## 3 Results and Discussion
### Effect of Steel Casing on Unit Skin Friction  
Unit skin friction of the pile shaft can be obtained by Equation (6).  
\[\text{T}=\frac{\text{P}_{\text{i}}-\text{P}_{\text{i}+1}}{\pi\text{Dh}} \tag{6}\]  
where T is the unit skin friction of the pile; P\({}_{\text{i}}\) is the axial force of the pile, i is the number of strain gauges, 1-6 from top to bottom; D is the diameter of the pile; h is the distance between two adjacent strain gauges.  
Figure 12: Axial force of steel tube-reinforced concrete piles with different steel casing lengths: (**a**) Steel casing length is 0 cm; (**b**) Steel casing length is 8 cm; (**c**) Steel casing length is 12 cm; (**d**) Steel casing length is 16 cm; and (**e**) Steel casing length is 20 cm.  
Unit skin friction of steel tube-reinforced concrete piles with different steel casing lengths is shown in Figure 13.  
As shown in Figure 13b,c, when the steel casing length is 8 cm or 12 cm, the unit skin friction of steel tube-reinforced concrete piles will reduce first, then increase, and finally decrease. Between the upper steel casing section and the lower reinforced concrete section, a smaller value of the unit skin friction is generated, and the maximum unit skin friction of the upper steel casing section is significantly larger than the lower reinforced concrete section. The main reason is the downward and lateral extrusion of the soil at the end of the upper steel casing section, so that a relatively small pile-soil displacement appears near the junction of the upper steel casing section and the lower reinforced concrete section, so the unit skin friction of this part is small. As for the main part of the unit skin, the friction of the upper steel casing section is significantly larger than the lower reinforced concrete section; the reason is that the superficial area of the upper steel casing section is larger than that of the lower reinforced concrete section.  
As shown in Figure 13d,e, when the steel casing length is 16 cm or 20 cm, the unit skin friction of steel tube-reinforced concrete piles will gradually reduce. Between the upper steel casing section and the lower reinforced concrete section, a smaller value of the unit skin friction is not generated. The main reason is that when the steel casing length is large, the lateral earth pressure at the end of the steel casing section is bigger [36], which will hinder the settlement and deformation of the soil at the end of the steel casing section; the upper steel casing section, the lower reinforced concrete section and the soil below the bottom of the steel casing section form a whole, hence the original unit skin friction of the smaller section will disappear.  
As shown in Figure 13a, the maximum unit skin friction of the reinforced concrete pile without steel casing is smaller than that of steel tube-reinforced concrete piles. The main reason is that the diameter of the steel casing is slightly larger than the borehole size, which has 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 shaft resistance.  
Figure 13: Unit skin friction of steel tube-reinforced concrete piles with different steel casing lengths: (**a**) Steel casing length is 0 cm; (**b**) Steel casing length is 8 cm; (**c**) Steel casing length is 12 cm; (**d**) Steel casing length is 16 cm; and (**e**) Steel casing length is 20 cm.