# Experimental Study on the Effect of Pile-End Soil on the Pile Load Transfer Law
## 1 Introduction  
The working principle of pile foundations is that the pile foundation transfers the upper load to the soil around the pile and the soil at the bottom of the pile through contact between the side and bottom of the pile and soil, thus ensuring that the building meets the requirements of foundation stability and the allowable amount of deformation. Therefore, pile-soil interaction [1; 2] has become a problem that must be considered in the design of pile foundations. The pile transfers part of the load to the surrounding soil through the frictional shearing action between the pile side and the soil, and the rest of the load is transferred to the pile-end soil through the pile body. Pile section properties [3; 4; 5], the pile-soil contact surface [6; 7; 8], and pile-soil shear mode [9; 10; 11] all affect the interaction between the pile and the surrounding soil.  
The single pile bearing capacity calculation of the existing pile foundation is to simply add the pile-end resistance and the skin friction; the default pile-end resistance and pile-side friction resistance do not affect each other. However, a large number of existing research practices show that the pile-end soil affects the exertion of the skin friction, and the measured skin friction of a single pile varies with the strength of the pile-end soil or the pile-bearing stratum. Some studies [12; 13; 14; 15] have shown that there is an interaction between skin friction and pile-end resistance; it is mainly manifested in the local strengthening effect of the skin friction at the pile-end. The effect of this strengthening effect is related to the strength of the pile-end soil. The greater the strength of the soil at the pile-end, the more obvious the strengthening effect. Additionally, the mechanism of the strengthening effect of skin friction is explained through [PERSON]'s [16] failure mode of pile foundation. [PERSON] et al. [17] affirmed that the increase of soil strength at the pile-end has a strengthening effect on the skin friction through the measured skin frictionof the static load test of super-long piles and used the Mohr-Coulomb theory to analyze the effect of pile-end soil strength on skin friction. By quantifying the test results of the model test, [PERSON] et al. [18] found that there is a relationship between the frictional shear strength of the pile side and the strength of the pile-end soil. Some studies pointed out that there is a weakening effect of the skin friction at the pile-end. [PERSON] [19] and [PERSON] [20] pointed out that there is a weakening effect of the skin friction at the pile-end when the soil strength at the pile-end is low through the static load test of the grouted pile. The lower the strength of the soil at the end of the pile, the more obvious the weakening effect. Through numerical analysis, [PERSON] et al. [21] found that when the water content of the sediment at the pile-end is large, excess pore water pressure will appear on the pile-soil contact surface near the pile-end, thereby reducing skin friction. Skin friction at the pile-end shows a weakening phenomenon.  
The existing research on the different performances of skin friction at the pile-end is mainly manifested as the strengthening effect and the weakening effect. The studies are based on the presence of pile-end soils, and there are few quantitative studies of the effect of the presence or absence of pile-end soil on the skin friction of pile. This test compares the load transfer laws of two pile types, pile-end soilless compressive pile and conventional compressive pile, in a layered foundation with mainly clay soil. Our aim was to study the load transfer mechanism under vertical load and the difference in the performance of the skin friction traits to analyze and discuss the quantitative effect of the presence of pile-end soil on skin friction.