Low Strain Pile Integrity Testing
Low Strain Pile Integrity Testing
Low Strain Pile Integrity Testing
We are considered an industry standard for Low Strain Integrity Testing due to our consistent results
and performance. Geo Dynamics owns 16 sets of pile integrity testers available at various locations
across the country. We are equipped with all the current models and our specific expertise includes
testing large scale projects efficiently backed by good equipment / manpower ratio.
Our database of testing more than 20,000 piles annually helps us evaluate critical information like
necking, soil inclusion, cold joints, concrete quality in quick time. The testing conforms to ASTM
D5882. With proper site arrangements, 30 to 50 piles can be tested per day. Testing is conducted
through our offices or alongside major projects across the country. Often our team is fully equipped
to provide site reports and analysis so as to avoid delays in construction activity.
Geo Dynamics has the complete setup and expertise in use of Pulse Echo Method, Force-Velocity
and Transient Dynamic Response Approach, although the Pulse Echo Method is most widely used
and recommended.
PRINCIPLE
The method involves generation of a low stress wave with a help of a specialized hand held hammer.
The waves are then collected in acceleration form by an accelerometer and integrated to velocity for
further analysis. The accelerations generated by the impact are measured by the accelerometer
attached on the pile top and are converted to velocity form for display onto 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. Since the strains during the test are
of extremely low magnitude, the method is known as Low Strain Test method. Generally, the Pulse
Echo Method is used, although we also have the facility to use other methods.
1. The test is fast economical and reliable and gives cardiogram of pile health.
2. Helps locate pile defects & its location and ascertain concrete quality.
3. Helps to select piles for static & dynamic testing, since atleast 30 piles can be tested in a day.
4. Applicable for piles of flyovers, bridges, jetties, multistorey buildings or where pile integrity
is questionable.
5. In some cases where testing can be done from top of pile cap, it maybe possible to evaluate
integrity of existing piles used for bridges, flyovers, buildings and other structures. Geo
Dynamics has executed several such projects in India for testing from top of pile cap.
6. With some information about soil and pile, it can be used to ascertain pile length within a
range of + 5% to 10%
7. Owner / client / contractor is assured of reliable pile foundation at minimum cost and thus
avoids extensive arguments & litigations.
LIMITATIONS
1. The test cannot be used for pre-cast concrete piles beyond the first joint. Similarly it cannot
be used for steel H or pipe piles as the equipment is not calibrated for it.
2. The test cannot be generally used for pre-cast jointed piles beyond the first segment as the
wave gets reflected from the first joint.
3. Testing micro piles is difficult and most of the times not possible due to presence of high
percentage reinforcement that affect the signal.
4. The method does not generally tell about the integrity of the pile beyond the first major
defect and sometimes even a bulb.
Low Strain integrity testing is state of art and requires experience in data collection and analysis. The
practice of awarding the work at low costs to agencies without experience often leads to
questionable data and practices. This eventually leads to more confusion and unintended losses.
Hence evaluation of testing agency is important in ensuring good quality foundation and avoiding
losses at project site. Testing of 100 or more piles in one day is not physically possible many times
and also leads to questionable practices.
TESTING PROCEDURE
This testing requires the attachment of the highly sensitive accelerometer to the pile top with
viscous material. Accelerometer is connected to PIT Collector - computer with special purpose signal
conditioning and A/D converter. After hammer impact downward compressive wave is generated
traveling with wave speed "c". When this initial wave encounters a cross section change or concrete
quality change at depth x, it generates an upward traveling wave which is observed at the pile top at
a time equal to twice the distance of the cross section change from the top divided by the wave
speed c (2x/c). The rest of the initial wave travels down to the pile toe and reflects. It is observed at
the pile toe at time twice the pile length divided by wave speed (2L/C, L= pile length). Reductions in
pile cross and concrete quality section generate tension upward traveling waves, while soil
resistance and bigger pile cross section generates compression upward traveling waves. From the
known pile length and material the expected arrival time of pile toe reflect could be calculated. If the
positive reflection is observed prior to the expected toe reflect time, the approximate defect
location can be estimated from the assumed wave speed. The size of the defect can be estimated
from the magnitude of the early reflection. Pile top velocity (integration of measured acceleration)
for each hammer impact is recorded. It is possible to perform fast Fourier analyze on time domain
record to obtain frequency domain response, on which it is also possible to interpret pile cross
section changes and pile concrete quality. With PIT measurements it is possible to perform pile
integrity control on bored and nearly all cast in place piles and driven concrete piles. Piles with
greatly varying cross-sectional areas may make it difficult to distinguish between reflections from
significant discontinuities and those caused by the construction method. However, if large numbers
of piles are tested, it is generally easy to spot the piles heaving unusual responses. Suspected piles
can be subjected to further tests as wave analysis, borings etc.
Limitations: L/D ratio <=30 (L=pile length, D = pile diameter) Cracks it is impossible to detect pile
under the crack
Pile preparation before test: All bad concrete from pile head should be removed, pile head should
be clean and without cracks for successive test.
MEASUREMENTS AND ANALYSIS RESULTS
Method and testing procedure is standardized with ASTM D-5882-00 Standard test Method
Pile Integrity results are presented on table and graph. Table data for each pile:
Pile Name
Date Collected
Pile Lengthas reported on site
For each tested pile v (t) graph is presented. Below is basic v (t) graph explanation:
Wikipedia