## 1. Introduction

Pile foundations have been used for several years in bridge and building engineering to support lateral and vertical loads. Some projects, such as bridges, accommodation projects, and others located in complex terrain and landforms typically set piles in sloping ground to reduce the amount of earthwork required. These pile foundations are affected by both the upper vertical and lateral loads as well as the pile-side landslide thrust. As a result, their formation mechanism, load distribution, and horizontal resistance influenced by the lateral soil are highly complex [

1,

2,

3,

4], and the soil–structure interaction is also very important [

5,

6,

7,

8]. Furthermore, the pile foundation requires different bearing performances when it is located on slopes with different angles [

9].

An early study on a single pile was mainly aimed at the estimation of ultimate bearing capacity. Broms [

10] assumed the lateral pile-soil pressure’s distribution and then used statistical analysis as an approximate solution for the single pile’s ultimate lateral resistance. Randolph [

11] put forward a 2D Finite Element Method (FEM) solution that modelled the soil and the pile as an elastic continuum and elastic beam, respectively. To overcome the limitations of 2D methods, many attempts were made to perform the pile under combined loading by converting 2D to 3D cases based on FEM. Fan and Long [

12] modelled the soil and the pile as elastic–plastic material and linear–elastic material, respectively, then introduced 3D FEM analysis method for laterally-loaded piles in soil. Chik et al. [

13] developed this further with the introduction of PLAXIS software; moreover, the Mohr–Coulomb elastoplastic model was used for the soil model.

In recent years, many analytical methods for the response analysis of piles have been developed under combined loads. Comodromos et al. [

14] presented a simple method for designing a pile foundation under combined loading. Begum and Muthukkumaran [

15] achieved the research of laterally-loaded piles on a sloped surface through changing the slope angle and the relative density of the soil. Chien et al. [

16] reported the full-scale shaft load tests with axial load or lateral load only. It could be seen from the test results that 63% of the pile head displacement produced a combined load corresponding to the lateral load under the same lateral load. Xie et al. [

17] performed tests on a single pile located at different positions on the slope with horizontal loading as well as unloading, which concluded that the pile strain was reduced as the distance from the slope increased. Zhang et al. [

18] simulated the pile load and deformation with the combined effect of the slope’s vertical and horizontal loads. The study pointed out that the vertical load could effectively make the pile’s bending moment and lateral displacement decrease under combined force. Kershaw and Luna [

19] explored some combined-load field tests by applying a static axial and lateral load on the micropiles. The concrete-embedded strain gauges were used to measure the distribution of load and pile-head, load-deformation response and bending moment with depth along the micropile length. Zuo et al. [

20] analyzed the force on an all-straight pile foundation under the action of a two-way cyclic load, which illustrated that the force on a pile became redistributed, and the pile body axial force increased gradually while the side friction resistance decreased with the combined action of horizontal and vertical cyclic loading. Muthukkumaran and Begum [

21] studied the p–y curves of the slope, the embedded depth of the pile, and the soil’s density by using laboratory experiments. It revealed that the soil resistance increased with the pile’s buried length and the growth of soil density. Jegatheeswaran and Muthukkumaran [

22] explored the pile’s horizontal displacement by examining the vertical and horizontal loads on the pile foundation from different angles.

The previous studies, mentioned above, investigated the relationship between the pile-soil failure mode and the pile-soil system’s stiffness in the plane. However, there is little research on the failure mode of lateral load piles on slopes. The passive soil pressure area of the lateral load pile on the slope is smaller than that of the pile in the ground, which will affect the displacement and the pile’s bending moment, thereby affecting the structural design of the pile. Therefore, it is necessary to deeply research the piles’ bending moment and lateral displacement for varying slope angles. In this paper, model tests and ABAQUS FE analysis software were used to analyze the distribution of soil pressure around piles, the difference of pile head displacement under different loads, and the variation of internal force of pile on different slopes.

## 7. Conclusions

The mechanical performance of a single pile under combined load is very significant in engineering design. This article used experimental and finite element numerical calculation methods to explain the slope angle’s influence on the single pile lateral response under combined loads. The variation law of earth pressure around the pile, pile head displacement change with load, and the change of the pile internal force were fully studied.

A self-made loading system was used to conduct the experiment. With the slope gradient increase, the horizontal thrust force acting on a pile generally increased; however, the single pile’s horizontal bearing capacity was gradually reduced. The soil’s thrust behind the pile increased with the slope angle increase, while the resistance of the pile decreased. Increased slope gradient and slope risk caused an increment of the bending moment and the horizontal displacement.