1. Acoustic tube burying technology
(1) D≤1500mm, three acoustic measuring tubes are buried in an equilateral triangle arrangement; D>1500mm, four acoustic measuring tubes are buried in a square arrangement.
(2) The metal tube is used, the inner diameter is 15mm larger than the outer diameter of the transducer, and the threaded connection is adopted to prevent water leakage.
(3) Firmly welded or tied to the inner side of the steel cage, parallel to each other, accurate positioning, buried to the bottom of the pile, and the nozzle is more than 300mm above the top of the pile. The bottom of the tube is closed and the mouth of the tube is capped.
(4) With the apex of the forward direction of the route as the starting point, number and group in a clockwise direction.
2. Preparation before testing
(1) Collect engineering geological data, foundation pile design drawings and construction records, supervision logs, etc. to understand the construction process and abnormal conditions during the construction process;
(2) Develop a testing plan;
(3) Concrete age is greater than 14d;
(4) The sound measuring tube is filled with clear water (attenuation, delay) to ensure smooth flow;
(5) Calibrate the system delay time from transmission to reception of the acoustic wave instrument;
(6) Measure the distance between the inner and outer diameter of the acoustic measuring tube and the outer wall of the adjacent acoustic measuring tube, ±1mm;
3. Detection method
The detection method is shown in Figure 2. The distance between the measuring points should not be greater than 250mm; the transmitter and receiver transducers rise and fall synchronously at the same elevation, and the cumulative error should not be greater than 20mm, which can be corrected at any time; during the detection process of the same pile, the transmitter voltage remains unchanged (comparability). The abnormal parts are carefully measured by horizontal encryption, equal difference synchronization or fan-shaped scanning, combined with the corresponding waveform analysis of the test to determine the severity of the pile concrete and the location of the defects. When laying the receiving and transmitting transducers, the horizontal angle of the connection should be between 30-40.
4. Judgment basis for completeness
(1) Judgment basis for sound speed
The critical value of sound velocity is determined based on the probability method, that is, the measured value of the sound velocity of the pile foundation without defects has a certain degree of dispersion due to its own inhomogeneity, but conforms to the normal distribution; the abnormal low sound velocity caused by the defect does not conform to the normal distributed. Therefore, the average value and standard deviation of the normal concrete sound velocity must be used when determining the critical value, otherwise the calculated average sound velocity will be too small, which will easily cause missed judgment. At the same time, the influence of the error caused by the non-parallelism between the acoustic measuring tubes should also be considered.
The speed of sound is one of the basic physical quantities of the material and is related to the strength of concrete. When the numerical value of the sound speed of the corresponding measuring point of the detection plane is very small and generally low, it can be judged by the method of low sound speed limit. The low sound velocity limit is determined by comparing the sound velocity and compressive strength of concrete test blocks with the same conditions, and then combining the actual experience in the region. The concrete strength corresponding to the low limit of sound velocity should not be lower than 0.9R (design strength). If the test piece is a drilled core sample, it should not be lower than 0.85R.
(2) Basis for volatility judgment
The magnitude of the critical value of the amplitude is reduced by 6dB from the average value of the amplitude. When the actual measured amplitude is found to be less than the critical value of the amplitude, the detection section should be regarded as a suspicious defect area. The wave amplitude is obtained by relative testing. Because of the large variability of the internal structure of the pile body concrete, it is difficult to find a better statistical regularity of the wave amplitude.
(3) PSD judgment basis
The judgment basis of the PSD method is that the slope of the sound wave depth curve will increase significantly when the sound wave passes through the defect, and the degree of the defect is closely related to the size of the sound jet lag. The slope method is often used as an auxiliary abnormality criterion. When an abnormal change in the PSD value is found near a certain measuring point, it should be regarded as a suspicious defect area and further tested.
5. Inspection precautions
When the speed of sound, amplitude and PSD value exceed the critical abnormality or sudden change, the defect should be tested carefully. At the same time, a comprehensive analysis is carried out in combination with relevant data such as waveforms, construction technology and construction records to determine the location and extent of the pile body concrete defects. When the sound velocity is generally lower than the low limit, the concrete strength of the foundation pile should be tested by the drilling core method. Since ultrasonic waves can only detect the concrete quality of the pile body, for supporting piles or rock-socketed piles, the low-strain reflected wave method should also be used to detect the supporting conditions of the pile ends to ensure that the bearing capacity of the pile meets the design requirements.
Using ultrasonic to detect foundation piles can more accurately and reliably evaluate the integrity and bearing capacity of foundation piles. When necessary, check the bearing capacity of foundation piles with hidden safety hazards, and carry out corresponding reinforcement treatment for foundation piles that cannot meet the requirements of various bearing capacity to avoid safety accidents and unnecessary losses. 
