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| Ultrasonic time-frequency characteristics of water-rich fine sand during unidirectional freezing process#br# |
| ZHANG Jiwei1，2，3，LIU Shujie1，2，3，4，ZHANG Song1，2，3，5#br# |
(1. Shaft Branch of China Coal Research Institute，Beijing 100013，China；2. Beijing China Coal Mining Engineering Co.，Ltd.，Beijing 100013，China；3. National Engineering Laboratory of Deep Well Construction Technology in Coal Mine，Beijing 100013，China；4. School of Resources and Civil Engineering，University of Science and Technology Beijing，Beijing 100083，China；
5. School of Civil Engineering，Shijiazhuang Tiedao University，Shijiazhuang，Hebei 050043，China)
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Abstract To study time-frequency characteristics of ultrasonic P-wave of water-rich fine sand under unidirectional freezing，ultrasonic tests on different freezing front positions of water-rich fine sand under unidirectional freezing were conducted by using NM-4A nonmetal ultrasonic test meter，and the characteristics of acoustic waveform and wave frequency spectrum of partial frozen soil were analyzed through wavelet decomposition，Fourier transform and continuous wavelet transform. The experimental results show that，due to water migration，water-rich fine sand under unidirectional freezing presents three acoustic impedance regions including frozen and water-rich bottom area，frozen and loss water middle area and unfrozen and loss water upper area，and that the time-frequency characteristics of received waves are influenced by scattering effect of unfrozen and loss water upper area during unidirectional freezing. With increasing the height of the freezing front，the wave period number of partial freezing soil increases but the scattering effect of unidirectional freezing is more apparent than constant temperature freezing. The P-wave velocity of fine sand under unidirectional freezing increases with rising the height of the freezing，which relationship can be described by a cubic function with a good correlation. As the height of the freezing front increases，the frequency spectrum changes and is characterized by multi-peak，ambiguity dominant frequency，wide frequency band and the proportion of the high frequency equal to the low frequency，which indicates that，at the later period of unidirectional freezing，different frequency peak values can be used to represent the dominant frequency of specific acoustic impedance region. It is also shown that，with increasing the height of the freezing front，the scattering effect decreases gradually and the coda wave converts from comparative developed to non-developed. Continuous wavelet transform analysis reveals that the historical freezing model can be expressed by that the dominant frequency position of unidirectional frozen soil shifts to the cold side while the dominant frequency position of constant temperature frozen soil locates at the middle of overall time domain.
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2020, Vol. 39 
