Application Cases

Experimental Title: Study on the Acoustic Characteristics of Saline Artificially Frozen Soil

Research Focus: Artificial freezing method uses refrigeration technology to freeze water in the ground, forming frozen soil to isolate groundwater from underground engineering construction, allowing work to proceed under the protection of a frozen wall. Typically, circulating a low-temperature coolant (calcium chloride solution is often used in freezing method construction) through freeze pipes converts natural soil into artificially frozen soil. However, accidents involving freeze pipe fractures occur occasionally due to ground conditions, freeze hole deviation, freezing techniques, and excavation methods. The resulting brine leakage compromises frozen wall stability. Current methods for assessing frozen wall development include the diagram multiplication method, empirical formula method, and numerical simulation method. The drawback of these methods is their high reliance on temperature measurement data from temperature holes, leading to inaccuracies at distances far from these holes and potential oversight of critical issues like local frozen wall 'windows' or insufficient strength. For artificial ground freezing under saline conditions, the temperature field changes are complexly influenced by salt content, making anomaly detection more difficult with existing methods. For instance, uneven salt distribution can lead to irregular frozen wall formation or uncertain risks of 'windows' due to brine leakage. Understanding the development and engineering performance of the frozen wall is crucial for the safe construction using the artificial freezing method under these special saline conditions.

The propagation of acoustic waves in geomedia is closely related to their physical and mechanical properties. Understanding the acoustic characteristics of saline artificially frozen soil and establishing the relationship between acoustic parameters and the physical-mechanical properties of saline artificially frozen soil in artificial ground freezing projects can provide a necessary basis for acoustic detection of anomalies in brine-infiltrated frozen walls. Acoustic testing, as a convenient and rapid non-destructive testing technique, shows promise for achieving this goal. When acoustic waves propagate through frozen soil, their characteristics serve as a comprehensive indicator of factors such as soil temperature, water content, density, presence of fissures, and stress state. Simultaneously, frozen soil strength is also a comprehensive reflection of these factors. Through the relationship between the two, rapid in-situ determination of acoustic parameters can enable the estimation of the mechanical properties of frozen soil on site.

Experimental Objective: To verify the acoustic characteristics of saline artificially frozen soil.

Test Equipment: Signal generator, ATA-2022H high-voltage amplifier, oscilloscope, charge amplifier, piezoelectric ceramic sensors, etc.

Experimental Process: First, a single-cycle sine wave is generated by the function generator. The excitation signal is then amplified by the ATA-2022H high-voltage amplifier and split into two paths. One path is directly output to the digital oscilloscope as the excitation waveform signal. The other path is used to drive the piezoelectric ceramic transmitter to generate vibrations and excite acoustic waves. These waves propagate through the test specimen. On the other end, the signal received by the piezoelectric ceramic receiver is filtered by the charge amplifier, amplified again, and transmitted to the digital oscilloscope as the received waveform signal. Finally, the oscilloscope's storage function is used to import the waveform data into a computer for determination of the propagation time. The experimental system setup is shown in Figure 1-1.

Figure 1-1: Experimental System Diagram

Experimental Results:

Figure 1-2: Diagram of Wave Velocity Ratio Variation Pattern

From Figure 1-2(a), it can be observed that under unconfined conditions, the wave velocity ratio slightly decreases with increasing salt content, with a variation range of 1.82~2.41. This indicates that the wave velocity ratio is not sensitive to changes in salt content under unconfined conditions. The wave velocity ratio of fully saturated frozen soil is greater than that of unsaturated frozen soil, suggesting that the wave velocity ratio is related not only to soil type but also to the degree of saturation. Under low confining pressures (4MPa, 6MPa), the wave velocity ratio increases with increasing salt content. Under high confining pressure (8MPa), the wave velocity ratio decreases with increasing salt content. From Figure 1-2(b), it can be seen that for frozen silt with the same salt content, the wave velocity ratio generally increases with increasing confining pressure.

Power Amplifier Recommendation: ATA-2022B High-Voltage Amplifier

Figure: ATA-2022B High-Voltage Amplifier Specifications and Parameters

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