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Example analysis of double velocity integrity detection of active piles under bridge
Release time: 2018-06-23

Han   bright

   

100062) (Europe and the United States Earth Instrument Equipment China Co., Ltd., Beijing 100062)


Abstract: Integrity testing of active piles under bridges requires an effective test method, but it is very difficult to test because it is connected to the upper mechanism, which has become an engineering problem. The use of two-speed test technology provides new ideas and test methods for solving the integrity of such foundation piles. The discussion of the actual test cases provided in this article shows that after overcoming various influencing factors, the dual-speed method can effectively extract the upward wave from the measured signal, and then analyze the existence of the defect and determine its location. In addition to the necessary calculation and analysis techniques for the two-speed method, testers should also have a solid traveling wave theory, clever layout of field observation systems, and rich engineering experience.

Keywords: two-speed test, in-service pile integrity defect, upgoing wave

前言 1 Preface

The reinforcement and reconstruction of a bridge requires an evaluation of the original foundation. The investigation of the status of the foundation pile integrity is the most important link. It is the basis for the preparation of a new design scheme for the upper and lower structures of the bridge. Because the bridge was built earlier and lacked information about the integrity of the pile foundation, it is necessary to test the integrity of these active piles on site without affecting the normal use of the bridge. As is known to all, in-service piles are often buried deep at the top of the pile and connected to the superstructure, and conventional low-strain methods cannot perform inspection work. The two-speed method is a new technique for solving the integrity of piles in service. Before the large-scale testing work was carried out in this project, the two-speed method was tested.

The test site is close to the river and the water level is high, and the industrial and mining conditions are very complicated. The test pile is exposed to the side of the pile with the cooperation of the excavator to facilitate the selection of the installation location of the sensor and the stress wave excited by the hammer. Ф1200mm 的钻孔灌注桩,位于墩柱之下,墩柱与盖梁相连,现场情况如图 1 所示。 The test pile is a bored cast-in-situ pile with a diameter of 1200mm , which is located below the pier column, and the pier column is connected to the cap beam. The site situation is shown in Figure 1 . Before the actual measurement, various influencing factors and test conditions were considered, and a test plan was planned to fully prepare for the test work under such complicated industrial and mining conditions.

The test uses a pile integrity tester with a wireless transmission function, which is beneficial to collect signals in real time at a certain distance outside the test pit. Before collecting signals, the sides of the test pile were smoothed and dried to install two side-mounted acceleration sensors.



DSC03856.JPG
1 现场概貌 Figure 1 site overview

基本原理 2 basic principles

    When the pile top of the existing foundation is connected to the superstructure, the pile top is a non-free end, and the conventional overhead reflection method cannot achieve the integrity of the pile body. In addition, due to the increase in the impedance interface of different structures above the pile top, The stress wave has both an ineffective downward wave and an effective upward wave in the transmission process, which causes the measured curve to be very complicated, and it is impossible to distinguish between the upward wave and the downward wave. Using the two-speed method can not only extract the upward waves, but also accurately determine the average wave speed of the pile body. The two-speed method is to install two acceleration sensors vertically within a certain distance along the side surface of the pile, collect two acceleration signals at the same time, and separate the ascending wave from the measured curve by the traveling wave calculation method to identify the defect; Or reflection time to obtain the average wave speed of the pile body between the two sensors.

    A new type of multi-channel pile integrity tester with wireless transmission function is used, that is, there is no cable connection between the sensor and the host, which is conducive to the detection of foundation piles in complex structures or test environments. A variety of special hammers are used to excite the incident wavelets with different main frequencies.


分析 3 Analysis


Due to the large buried depth of the pile body, the high water level, and small changes in the cross-section dimensions of the pier and the pile body, the sensor is installed on the side of the pier. The sensor is coupled with a special coupling agent, and the coupling effect is good. 2 所示。 The width of the cover beam is larger than that of the pier column, there is no hammering position, and the space above the cover beam is large, so the hammering position is selected on the upper surface of the cover beam. The field operation is shown in Figure 2 . 11.0m 14.5m 位置,存在拉伸反射,该部位与桩身阻抗存在明显差异,判断存在缺陷;桩端附近没有明显的反射,其原因是该桩工作龄期很长,桩端与桩端土体耦合完好,桩端界面阻抗差异较小所致。 The measured curve reflects that there are tensile reflections at the positions of about 11.0m and 14.5m under the first sensor. There is a significant difference between the impedance of this part and the pile body, which is judged to be defective; there is no obvious reflection near the end of the pile, which is due to the work The age is very long, the coupling between the pile end and the soil at the pile end is intact, and the difference in impedance at the pile end interface is small.


             2 现场测试场景 Figure 2 Field test scenario                     

3 实测曲线(上图)和上行波曲线(下图) Figure 3 Measured curve (upper) and upward wave curve (lower)



   
 
3 所示。 The measured double velocity curve and the calculated upward wave curve are shown in Figure 3 . 11.0m 14.5m 位置出现明显的上行反射特征,其它位置均没有明显反射,说明这两处桩身存在明显缺陷,按照现行规范规定桩身完整性应判定为 类桩。 The upward wave curve shows obvious upward reflection characteristics at about 11.0m and 14.5m below the first sensor, and there is no obvious reflection at other positions, indicating that there are obvious defects in the two piles. According to the current code, the integrity of the piles should be Judged as a type III pile. Such foundation piles should be reinforced or re-piled to ensure the safety of the new design scheme of the bridge.


结论 4 Conclusion

40m 为宜。 The results of this test show that the two-speed method is more suitable for testing the integrity of the pile in service under the bridge pier in similar industrial and mining conditions, and the pile length is generally not more than 40m . The integrity testing of in-service piles is still a very difficult engineering problem commonly encountered at home and abroad. There are undoubtedly areas for improvement in the dual-speed method. At the same time, testers not only need to have the necessary data analysis and processing capabilities, but also need a solid traveling wave theory foundation, clever on-site observation system layout, and rich practical engineering tests. experience.


references


[1] . 既有基础下基桩完整性双速度测试技术 [J]. 《建筑结构》第 37 卷增刊 ,2007 Han Liang.Two-speed test technology for foundation pile integrity under existing foundation [J]. Building Structure, Vol. 37 Supplement , 2007

韩亮译 . PIT-W 使用手册 [M] . 欧美大地仪器设备中国有限公司 ,2005 [2] Translated by Han Liang . PIT-W manual [M] . Europe and the United States Earth Instrument Equipment China Co. , Ltd. , 2005

[3] Johnson, M., and Rausche, F., (1996), "Low Strain Testing of Piles Utilizing Two Acceleration Signals," StressWave 1996, Orlando , FL , p.859-869

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