## 3 Results and Discussion
### Effect of Steel Casing on Shaft Resistance  
Shaft resistance of the pile shaft can be obtained by Equation (8).  
\[\mathrm{Q}_{\mathrm{s}}=\mathrm{Q}_{\mathrm{u}}-\mathrm{Q}_{\mathrm{t}} \tag{8}\]  
where \(\mathrm{Q}_{\mathrm{s}}\) is the shaft resistance of the pile; \(\mathrm{Q}_{\mathrm{u}}\) is vertical loading of the pile; and \(\mathrm{Q}_{\mathrm{t}}\) is the tip resistance of the pile.  
The effect of steel casing on the shaft resistance of steel tube-reinforced concrete piles is shown in Figure 16.  
Figure 15: Effect of steel casing on tip resistance under ultimate bearing capacity: (**a**) Tip resistance under the ultimate bearing capacity; (**b**) Reduction rate of tip resistance under ultimate bearing capacity.  
As shown in Figure 16, the shaft resistance of the pile increases gradually with the increase of pile top load; when the pile top load is small, the shaft resistance of the pile increases linearly; when the pile top load is large, the increase of shaft resistance gradually slows down. The main reason is that when the pile top load increases gradually, the shaft resistance of the pile side gradually tends to limit. The shaft resistance of the pile without steel casing is smaller than steel tube-reinforced concrete piles with different steel casing lengths, which is due to the fact that the diameter of the steel casing is slightly larger than the borehole size, which has an extrusion effect on the soil, and with the increase of the length of the steel casing, the extrusion area of the steel casing and the soil increases, and more axial forces are transmitted to the soil in the form of the shaft resistance. In a word, the presence of steel casing can enhance the shaft resistance of the pile foundation.  
The shaft resistance under the ultimate bearing capacity will be further analyzed, as shown in Figure 17.  
As shown in Figure 17, when the steel casing length is 0 cm, 8 cm, 12 cm, 16 cm, and 20 cm respectively, the shaft resistance under the ultimate bearing capacity is 745.6 N, 845.0 N, 881.4 N, 895.5 N, and 915.2 N, and the ratios of the shaft resistance to the ultimate bearing capacity are 83.5%, 90.1%, 92.1%, 92.8%, and 94.5%. With the increase of steel casing length, the shaft resistance of steel tube-reinforced concrete piles shows a gradual increasing trend. The main reason is that the diameter of the steel casing is slightly larger than the borehole size, which has an extrusion effect on the soil, and with the increase of the length of the steel casing, the extrusion area of the steel casing and the soil increases, and the larger the shaft resistance that can be obtained. When the steel casing length is  
Figure 16: Effect of steel casing on shaft resistance.  
Figure 17: Effect of steel casing on shaft resistance under ultimate bearing capacity: (**a**) Shaft resistance under the ultimate bearing capacity; (**b**) Lifting rate of shaft resistance under ultimate bearing capacity.  
8 cm, 12 cm, 16 cm, and 20 cm respectively, the lifting rate of the shaft resistance under the ultimate bearing capacity is 13.3%, 18.2%, 20.1%, and 22.7%, see Figure 17 and Equation (9). From the above research, it can be seen that the existence of steel casing will increase the shaft resistance of the piles. However, when the length of the steel casing exceeds 12 cm, the increase of the length of the steel casing has little effect on the shaft resistance.  
\[\alpha_{\mathrm{s}}=\frac{\mathrm{F_{si}-F_{s0}}}{\mathrm{F_{s0}}} \tag{9}\]  
where \(\alpha_{\mathrm{s}}\) is the lifting rate of the shaft resistance under the ultimate bearing capacity; \(\mathrm{F_{s0}}\) is the shaft resistance of the conventional reinforced concrete pile; and \(\mathrm{F_{si}}\) is the shaft resistance of the steel tube-reinforced concrete piles with steel casing of 8 cm, 12 cm, 16 cm, and 20 cm.