CROSS-HOLE SONIC LOGGING (CSL) Test.
This article describes the procedure of Cross-Hole Sonic Logging (CSL) as method for assessment of the homogeneity and integrity of concrete (quality of concrete) in a deep foundation.
PROCEDURE.
Test to be carried out not less than 7 days from pile age.
Cross-hole Sonic Logging (CSL) techniques have been designed to assess the
quality of piles, drilled shafts, and slurry trench walls after installation,
without destroying what has been built and without development of more efficient
and reliable constructed systems.
CSL provides the engineer with an inexpensive
method of testing concrete and of locating and assessing the extent of any
irregularity. This method is most applicable when performed between tubes that
are installed during concreting by the pile contractor.
CSL method for testing deep foundations was first developed
in France by the Experimental Center for Research and Studies in Building and
Public Works (CEBTP) in the late 60's. The basic principle of the test is that
the velocity of an ultrasonic pulse through concrete is inversely proportional to the density of the concrete.
By measuring the travel times of a pulse along a known distance, a measure of
the density, and, hence, quality of concrete can be determined. This method
does not give the exact type of defect, but rather only that a defect
exists.
Cross-hole sonic logging basics.
The cross-hole sonic logging (CSL) method is an ultrasonic
test that involves measuring the propagation time of ultrasonic signals between
two probes in vertical holes in a shaft. This test measures the apparent time
between probes.
The apparent velocity incorporates all the concrete conditions
between the probes including the concrete, water and tubes. Non uniformity such as contamination, soft concrete, and honeycombing, voids, or inclusions
exhibit delayed arrival time with reduced signal strength.
Cross-hole sonic logging is a derivative of the ultra-sonic
pulse velocity (UPV) test. The propagation velocity, VP, is a function of
Poisson's ratio (n), the density (r), and the elastic modulus of
the material (E).
vp = √ E (1-n)/ [r (l+n) (1-2n)] Access and Testing Equipment.
Access tubes.
The total number of installed access ducts in
the pile shall be closer consistent with good coverage of cross section.
The
recommended access tubes are nominal 50 mm (1.5 to 2.0 inch) inside diameter are
attached to the reinforcement cage of the pile distributed at sensibly constant
spacing. As guide, the number of access
ducts is of often selected as one duct for every 0.25 to 0.3 m of pile diameter, with a minimum of three access
ducts, spaced equally around the circumference and filled with water prior to the concrete
placement.
The water acts as a coupling medium between the transducer and the
access tube. Tubes should be free from corrosion with clean internal and external
faces to ensure a good bond between the concrete and the tubes. Tubes should be
installed by the Contractor such that the CSL probes will pass through the
entire length of the tube without binding.
The test equipment comprises:
Impulse generator.
Two piezo-electric probes having an outside
diameter of 25 mm. Two piezoelectric
probes shall be placed in the tubes, one to emit
the ultrasonic pulse and the other to
record the pulse.
These probes consist of
ceramic transducers in sealed tubes. Ceramic
transducers are chosen because of their
mechanical impedance close to that of concrete.
The transmitter shall be robust, to send
acoustic signals as clear as possible, and to have
high power at low voltage to prevent
interference from the cables.
The receiver is capable of amplifying the sound waves focused by the
concrete-water interface at a point located just behind the center of the tube. Probes shall
allow a generated or detected pulse within 100 mm of the bottom of access tubes.
The Encoder is linked via a depth-related
voltage control to the data acquisition system so
that the depth of the probes can be recorded.
The depth measuring device shall be
accurate to within 1% of the access duct length,
or 0.25 m whichever is larger.
Testing procedures.
Test sequence.
Each pair of tubes shall be sounded in turn, and
the results for each sounding shall be recorded, analyzed, and reported. Therefore, three tube systems will have three
paths (profiles), four tube systems will have six paths as illustrated in below
and five tube systems will have ten paths.
Preparation.
The Sonic Logging Test shall be performed not
less than 7 days after casting depending on concrete strength and shaft
diameter (larger diameter shafts may take closer to 7 days) unless agreed
earlier.
The access tubes shall be filled with water
prior to test, in order to provide an acoustic coupling between the probes and
the pile concrete.
Testing.
The source and receiver probes are lowered to
the bottom of their respective tubes and placed such that they are in the same
horizontal plane.
The emitter transducer generates a sonic pulse (on
the order of 10 pulses per second), which is detected by the receiver in the
adjacent tube. The two transducers are
simultaneously raised at a rate of about 300 mm/sec (1 ft/sec) until they reach
the top of the drilled shaft.
Typically, this process is repeated for each
possible tube pair combination (perimeter and diagonals). Thus, the
measurements are taken at approximately every 5 cm of pile depth or as per
project requirement.
As the probes are lifted, therefore, a vertical
picture or ‘sonic profile’ of the zone of concrete between the pair of probes
can be built up.
Data Capture and Processing.
The main measurement of cross-hole sonic logging
techniques is the transit time of the signal from emitter to receiver. In general variations in arrival time of
+/-20% are within the normal range for the Cross-Hole Logger technique, due to
the variation in probe location within the pipes, pipe alignment and other
factors. Defect, if present would be indicated by an increase in arrival time
(greater than 20% of the average arrival time) and a corresponding decrease in
relative signal energy.
The variation in signal arrival time enables one
to assess and locate areas of low density, and so damaged concrete. Concrete
containing soil inclusions, gravel, and bentonite or honeycombing has a much
lower propagation velocity so that the presence of these irregularities is
immediately obvious. The amplitude and the sinusoidal shape of the signal will
change as well if there is anything but sound concrete along the path of the
wave path.
Signal attenuation, or loss
of energy, is sign of poor-quality zones because more energy is transmitted
through sound concrete than through poor concrete. The test is more suitable
for integrity testing of deep drilled shafts because signal attenuation
problems do not occur as with the surface methods, and the travel distance
between the two probes at any depth is relatively short and essentially
constant.
Interpretation.
The Results of cross hole sonic test are presented for each
tested pile in standard format. The FAT, Energy and Waterfall diagrams are
presented for each direction (profile). A brief description and the
significance of each graph are given below:
First Arrival
Time
The first arrival time from transmitter to receiver verses
depth is the most significant parameter measured by cross hole sonic test.
Shaft integrity may be identified by a consistent wave arrival travel time
between access tubes. In general variations in arrival time of +/-20% are
within the normal range for the Cross-Hole Logger technique, due to the
variation in probe location within the pipes, pipe alignment and other minor
factors.
A defect, if present would be indicated by an increase in arrival
(usually greater than 20% of the average arrival time) and a corresponding
decrease in relative signal energy of wave speed.
Energy
Energy is calculated by integrating the absolute value of
the acquired data over a user –specified interval. Energy is therefore
displayed in “relative” terms on log scale (low values to right and high values
to left), along X-axis and depth along Y-axis. High energy corresponds to low
FAT and therefore indicates good integrity.
Sonic Map (or Waterfall Diagram).
A sonic map is a three-dimensional display of acquired data (center main graph). It is representing a “nesting” of the individual data signals from all depths. The vertical axis is the depth of the pile.The horizontal axis is time. The third dimension represents “signal strength” detected by the receiver. High signal strength corresponds to low FAT and hence indicates sound and integral concrete.
Severity and significance of anomalies shall be assessed carefully. Factors such as: deviation from average FAT, Depth range of existence of the anomaly, existence of the anomaly in more than one direction (profile) at same depth range, etc. shall be considered.
Frequency.
Procedure and frequency of the cross hole sonic logging
testing shall be as per project specification and structural drawings.
Reporting.
The report shall include the following information as a
minimum:
Identification of site and locations, date of testing.
Identification of the piles tested and their type. Pile reference numbers should be related
to
specifically identified contract drawings.
Cross-hole Sonic Logging test results related to each
individual pile under test.
A statement
mentioning whether the tested piles exhibit any structural problem and the
exact location of defects, if any.
The method of test adopted; equipment used.
Length of piles & Logging depth.
Date of test.
Description of the testing apparatus units and probes.
Identification of test staff.
Any failure of the probes to penetrate the full depth of
access tubes.