## 3 Results and Discussion
### Effect of Steel Casing on Bearing Capacity  
The load-settlement curve of steel tube-reinforced concrete piles is shown in Figure 9.  
As shown in Figure 9, under the same settlement, the longer the length of the steel casing, the greater the load the pile can bear. The load-settlement curve is always used to judge the ultimate bearing capacity of piles. There are two different criteria. In the first, the ultimate bearing capacity is the load when the settlement mutates. In the second, the ultimate bearing capacity refers to the load corresponding to the settlement of 5% of the pile diameter [35]. Since the settlement in Figure 9 does not mutate, the second method is used to determine the ultimate bearing capacity of piles. The ultimate bearing capacity of steel tube-reinforced concrete piles is shown in Figure 10.  
As shown in Figure 10, when the steel casing length is 0 cm, 8 cm, 12 cm, 16 cm, and 20 cm respectively, the vertical ultimate bearing capacity is 892.6 N, 937.8 N, 957.5 N, 964.6 N, and 968.2 N. With the increase of the steel casing length, the vertical ultimate bearing capacity 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 less than 12 cm, the vertical ultimate bearing capacity of steel tube-reinforced concrete piles increases rapidly, and when the steel casing length is greater than 12 cm, the vertical ultimate bearing capacity of steel  
Figure 10: Effect of steel casing on ultimate bearing capacity: (**a**) Ultimate bearing capacity; (**b**) Lifting rate of ultimate bearing capacity. Figure 9: Load–settlement curve. tube-reinforced concrete piles increase slowly. Additionally, when the steel casing length of steel tube-reinforced concrete piles is 8 cm, the lifting rate of ultimate bearing capacity is 5.1%, see Figure 10 and Equation (2), and it can be seen that the existence of the steel casing can improve the ultimate bearing capacity. When the steel casing length of steel tube-reinforced concrete piles is 16 cm, the lifting rate of ultimate bearing capacity is 8.1%; it is only 0.8% more than steel tube-reinforced concrete piles whose steel casing length is 12 cm. The main reason is that when the length of the steel casing exceeds a certain value, this means that the growth of length has not had much effect on the bearing capacity. \[\alpha_{v}=\frac{F_{i}-F_{0}}{F_{0}} \tag{2}\]  
where \(\alpha_{v}\) is the lifting rate of the ultimate bearing capacity; \(F_{0}\) is the ultimate bearing capacity of the conventional reinforced concrete pile; and \(F_{i}\) is the ultimate bearing capacity of the steel tube-reinforced concrete piles with steel casing of 8 cm, 12 cm, 16 cm, and 20 cm. According to Table 1, a similitude ratio of force is 1: n\({}^{2}\) and n is 100 in this study, so the prototype force is 10,000 times the model force. When the steel casing length of model piles is 0 cm, 8 cm, 12 cm, 16 cm, and 20 cm respectively, the vertical ultimate bearing capacity of model piles is 892.6 N, 937.8 N, 957.5 N, 964.6 N, and 968.2 N. So when the steel casing length of prototype piles is 0 m, 8 m, 12 m, 16 m, and 20 m, respectively, the vertical ultimate bearing capacity of prototype piles is 8926 kN, 9378 kN, 9575 kN, 9646 kN, and 9682 kN. The lifting value of ultimate bearing capacity is 452 kN, 649 kN, 720 kN, and 756 kN, see Equation (3).