Key aspects and common methods of pile foundation inspection
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After a pile is driven into the ground, it is necessary to check whether it meets the load-bearing requirements; factors such as cement strength, pile depth, and whether the pile is tilted all need to be carefully considered – it’s not enough to simply drive it in and assume it’s fine. Therefore, after normal construction is completed, a series of testing methods are used to assess the quality of the pile foundations. First, we need to determine what aspects need to be inspected: 1. Precast piles and steel piles must be tested; it is necessary to know the elevation and deviation of the pile tops, as well as information regarding the depth to which they have been driven. 2. It is necessary to inspect the driven piles, primarily by checking the deviation of their positions as well as the quality of their shafts. 3. It is necessary to conduct inspections on manually dug piles. During these inspections, the depth of the excavation as well as the conditions of the rock and soil are key factors to consider. When necessary, additional test specimens must be used, and their number should not be less than 1% of the total number of specimens; therefore, at least three specimens are required. The testing items listed above are determined based on actual needs; different projects may require different tests. However, the purpose of testing the quality of pile foundations is to verify the reliability of the piles themselves, which is why such testing is essential.
First: static load test method； This is currently the widely accepted reliable method, as it directly measures the vertical compressive bearing capacity. It involves using sandbags of a specified weight or other heavy objects to press on the foundation slab, then supporting it with pile foundations for a certain period of time; after that, it is checked whether there is any settlement or other problems. If no such problems occur, it indicates that the pile foundations have passed the inspection. Second: reflected wave method ； In China, the vast majority of testing agencies use the reflected wave method (transient time-domain analysis) to assess the integrity of pile shafts. The main reason for this is that the equipment used is lightweight, allowing for rapid on-site testing; moreover, methods such as excitation techniques and frequency-domain analysis are incorporated as tools for testing and supplementary analysis. Of course, when using the low-strain method for detection, regardless of the type of defect, the overall effect is a decrease in the pile’s impedance; it is difficult to distinguish the nature of the defect, and this constitutes its biggest limitation. Third: Ultrasound testing method ； The non-metallic ultrasonic detector uses the ultrasonic rebound combined method to determine concrete strength, detect and locate internal defects in concrete, measure the depth of concrete cracks (using an optimized crack-crossing detection approach), measure the width of concrete cracks, and enables automatic reading along with photographic capabilities via ultrasonic transmission testing; it can also assess the integrity of pile foundations (with a one-transmission-two-reception function). Fourth: Core drilling method ； This method is characterized by its scientific nature, intuitiveness, and practicality, and it is widely used in the inspection of concrete cast-in-place piles. A complete and successful core drilling test can provide information on the pile length, the strength of the concrete in the pile shaft, the thickness of the sediment at the pile bottom, and the integrity of the pile shaft, while also allowing for the determination or identification of the geological properties of the bearing stratum at the pile tip. The specific procedure involves using specialized core extraction tools to reach the designed depth from the top of the pile, then arranging the pile cores in order and checking for any deviations or variations in the depth of the mud layer. If soil is reached at the predetermined depth, it indicates either that the pile is not deep enough or that the pile is tilted; in such cases, it may be necessary to drive another pile nearby to provide stability.
The ultrasonic transmission method is primarily used to detect the density of the concrete in the pile shaft as well as the integrity of the pile shaft； The static load test method is currently recognized as the most direct and reliable test method for determining the vertical compressive bearing capacity of foundation piles ； The core drilling method is characterized by being scientific, intuitive, and practical, and it is widely used in the inspection of concrete cast-in-place piles ； The reflected wave method is primarily used to detect the integrity of the pile shaft ； The main function of the high-strain method is to determine whether the vertical compressive bearing capacity of the pile meets the design requirements.
04 Data Analysis: Based on the sound-time criteria, sound-amplitude criteria, and PDS criteria obtained from the tests, the critical values of various acoustic parameters are analyzed. By integrating the results of these various criteria along with the actual waveform diagrams, an assessment of the integrity of the pile is made. In addition to the above criteria, there are also the main frequency criterion and the sound speed criterion, which evolved from the sound time criterion. Analysis and determination -- Measurement values: sound time, sound speed, amplitude, and fundamental frequency. Analysis and determination -- Critical sound speed value: ① Data sorting ； ②Truncate obvious small values ； ③Calculation criterion ； ④inspection ； ⑤Truncate the smaller value ； ⑥Repeat steps ③④⑤ ； ⑦The critical value is obtained. Analysis and determination – Amplitude threshold: The amplitude threshold is determined by subtracting 6 dB from the average amplitude; when the measured amplitude is below this threshold, it should be considered a suspect defect area. Analysis and determination — Slope criterion: The product of the slope on the sound-time depth curve and the sound time at the adjacent measurement point; this is a parameter sensitive to the edges of defects, and it is used in conjunction with sound-time or sound-amplitude criteria.