# Experimental Study on Shear Mechanical Properties of Pile-Soil Interface Under Freezing Conditions
## 4 Analysis of Test Results
### P-S Curve Measured from Test  
Figure 5 shows the P-S (axial pressure-vertical displacement) curves of the pile from the pile-pressing test and the field test. The field test was conducted in permafrost regions on the Chumar'he High Plain between the Kunlun and Tanggula mountains [21]. From the comparison, it was found that the results obtained from the pile-pressing test have a good  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Depth/cm** & **0** & **8** & **16** \\ \hline
5 & \(-5.18\) & \(-5.24\) & \(-5.33\) \\
10 & \(-5.16\) & \(-5.18\) & \(-5.27\) \\
15 & \(-5.14\) & \(-5.17\) & \(-5.23\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of soil temperatures at different measuring points (unit: \({}^{\circ}\)C).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Location Depth** & **Moisture Content** & **Moisture Content** & **Rate of Change** \\ /**cm** & **Before Test** & **After Test** & **Rate of Change** \\ \hline
5 & 20.81\% & 19.30\% & 7.25\% \\
10 & 19.13\% & 18.82\% & 1.62\% \\
15 & 18.57\% & 17.51\% & 5.7\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of soil moisture content before and after test.  
agreement with the field test, which indicated that the observations from the laboratory-scale tests have a certain practical significance for actual pile systems in permafrost.  
The P-S curve obtained from the pile-pressing test was consistent with the data taken directly from field tests. When the load was small, the vertical displacement at the top of the pile gradually increased with the increasing load, presenting a rising trend in the ascending stage. At the initial stage of loading, the pile-frozen soil interface was well cemented, and the bond structure of ice crystal was not destroyed. Therefore, the shear strength of the pile-soil interface was great, which indicated that the vertical displacement could be produced only after multi-stage loading. When the load reached a certain value, the P-S curve showed an apparent steep drop, which meant that the frozen state of the pile-frozen soil interface appeared to be damaged. Subsequently, the shear strength of the interface began to decline.  
With the complete destruction of the frozen state between the interface, there existed a shear failure at the pile-frozen soil interface. At that moment, the pile continuously moved downward when the load was kept constant. In this case, the axial load on the top of the pile was entirely provided by the adfreezing strength at the interface between frozen soil and pile. The vertical bearing capacity of piles and the average adfreezing strength of the interface, under different test conditions, are given in Table 6. The average adfreezing strength increased with increasing moisture content, and this value also increased with the decreasing temperature. When moisture content varied from 15 to 20%, the average adfreezing strength increased from 50.76 kPa to 67.59 kPa under the test condition of a \(-2\)\({}^{\circ}\)C temperature, and this value increased from 75.58 kPa to 86.31 kPa under the test condition of a \(-5\)\({}^{\circ}\)C temperature. The variation in adfreezing strength change rate with temperature reached 8.27 kPa/\({}^{\circ}\)C under the test condition of a 15% moisture content, and this variation was 6.24 kPa/\({}^{\circ}\)C under the test condition of a 20% moisture content. Through a comparison of the adfreezing strength change rate, it was found that the temperature had a more significant impact on adfreezing strength.  
Figure 5: The vertical pressure–pile displacement curve: (**a**) pile-pressing test and (**b**) field test.