## 3 Experimental Program  
To ensure the orientation of sample particles, the sand pluviation method is used for sample reconstitution by taking compaction degrees, fine contents, etc., into account; then, the reconstituted samples were frozen and thawed under different confining pressures and freezing temperatures; finally, scanning electron microscope (SEM) and laser scattering and transmissometry (LST) tests were both conducted to comprehensively explore the particle characteristics of saturated silt under long-term freeze-thaw cycles. The testing procedures were as follows:  
* Firstly, dry and smash the undisturbed soil into soil particles, and pass it through 2.0 mm fine sieve for particle separation;
* Use the sample preparation device of sand pluviation method (as shown in Figure 4) for sample preparation:  
The sample preparation device consists of a sand-spreading device, a water tank, a movable trolley platform, and some double valves. The water level of water tank is 1 m higher than the double valves [13]. To avoid sorting due to different particle diameters during the initial spreading period, a movable trolley platform is specially set up. After sand spreading for a period of time (30 s) move the trolley platform to the sand accumulation area to accumulate sand. The compactness degree of sand samples could be controlled initially by falling distance and falling amount, and then accurately controlled by sample compactor. The length and diameter of sand samples are 80.0 mm and 39.1 mm, respectively. Store the reconstituted samples in a vacuum saturator to saturate for 12 h;  
Place the saturated samples on the freeze-thaw device (as shown in Figure 5) and set the preloading:  
First, place the saturated samples on the freeze-thaw device. Open the drainage channel of freeze-thaw device and put an appropriate amount of weight on loading frame according to the effective confining pressure of the experimental program. Close the drainage channel after loading for 2 h. Then add another appropriate amount of weight to make the loading reach the designed total overburden stress value. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & **Water** & **Unit** & **Natural** & **Particles** & **Maximum** & **Minimum** & **Cohesion** & **Internal** & **Compression** & **Uniformity** & **Fine** \\  & **Content** & **Weight** & **Void** & **Specific** & **Void** & **Void** & **kPa** & **Friction** & **Modulus/** & **Cofficient** & **Cofficient** & **Particle** \\  & **\%** & **(kN/m3)** & **Ratio** & **Gravity** & **Ratio** & **Ratio** & **Angle**/ & **/** & **kPa** & **Cofficient** & **Content/\%** \\ \hline silt & 27.3 & 18.9 & 0.797 & 2.69 & 1.31 & 0.69 & 7.2 * & 28.1 * & 10.27 & 6.09 & 88.3 \\ silty sand & 24.8 & 19.4 & 0.734 & 2.70 & 1.18 & 0.65 & 5.3 * & 30.6 * & 10.82 & 1.89 & 7.2 \\ \hline \hline \end{tabular} Note. * is obtained by direct shear test.  
\end{table} 
Table 1: Physical and mechanical properties of soils. Figure 4: Sample preparation device of sand pluviation method for soil reconstitution. Put the freeze-thaw device with sample under a certain confining pressure of weights into a DW-40 type low temperature test chamber and freeze for 12 h. The freezing temperature matches the experimental program. After freezing, take it out to room temperature (constant temperature 20 \({}^{\circ}\)C by air conditioner) and thawing for 12 h. In this way, cycle the freeze-thaw operation until the design freeze-thaw cycles are reached. Make the SEM sample and observe its microstructure:  
The soil structure of silt can hardly be preserved without special device and techniques. In this paper a special SEM sampling device and a kind of curing agent, menthol, are used for SEM sample preparation. Evenly press the SEM sampling device by using a pallet into the soil sample. The SEM sampling device is 10 mm diameter, 5 mm height, and 0.1 mm thickness by No.