## 2 Materials and Methods
### Testing Progress  
Both the uniaxial compression testing and Brazilian testing for the FSRM samples were conducted using a Servotronic system with a capacity of 100 kN and a resolution of 0.1 kN [39]. The radial displacement was measured using a contact strain acquisition sensor at a resolution of 0.05 mm. In this experiment, the test was conducted in a lab environment (20 \({}^{\circ}\)C). To prevent the loading interface from melting, we replaced the top and bottom metal load platens with high-density acrylic plates (PMMA). In addition, the cylinder samples were capped at both ends with a Vaseline film. Meanwhile, the cylindrical samples were wrapped with aluminum silicate thermal insulation cotton during the uniaxial compression testing. For the Brazilian testing, each cylindrical disk was placed directly between the platens of the compression testing machine, as shown in Figure 3. All the tests were carried out under displacement control at a rate of 1.236 mm/min. The samples were taken out from the low-temperature environment box and were placed on a loading table. Axial loading was applied via an electric motor and the axial loading and displacement were recorded. Moreover, an electronic thermometer was fixed on the sample surface to monitor the temperature variation of the sample surface during compression, as shown in Figure 3.  
Acoustic emission (AE) techniques can record ultrasound signals when emitted by a defect within a sample when it is produced. To clearly understand the internal deformation properties of rock blocks and the strain field characteristics under the uniaxial compression and Brazilian tests, we used an acoustic emission signal measuring system for obtaining the AE characteristics of a typical FSRM with the frozen temperature of \(-20\)\({}^{\circ}\)C during testing (Figure 4a). A piezoelectric wide-band sensor (RS-2A) with a frequency spectrum of 50-400 kHz was installed in the middle of the specimen surface to pick up the high-frequency stress waves, which were then converted to electric signals. The background noise from the uniaxial compressive apparatus and surrounding environment approximately ranged from 6 to 25 mV. The trigger level voltage of 30 mV was selected on the basis of the preliminary analysis of waveforms. The signals detected by the sensor that were greater than the trigger level of 30 mV were amplified by a pre-amplifier by 40 dB and were then processed and recorded. The AE energy was recorded as a function of time. In addition, the ultrasonic plus velocity (UPV) is commonly used to evaluate cracks or  
Figure 2: Fabricated samples: (**a**) cylindrical frozen soil–rock mixture sample; (**b**) cylindrical ice–rock mixture sample for compression testing; (**c**) disk soil–rock mixture samples for Brazilian testing; (**d**) disk ice–rock mixture sample for Brazilian testing.  
defects inside materials. A P-wave transducer and associated equipment were employed to three similar FSRM samples to record the axial UPV during the compression process. The piezoelectric transducers were deployed at the 2 ends of the sample, as shown in Figure 4b. The piezoelectric wide-band transducers featured a frequency spectrum of 1-125 kHz. Pulses were emitted at a pulse width of 0.04 ms and a voltage of 500 V, and the sampling period was 0.2 us. The transducer at the bottom emitted a pulse signal and the receiving transducer collected the signal at the top. An initial wave was selected to obtain the travel time at each loading step. The P-wave time to travel through the samples and acrylic end-caps platens were recorded (t\({}_{1}\)). The travel time in the acrylic plates was also recorded (t\({}_{0}\)). After measuring the sample length (L), we calculated the velocities as UPV = L / (t\({}_{1}\)-t\({}_{0}\)).