Application of ultrasonic method in bridge pile foundation detection
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Application of ultrasonic method in bridge pile foundation detection
With the rapid development of infrastructure construction in my country, pile foundation has become the most commonly used foundation form for bridge projects. Because the quality of pile formation is affected by many factors such as geological conditions, pile formation technology, mechanical equipment, construction personnel, and management level, it is more likely to cause mud inclusions, fractures, necking, concrete segregation, thick sediment at the bottom of the pile, and concrete at the top of the pile. Quality defects such as poor density endanger the normal use and safety of the main structure and even cause engineering quality accidents. Therefore, how to determine the location of the defect and accurately evaluate it has become a core issue in foundation pile quality inspection.
Based on the example of the Punan Expressway project in Fujian Province, this article introduces the application of ultrasonic method in bridge pile foundation detection.
Punan Expressway is part of the Beijing-Taipei Expressway, the second radial line of the National Expressway Planning Network. It is the expressway with the longest construction mileage and the largest investment in a single district or city in our province. project. There are 99 large and medium-sized bridges and more than 5,400 pile foundations across the entire line, of which 1,100 were tested using ultrasonic methods. Our unit is responsible for pile foundation testing for all bridges and evaluates the integrity of the pile concrete.
2 Ultrasonic detection principle and technology
(1) The basic principle of ultrasonic detection is: the ultrasonic pulse emission source excites high-frequency elastic pulse waves in the concrete, and uses high-precision The receiving system records the wave characteristics of the pulse wave during its propagation in the concrete; when there is a discontinuous or damaged interface in the concrete, the defective surface forms a wave impedance interface. When the wave reaches the interface, wave transmission and reflection occur, causing The received transmitted energy is significantly reduced; when there are serious defects such as looseness, honeycombs, and holes in the concrete, wave scattering and diffraction will occur; according to the first arrival time of the wave and the energy attenuation characteristics of the wave, frequency changes and waveform distortion Degree and other characteristics can be used to obtain the density parameters of concrete within the measurement area. Test and record the ultrasonic wave characteristics on different sides and different heights. After processing and analysis, the nature, size and spatial location of defects within the concrete in the measured area can be determined, and the overall homogeneity and integrity of the concrete can be evaluated.
(2) Before foundation pile construction, a certain number of acoustic tubes (usually steel pipes or galvanized pipes, with the bottom end closed and the top capped) are pre-buried according to the diameter of the pile as transducers. channel. During the test, every two acoustic detection tubes are in a group. The acoustic detection tubes are filled with clean water. Through the coupling of the water, the ultrasonic pulse signal is emitted from the transducer in one acoustic detection tube, and the ultrasonic pulse signal is emitted from the transducer in the other acoustic detection tube. The transducer receives the signal, measures the relevant parameters, collects the records and stores them. The transmitting and receiving transducers are raised simultaneously for detection. When an abnormality is encountered, methods such as horizontal encryption, arithmetic synchronization, and sector scanning can be used to perform encryption and detailed testing.
3 Data Analysis and Judgment
The testing is carried out in accordance with the relevant ultrasonic law provisions in the "Technical Regulations for Dynamic Testing of Highway Engineering Piles" (JTG/T F81-01-2004):
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(1) Pile body defects are comprehensively judged based on the critical value of sound speed, critical value of wave amplitude and PSD (slope method) criteria. The PSD value Kt is calculated according to the following formula:
Kt=K·Δt
K=（tci-tci-1）/zi-zi-1Δt=tci-tci-1
In the formula, tci is the sound time of the i-th measuring point; tci-1 is the sound time of the i-1th measuring point;
zi is the depth of the i-th measuring point; zi-1 the i-th measuring point -1 measuring point depth.
(2) The uniformity of the pile body is divided into levels A to D4 according to the sound speed dispersion coefficient Cv (Table 1).
Table 1 Sound speed dispersion coefficient level
(3) In the relevant specifications for foundation pile inspection, the integrity of the pile is divided into categories according to whether there are defects in the pile body and the severity of the defects. There are four categories: I, II, III and IV, and the integrity category of the pile under test is determined based on the distribution of abnormal points of acoustic parameters in each detection profile and the degree of deviation of the abnormal points.
However, because concrete is an aggregated composite material, a multi-phase composite system, and a complex interface (aggregate, bubbles, various defects), the acoustic parameter data detected fluctuates greatly; in addition, the concrete of cast-in-place piles is subject to self-densification, Due to the influence of geological conditions and pile-forming technology, the fluctuation of acoustic parameters is even greater. Therefore, it is less likely that abnormal measuring points will not appear at all during the actual test. Therefore, the pile body in the specification cannot be mechanically understood and implemented. The criterion for judging integrity (standards adopt "judgable" for abnormal sound parameter judgments), otherwise it would be difficult to have Class I piles in engineering, and it would not meet the definition of pile integrity classification. Therefore, the above theoretical abnormal points are just suspicious defect points, which can be comprehensively judged based on the following five aspects:
① The degree of deviation between the measured sound speed of the abnormal point and the normal concrete sound speed;
② Abnormality The deviation degree of the measured amplitude of the point from the normal concrete amplitude in the same section;
③ The degree of distortion of the waveform of the abnormal point compared with the waveform of the normal concrete;
④ The distribution of the abnormal point Scope and distribution of other profile abnormal points;
⑤ Pile type (friction type or end-bearing type), geological conditions and pile forming technology. The type of pile and geological conditions determine how the compressive stress and bending moment of the concrete on the pile body change with depth. Therefore, defects of the same size and degree at different depths of the pile body have a greater impact on whether the pile meets the design requirements. Appropriate distinction should be made.
4 Engineering Inspection Examples and Analysis
Pile 2l-1 of a bridge on Punan Expressway is a bored friction pile with a designed pile diameter of 1.5m and a designed pile length of 49.5m. Four acoustic measuring tubes are pre-embedded and tested using the ultrasonic flat measurement method with a spacing of 0.25m between measuring points. Among them, acoustic parameter anomalies appeared simultaneously in sections 1-2, 1-3, and 1-4 at 13.2-14m (as shown in Figure 1). The wave speed in the abnormal range dropped by 15% compared with the average wave speed, and the amplitude dropped by 30dB compared with the average amplitude. There are no obvious abnormalities in other sections at this location. It is preliminarily determined that there is an abnormality (defect) in the pile at 13-14m, and the defective area is in the direction of the No. 1 acoustic tube, but the scope of the defect cannot be determined, and it is classified into Category II. Still a Class III pile. In order to determine the severity and scope of the defect, two-way oblique measurements were performed on sections 1-2, 1-3, and 1-4, from 9 to 19 meters, with the receiving and transmitting transducers tilted at approximately 45°, and the measuring points The spacing is 10cm, and the oblique measurement results are shown in Figure 2. Through the oblique measurement data of each section and each direction, the measurement lines with abnormal acoustic parameters of the oblique measurement are determined. The envelope range of the abnormal measurement lines of each section is shown in the shaded part in the figure. It can be seen that 1-3 The sizes of radial defects in sections 1-2 and 1-4 increase in sequence, and sections 1-3 and 1-2 do not exceed 1/2 of the measurement distance, so this defect is a diameter reduction type close to the direction of No. 1 acoustic detection tube. Defects; judging from the scope of the defects, the longitudinal size is about 0.8m, and the radial size is less than 1/4 of the pile diameter; judging from the sound parameters and waveforms in the defect area, the amplitude of the sound parameters is not too large, and the waveform is basically complete. Therefore, this defect is judged as a minor defect and the pile is classified as a Class II pile.
Figure 1 Acoustic measurement curve
Figure 2 Schematic diagram of oblique measurement results
5 Several issues that should be paid attention to during testing
(1 ) The influence of pile concrete age.
During the inspection of 13-2 piles of a certain bridge, due to the tight construction schedule, the inspection was carried out on the 5th day after pouring. The test found that the received signal was quite weak and the waveform was severely attenuated. This situation is common in all measuring points. Preliminary analysis shows that it is an age issue. The pile was tested again on the 10th day after pouring. The signal and waveform were good, and the pile was judged to be complete and defect-free. It can be seen that age has a great influence on the results of acoustic testing. It is recommended that the testing time should not be less than 14 days of age.
(2) Ultrasonic method and drilling coring method are mutually reinforcing and can be applied comprehensively.
The designed pile diameter of 5-2 piles of a certain bridge is 1.5m, and 3 acoustic tubes are pre-embedded. Ultrasonic testing found that the concrete 3.5m below the top of the pile was seriously sand-incorporated. When verified by drilling core method, it is reflected as a complete pile. The excavation inspection confirmed that the results of the ultrasonic method were correct. The defective part happened to be in the quicksand layer. The two acoustic tubes were wrapped in the quicksand, so the defect cross section shown in the acoustic test results was a bit too large. The drilling site is close to the center of the pile, avoiding the scope of defects and does not reflect the true condition of the pile body. Therefore, the drill core sampling location should be chosen as close as possible to the interface where problems are found during the acoustic measurement process.
When abnormal conditions are found during ultrasonic testing, blind identification cannot be made. Geological and related data must be combined with different testing methods to reasonably determine the quality of foundation piles.
(3) Acoustic tube problem.
The acoustic tube is the channel through which the transducer enters the pile body during acoustic measurement. It is an important part of the ultrasonic pulse detection system for cast-in-place piles. Its pre-embedding method in the pile and its layout on the pile cross-section will directly affect the detection results. Therefore, the layout and embedding method of the acoustic detection tubes should be marked on the design drawings of the detection piles. The number of buried acoustic detection tubes and their layout on the cross-section of the pile should take into account the detection control area.
In actual measurements, it is often encountered that the acoustic tube is blocked or the probe is stuck. This is caused by improper installation of the acoustic tube. Acoustic detection pipes are generally installed in sections with steel cages. The joints between each section can be welded with reverse-threaded sleeve interfaces or casings to ensure no leakage of slurry under high hydrostatic pressure. The inner wall of the interface remains flat and is closed after installation. Nozzle. Non-parallel installation of acoustic detection tubes is also a common problem. Since the steel cage is prone to twisting and deformation during construction, the displacement of the acoustic detection tube is very large, which leads to deviations in the detected sound time value, mean square error, dispersion coefficient, average sound velocity, etc. You can use PSD method judgment is used to eliminate the influence of these non-defect factors. The turbid water in the acoustic detection tube will significantly or even seriously increase the attenuation of sound waves and prolong the propagation time, bringing errors to the sound wave detection. Therefore, the detection tube should be flushed and filled with clean water as coupling agent before testing.
6 Areas that need to be discussed and improved in practical work of the ultrasonic method:
(1) There is also widespread controversy over the use of sound velocity to estimate the concrete strength of pile bodies, and various methods are also being tried. Among them; mathematical statistics such as cluster analysis and statistical testing are used to determine the integrity of the piles. The better the construction quality of the piles, the stricter the judgment;
(2) Further develop the use of acoustic tubes or drill cores The technology of drilling holes to deal with pile body defects improves the reliability of reinforcement and reinforcement to deal with pile body defects, making the ultrasonic detection of foundation piles more economical and reliable.
For ultrasonic transmission method testing, the overall requirements for acoustic pipes are: the joints are firm and do not come apart, and the seal does not leak grout; the pipe wall is flat and not bent, smooth and without deformation; the pipe body is vertical and not skewed; the inside of the pipe Smooth and free of foreign matter.
When the material or installation process of the acoustic pipe is poor, it may cause accidents such as grout leakage, pipe blockage, breakage, bending, sinking, deformation, etc. The ultrasonic transmission method is used to detect pile foundation integrity. It will have a great impact, even making it impossible to carry out ultrasonic transmission detection.
Based on the above situation, through corresponding theoretical calculations and extensive engineering practice, Zhejiang Hongchang Technology Development Co., Ltd. launched our patented product: high-strength double-seal hydraulic acoustic tube (Patent No.: 200720112817.3) .
The high-strength double-seal hydraulic acoustic pipe is designed with two convex grooves at the end of the socket. The convex grooves are equipped with sealing rings. During installation, insert the socket end of this product into the socket end for 10cm, and then Special hydraulic pliers are used to squeeze two convex grooves at the same time. The pipe in the extruded part shrinks and deforms after being stressed. The outer pipe between the two convex grooves sinks deeply into the inner pipe, thus effectively realizing the reliability of this product. connection; at the same time, the rubber sealing ring deforms and fits between the two layers of pipes after being squeezed, which plays an extremely good double insurance sealing effect.