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First, the sound tubes used in road and bridge pile foundations are prone to several issues during installation:
1. The bottom of the pile is a common area where defects occur. Improper construction can cause the acoustic tube at the bottom to bend inward, reducing the spacing between tubes. This leads to the transducers not remaining parallel, resulting in abnormal sound velocity readings and reduced amplitude of the ultrasonic waves. The test curve becomes irregular, making it difficult to accurately assess the pile's condition.
2. If the sound tubes are not properly tied or spaced too far apart, they may tilt or bend due to concrete pressure during pouring. This changes the distance between the tubes, which affects the accuracy of the test results. In severe cases, it becomes impossible to determine the pile’s integrity without resorting to core drilling or other more reliable methods, which can delay construction.
3. When the steel casing is too long and the welding quality is poor, air pockets can form inside the tube. This forces the sound wave signal to travel through the air layer instead of directly through the concrete, causing serious signal distortion and affecting the determination of the pile’s integrity.
4. It is commonly assumed that the transducer is placed at the center of the sound tube. However, if the tube has a large diameter, the transducer may move within the tube, increasing the delay caused by the coupling water layer. This affects the time it takes for the sound wave to travel, leading to significant errors in the test results.
Second, there are specific operating standards for the sound tube testing of road and bridge pile foundations:
1. The sound tube should be embedded to the bottom of the cast-in-place pile, with the top extending 300–500 mm above the pile’s surface. All sound tubes on the same pile must have the same exposed height to ensure consistency.
2. The bottom of the sound tube must be sealed with a blind cover or steel plate to prevent slurry leakage. After installation, the top of the tube should be capped to avoid foreign objects falling in during concrete pouring, which could block the tube.
3. The sound tubes should be fixed using spot welding or wire binding to the inner side of the reinforcement cage. The fixing points should be no more than 2 meters apart, especially near the ends and joints. For areas without a cage, they can be fixed with additional steel bars. To maintain parallel alignment, triangular steel frames can be spot-welded between the tubes.
4. During installation, the sound tubes should be connected in stages along with the reinforcement cage. The lower section is fixed on the first segment of the cage, while the rest are temporarily held in place. When lowering the cage, the upper sections are aligned and inserted. Each joint should be connected using reverse-thread sleeves or welded sleeves. The reverse-thread sleeve must use a soft rubber seal ring, and the welded sleeve should be about 80 mm long, larger than the tube’s outer diameter. The tubes are then welded together. Regardless of the method, the joint must be strong enough to prevent bending and should be leak-proof under high hydrostatic pressure. The interface should be smooth, free of weld slag or burrs, so the transducer can move freely. If cutting is needed, a cutting machine should be used, followed by polishing to remove any burrs. Electric welding should not be used for cutting. When welding the reinforcement cage, care must be taken to prevent molten material from splashing onto the tube or joint.
5. Clean water should be filled into each sound tube as a coupling medium. River water should not be used, especially during flood season when it contains high levels of sediment. If used, it must be purified first. Before pouring underwater concrete, check the water level in the tube. If it’s not full, refill it to ensure proper coupling.
6. Testing can begin after the pile foundation concrete has cured for at least 14 days. Before testing, the pile head should be chiseled to the design elevation. A 20 cm-long steel bar should be used as a hammer to check the smoothness of the sound tube and ensure it is filled with clean water.
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