converted to velocity form for display on the collector screen. Reflections from either pile toe/pile discontinuities, cross-sectional changes, soil resistance changes, the wave speed through the pile, etc., are graphically displayed.
The test involves the collection of several blows during the stage of testing. All such similar blows are averaged before display. This averaging technique tends to cancel random signals in any particular blow while amplifying the effects of relevant repetitive responses. The signals are also exponentially amplified. Low strain signals generated due to hammer impact are often damped by skin friction. For long piles with high skin friction, reflections from the pile toe may be small. Amplifying the records exponentially with time will enhance the identification of relevant reflections that have low energy. Windows-based PC with respective software is then used for data analysis and reporting.
RESULT PRESENTATION
The testing engineer shall present a report within 5 working days after performing the field test to provide the final test results and integrity evaluation. For each pile tested, the averaged, amplified velocity versus time record shall be included in the report, with a table summarizing results and conclusions.
ACCEPTANCE AND REJECTION
Shafts with no significant reflections from locations above the pile toe and with a clear pile toe reflection may be accepted. Where no clear toe reflection is apparent, the experienced test engineer shall state to which shaft depth the test appears to be conclusive. Where reflections from locations with significant reductions in pile area or pile material strength or stiffness above the pile toe are observed, the pile has a serious defect.
If the record is complex, the results may be deemed inconclusive. Construction records (concrete usage, grout pressure records, soil borings) may be valuable in result interpretations or additional numerical analysis modeling may be used to quantify the record. The decision to reject and replace, or repair, any defective shaft is the sole responsibility of the engineer-of-record for the foundation.
REMEDIAL ACTION
Rejected or questionable piles may be replaced. Questionable piles may also be subjected to further testing, e.g., static load testing, dynamic load testing, core drilling, ultra-sonic logging, etc. Remedial action may include pressure grouting through core holes. If the pile top appears questionable, further pile top cut-off and retesting may be advisable. If a majority of piles diagnose as “inconclusive”, partial or even complete pile excavation or another test method may be necessary for pile acceptance.
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