Research On The Guided Wave Propagation And Reflection In Freezing Pipe Buried In Frozen Soils

The fracture of freezing pipe is one of the most important technical problems in the construction of freezing method for a long time,and it has not yet been completely solved.Once the accident of freezing pipe fracture occurs,the construction period will be delayed,and even the water will lead to the flooded well,which will seriously threaten the safety of people’s lives and engineering.Guided wave testing technology has many advantages,such as long-distance detection,wide-range detection and trenchless detection of buried pipelines.In order to explore the feasibility to detect the freezing pipe frecture with guided wave,this paper combines the numerical simulation and the physical test to study the guided wave propagation and fracture surface reflection in freezing pipe.Firstly,the numerical calculation method is used to explore the influence of frequency,waveform and frozen soil thickness on the propagation of guided waves.Then on the base of these,the freezing pipe buried in a single frozen soil layer is taken as the research objects,single factor analysis is carried out for three factors,including freezing pipe joints,frozen soil property and crack size.The study found that the existence of pipe joints will increase the guided wave attenuation.The attenuation coefficient of guided wave is about 4.54 times that without joints.The attenuation of the guided wave in pipe is influenced by the frozen soil property outer the pipe,and the attenuation coefficient is negatively correlated with the wave impedance of the frozen soil outside the pipe.The attenuation coefficient of guided wave is reduced from 0.1338dB/m to 0.1123dB/m when the wave impedance of the frozen soil is increased to 1/30 from the impedance of the freezing pipe wave impedance 1/90.The intensity of reflection wave is positively related to the circumferential length of crack,whereas the intensity of transmission wave is negatively related to the circumferential length of crack.When the circumferential length of crack reaches up to 100%,the intensity of reflected wave reaches the maximum and the reflectivity is about 91.1%.Meanwhile,the intensity of transmitted wave is reduced to a minimum and the transmissivity is about 0.Secondluy,the experimental method is also utilized to this research.Laboratory tests of guided waves propagation and attenuation in freezing pipe have been conducted in different working conditions,including unconstrained(working condition 1),buried in sand(working condition 2),similar materials of frozen soil for low elastic modulus(working condition 3)and for high elastic modulus(working condition 4).The results show that the group velocity of guided waves is basically not affected by working conditionshas,but the attenuation and propagation distance are related to the working conditions.The attenuation coefficient of working condition 2~4 is about 3.7 times,5.3 times and 3.4 times that of working condition 1.This paper also carrys out the experimental research on the law of reflection,transmission and energy attenuation of guided waves at pipe joints under the first three working conditions described above.Through comparative analysis betwween experimental results and numerical simulation results,it is found that the reflectivity and transmissivity of guided waves at pipe joints are related to the frequency and working conditions.Over the frequency range of 45~100 kHz,the transmissivity is in the range of 0.56~0.99、0.54~0.98、0.45~0.88 for three working conditions,respectively.According to the test results,the propagation distance is estimated with the amplitude residual ratio 0.018% as the detection limit.The maximum propagation distance of guided waves in the freezing pipe is 94 m,and the minimum propagation distance is 50 m when the high elastic modulus frozen soil is wrapped outside the pipe.At last,numerical calculation and physical test of the guided wave propagation and reflection in freezing pipe buried in composite frozen soil layers have been carried out for three factors,including elastic modulus difference between adjacent frozen soil layers,fracture forms of pipe joint,soil layer position of fracture surface.It is found that the existence of soil interfaces has no effect on the attenuation of guided wave.The propagation attenuation law of guided waves in freezing pipe buried in a composite frozen soil layers is similar to that of buried in a single frozen soil layer.The reflectivity of guided waves at crack type I and II increases generally as the frequency increases,and the transmissivity decreases with the increase of frequency.According to the test results,the detection distance is estimated with the amplitude residual ratio 0.018% as the detection limit.The maximum detection distance of guided waves in the freezing pipe that contains one joint per 9m is 49.5m,and the minimum detection distance is 22.5m.