## 3 Analysis
### Analysis of Vertical Bearing Characteristics of Bored Piles
#### 3.4.3 Test Analysis of the Negative Friction on the Pile Side  
The direction of the relative displacement between the pile and soil determines the direction of pile side friction resistance. When the pile displaces downward relative to the  
Figure 6: P-S curve of static load test of bored piles empty bottom piles. Figure 7: Q-S curve of static load test of bored piles. soil, the soil on the pile side exerts upward friction resistance on the pile, which is called positive friction. When the soil around the pile collapses and displaces downward relative to the pile, the pile side friction resistance is downward, which is called negative friction. To test the negative friction resistance value of the pile side when a deep collapsible loess site is immersed in water, the hanging technique is utilized to determine the downward weight of the soil surrounding the bored piles while it is submerged in water. The immersion test lasted 69 days, comprising 62 days of immersion observation and 7 days following water cessation, and the water level in the immersion test pit was maintained at 30 cm during the observation period. Generally speaking, the change law of the average negative friction of each pile with the immersion time is consistent. The variation in the negative friction of DP 1 bored piles with the immersion time is displayed in Figure 8.  
In the early stage of 0-10 days of water immersion, the negative friction increased steadily with an increase in the water infiltration depth. As the water penetration depth increased from 10 to 24 days, the soil surrounding the pile underwent considerable collapsible deformation, and the negative friction increased dramatically. At 24 days, the water reached the pile's base, and the negative friction reached its initial high of 28.47 kPa. With continuous water infiltration from 24 to 28 days, the shear strength of the soil decreased, and a portion of the soil adhered to the pile wall slides, resulting in a modest decrease in the negative friction of the pile side. Negative friction was typically steady between 28 and 62 days. After the water stopped in 62 days, the soil around the pile was solidified by water loss, the ability of the soil attached to the side wall of the pile was again strengthened, and the weight of the soil attached to the pile increased slightly, resulting in a second peak value of negative friction of 29.43 kPa on the fourth day after the water stopped. The largest impact of negative friction on the pile foundation occurred during the steady phase of immersion and consolidation of the soil around the pile following water stoppage. Table 6 lists the highest negative friction for each test pile. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Pile** & **Pile** & **Downward** & **Negative** & **Average** \\
**Number** & **Diameter/mm** & **Load/kN** & **Friction/kPa** & **Friction/kPa** \\ \hline BP 1 & 500 & 447 & 29.43 & 27.90 \\ BP 3 & 500 & 414 & 26.38 & \\ BP 2 & 800 & 672 & 26.75 & \\ BP 4 & 800 & 656 & 26.11 & \\ NP 1 & 800 & 540 & 21.50 & \\ NP 2 & 800 & 623 & 24.80 & 23.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Statistical table of negative friction resistance of test piles. Figure 8: Curve of negative friction of pile No. 1 with time. In Table 6, BP is the bored pile, and NP is the non-squeezed soil pouring pile. As listed in Table 2, when the pile diameter was 500 mm, the average negative friction of the bored piles was 27.9 kPa. When the pile diameter was 800 mm, the average negative friction of the bored piles and non-extruded pouring piles were 26.43 and 23.15 kPa, respectively. The investigation indicates that the diameter of the pile did not affect the negative soil friction surrounding it. The average negative friction of bored piles with a pile diameter of 800 mm was 14% greater than that of a non-extruded pouring pile with the same pile diameter.