Monitoring Technology Of Local Scour Depth For The Bridge Foundation Based On The Ultrasonic Principle

Foundation scour is the result of the water current erosion,which could result in the removal of constraining materials around the pier or the abutment foundation for bridges.The riverbed scour around the foundation is one of the most important reasons for the failure of the as-built bridge structure.The structural damage caused by foundation scour usually has no obvious foreboding,which can easily lead to the complete bridge collapse.In orde r to reduce the failure probability of the bridge collapse ca used by the foundation scour of bridges,it is necessary to conduct the real-time monitoring of the foundation scour for the as-built bridges and to evaluate the safety of bridge structures in service.This paper proposed an ultrasonic-based scour monitoring method for the bridge foundation,the model verification experiment and finite element numerical simulation analysis were conducted for ultrasonic-based scour monitoring method.The main resea rch contents are as follows:(1)Taking advantage of the advantages for the ultrasonic wave,such as the good direction,the high energy and the reflection at interface of two media with different acoustic impedances,a real-time monitoring ultrasonic-based method of the bridge foundation scour is proposed.The core module of the monitoring method consists of a high-frequency nonlinear acoustic detector and a self-transmitting and self-receiving ultrasonic piezoelectric transducer,which are all developed independently by our research group.The transducer introduces a wideband acoustic signal into the test object,the pulse propagates in the water and is reflected at the scour pit interface.The reflected signal is received by the receiving transducer and t he returned signal is displayed on the oscilloscope as a curve of amplitude and time.The depth of foundation scour can be directly determined by the reflection time of the receiving echo and the propagation velocity of ultrasonic waves in the water.(2)The local scour model of the bridge foundation was established in the laboratory by using the glass boxes to verify the scour test based on the ultrasonic method.The close-packed fine gravel is used to simulate the scour pit,the glass tank wall is used to simulate the bridge pier.First of all,the fine gravel is piled into an inclined plane with the bridge pier to test the influence of different inclined angles on the measurement results at the same reflection interface.Then,the influence of the reflect or for different materials on the scour measurement results under the same water depth is measured with silt,fine sand and marble.The results show that the measurement error of the same material reflecting interface at different inclination angles increa ses greatly compared with that of the flat interface.With th e increase of the inclination angle,the measurement error becomes larger,and the "peak value calculation method" can greatly reduce the measurement error of the corresponding scour depth.The normalized energy integral functions of various materials can be obtained by the energy normalization treatment of the waveforms of different interface echoes.The classification characteristics F of the energy integral function can be used to judge the und erwater soil type,so as to facilitate in the judgement of actual scour depth in the measurement.(3)The finite element analysis software COMSOL is used to establish the two-dimensional simplified model of the riverbed interfaces,including(a)the models with different tilt angles under the same material interface and(b)the models with 11 different interface materials under the same water depth.The echo signals of each measuring point are analyzed,compared and discussed with the experimental results in the previous chapter.The results show that the influence of different tilt angles on the simulation results under the same material interface is consistent with the experimental results,and the calculation error can be greatly reduced by using the "pea k value calculation method".The peak value of echo signal fo r different material interface increases with the increase of reflectance coefficient of riverbed interface material.The measurement results of different interface materials can be judged by combining the arrival time of echo and the peak value of signal under the ideal simulated conditions in this study.