Evaluation of freezing state of sandstone using ultrasonic time-frequency characteristics

Common problems in engineering projects that involve artificial ground freezing of soil or rock include inadequate thickness, strength and continuity of artificial frozen walls. It is difficult to evaluate the freezing state using only a few thermometer holes at fixed positions or with other existing approaches. Here we report a novel experimental design that investigates changes in ultrasonic properties (received waveform, wave velocity Vp, wave amplitude, frequency spectrum, centroid frequency fc, kurtosis of the frequency spectrum KFS, and quality factor Q) measured during upward freezing, compared with those during uniform freezing, in order to determine the freezing state in 150 mm cubic blocks of Ardingly sandstone. Water content, porosity and density were estimated during upward freezing to ascertain water migration and changes of porosity and density at different stages. The period of receiving the wave increased substantially and coda waves changed from loose to compact during both upward and uniform freezing. The trend of increasing Vp can be divided into three stages during uniform freezing. During upward freezing, Vp increased more or less uniformly. The frequency spectrum could be used as a convenient and rapid method to identify different freezing states of sandstone (unfrozen, upward frozen, and uniformly frozen). The continuous changes in reflection coefficient rf, refraction coefficient tf and acoustic impedance field are the major reason for larger reflection and refraction during upward freezing compared with uniform freezing. Wave velocity Vp, wave amplitude Ah, centroid frequency fc and quality factor Q were adopted as ultrasonic parameters to evaluate quantitatively the temperature T of uniformly frozen sandstone, and their application within a radar chart is recommended. Determination of Vp provides a convenient method to evaluate the freezing state and calculate the cryofront height and frozen section thickness of upward frozen sandstone, with accuracies of 73.37%–99.23%.