## 3 Results and Discussion
### Effect of Steel Casing on Tip Resistance  
Tip resistance of the pile can be obtained by earth pressure cell, which is installed at the bottom of the pile.  
The effect of steel casing on the tip resistance of steel tube-reinforced concrete piles is shown in Figure 14.  
As shown in Figure 14, the tip resistance of the pile increases gradually with the increase of pile top load; when the pile top load is small, the tip resistance of pile changes slowly; when the pile top load is large, the tip resistance of pile changes sharply. The main reason is that when the pile top load is small, the pile top load is mainly transmitted to the soil through the form of shaft resistance; when the pile top load exceeds a certain value, the shaft resistance has been fully utilized, and the pile top load will be transmitted to the soil mainly through the form of tip resistance [36]. In addition, the tip resistance of the pile without steel casing is larger than the steel tube-reinforced concrete piles with different steel casing lengths. The main reason is that pile top load is utilized through the form of shaft resistance and tip resistance; the shaft resistance of the pile without steel casing is smaller than the steel tube-reinforced concrete piles with different steel casing lengths, so the tip resistance of the pile without steel casing is larger than the steel tube-reinforced concrete piles. Moreover, the longer the steel casing is, the greater the shaft resistance is, and so the smaller the tip resistance is. In a word, the presence of steel casing will reduce the tip resistance of pile foundation.  
The tip resistance under the ultimate bearing capacity will be further analyzed, as shown in Figure 15.  
Figure 14: Effect of steel casing on tip resistance.  
As shown in Figure 15, when the steel casing length is 0 cm, 8 cm, 12 cm, 16 cm, and 20 cm respectively, the tip resistance under the ultimate bearing capacity is 147.0 N, 92.8 N, 76.1 N, 69.1 N, and 53.0 N; the ratios of the tip resistance to the ultimate bearing capacity are 16.5%, 9.9%, 7.9%, 7.2%, and 5.5%. With the increase of steel casing length, the tip resistance of steel tube-reinforced concrete piles shows a gradual decreasing 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. Because the bearing capacity is composed of shaft resistance and tip resistance, the tip resistance decreases as the length of the steel casing increases. When the steel casing length is 8 cm, 12 cm, 16 cm, and 20 cm respectively, the reduction rate of the tip resistance under the ultimate bearing capacity is 36.9%, 48.2%, 53.0%, and 63.9%, see Figure 15 and Equation (7). From the above research, it can be seen that the existence of steel casing will greatly reduce the tip resistance of piles.  
\[\alpha_{\mathrm{t}}=\frac{\mathrm{F}_{\mathrm{t0}}-\mathrm{F}_{\mathrm{ti}}}{ \mathrm{F}_{\mathrm{t0}}} \tag{7}\]  
where \(\alpha_{\mathrm{t}}\) is the reduction rate of the tip resistance under the ultimate bearing capacity; \(\mathrm{F}_{\mathrm{t0}}\) is the tip resistance of the conventional reinforced concrete pile; and \(\mathrm{F}_{\mathrm{ti}}\) is the tip resistance of the steel tube-reinforced concrete piles with steel casing of 8 cm, 12 cm, 16 cm, and 20 cm.