# Experimental Study on Shear Mechanical Properties of Pile-Soil Interface Under Freezing Conditions
## 4 Analysis of Test Results
### Relation Curves of Shear Stress and Shear Displacements  
According to the load transfer method, Hooke's Law is employed to calculate the axial force at the measured point based on the measured strain. Then, the average shear stress of the pile-soil interface can be determined, as the axial force of the pile can be balanced by the shear stress of the pile-soil interface [22]. Figure 6 shows the relation curves of shear stress-shear displacement of the pile side at different depths. Under different test conditions, the shear stress-shear displacement curves at the lower part of the pile had obvious strain-softening characteristics. These curves could be categorized into three stages--an elastic deformation stage, a yield failure stage, and a residual stage. The shear stress at the pile-soil interface was different at different locations, showing that the shear stress decreased with the increase in depth under the same shear displacement condition. Moreover, the peak and residual shear strength also decreased with the increase in depth. For this type of curve, the peak value of shear stress is defined as the interface shear strength. \begin{table}
\begin{tabular}{c c c} \hline
**Test Condition** & \
\begin{tabular}{c} **Vertical Bearing** \\ **Capacity/N** \\ \end{tabular} &
\begin{tabular}{c} **Average Adfreezing** \\ **Strength/kPa** \\ \end{tabular} \\ \hline T = \(-2\)\({}^{\circ}\)C, \(\omega\) = 15\(\%\) & 1275.17 & 50.76 \\ T = \(-2\)\({}^{\circ}\)C, \(\omega\) = 20\(\%\) & 1697.96 & 67.59 \\ T = \(-5\)\({}^{\circ}\)C, \(\omega\) = 15\(\%\) & 1898.56 & 75.58 \\ T = \(-5\)\({}^{\circ}\)C, \(\omega\) = 20\(\%\) & 2167.99 & 86.31 \\ \hline \end{tabular}
\end{table} 
Table 6: The maximum vertical pressure on the pile top under different test conditions. Figure 6: Relation curves of shear stress–shear displacement of pile side at different depths. With the change in test conditions, the shear stress-shear displacement curves at the upper part of the pile had significant strain-hardening characteristics [23]. One of the premier causes of this is that the soil experienced a top-down cooling process, and thus the near-surface temperature was much lower. With the decrease in the temperature and increase in moisture content, the cementation between the ice in the frozen soil and pile were further enhanced. According to the provisions of the \"Standard for Geotechnical Test Methods\" (GB/T50123-2019, China) [18], for this type of stress-strain relation curve, when the shear displacement reaches 4 mm, its corresponding shear stress can be termed as the interface shear strength. The shear strength at the interface between the frozen soil and pile, under different test conditions, is presented in Table 7. The interface shear strength attained the maximum value at the upper part of the pile, which decreased with the increasing pile depth. The results demonstrated that the interface shear strength increases with the increase in water content and the decrease in temperature. Under the test condition of a 15% moisture content, the interface shear strength reached its maximum value of 65.17 kPa at the temperature of \(-2~{}^{\circ}\)C, while this increased to 145.88 kPa when the temperature dropped to \(-5~{}^{\circ}\)C, with an increasing rate of 124%. Under the test condition of a 20% moisture content, the interface shear strength reached its maximum value of 121.62 kPa at the temperature of \(-2~{}^{\circ}\)C, while this increased to 178.85 kPa when the temperature dropped to \(-5~{}^{\circ}\)C, with an increasing rate of 47%. Under the test condition of a \(-2~{}^{\circ}\)C temperature, the increasing rate of the interface shear strength reached 87% when the moisture content increased from 15% to 20%. While under the test condition of a \(-5~{}^{\circ}\)C temperature, the increasing rate of interface shear strength reached 23% when the moisture content increased from 15% to 20%.