# A Study on the Vertical Bearing Characteristics of Screw Piles in Permafrost Regions
## 2 Model Test  
### Design of Experiment  
The body of the test pile is made of aluminum alloy. The length of the conventional, ordinary pile is 40 cm, and it has a diameter of 4.5 cm. The screw pile has a length of 40 cm, an inner diameter of 4.5 cm, an outer diameter of 6 cm, a pitch of 4 cm, and a thread width of 1.2 cm. Strain gauges were arranged symmetrically on both sides of the pile's body, with seven groups placed for a single pile. The pile's body was placed in the test chamber for loading. The diameter of the test chamber was 45 cm, and the depth was 65 cm. The soil width range around the pile was 45 cm, and the soil depth was 2 times the pile length, making it 80 cm, which can simulate the bearing capacity characteristics of the pile's body in the semi-infinite stratum, as indicated in Figure 1. Temperature sensors were installed every 10 cm in a downward direction along the depth of the soil in the model test chamber, with a total of 8 temperature sensors, to test the soil temperature. Standard loess from Lanzhou, China, was selected for this test, with a density of 1650 kg\(\cdot\)m\({}^{-3}\). The liquid limit was 24.6%, and the plastic limit was 17.7%. Figure 2 depicts the particle size distribution curve of standard loess. The collected standard loess soil sample was mixed with water, and the water content was set at 21%. A heavy hammer was used to compact every 5 cm of soil, and each layer's compaction work was controlled at approximately 50.6 kJ\(\cdot\)m\({}^{-3}\). Once the pile-soil model was created, it was placed in a low-temperature environmental test chamber and frozen for 3-4 days until the temperature reached the test value of \(-0.5\)\({}^{\circ}\)C. This temperature condition corresponds to high-temperature frozen soil [14].  
The low-temperature control system utilizes a GDJS-150 high- and low-temperature alternating wet heat test box. This test box can simulate a temperature range of \(-40\)\(-\)\(-\)\(-\)\(-\)\(150\)\({}^{\circ}\)C, with a temperature control error of \(\pm 0.1\)\({}^{\circ}\)C, which enables the simulation of a permafrost environment. The lever loading method is employed in the test, and the loading rod features three holes. By changing the hole positions, the system can be amplified and loaded to the top of the pile. Before loading, the pressure sensor is placed on top of the pile. The first level of the load is the weight of the loading device, and the second level is the weight of the weight box. The weight of the loading device is displayed through the pressure sensor reading. The lever loading system and the low-temperature control system are illustrated in Figure 3. Displacement data are collected by directly connecting the digital dial indicator to the computer. The test adopts the fast-loading method. The load is maintained for 2 h, and the settlement of the pile's top converges; the next stage is then applied. If the settlement amount is twice the previous stage's load, it is considered to be damaged [17].  
Figure 1: Schematic diagram of screw pile in permafrost region.  
Figure 2: The particle size distribution curve.