CROSS-HOLE SONIC LOGGING (CSL) Test. Pile testing
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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.
 
Soft copy of the report will be submitted.  


Reference.


ASTM D 6760.





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