Research on Deformation and Force of Bridge Pile Foundation on High and Steep Slope in Mountainous Area

With the rapid development of my country’s economy, the demand for infrastructure construction is also increasing. However, in most areas of China, the terrains are mountainous and hilly. Some projects have to be built on steep slopes. Choosing viaducts or half-bridges on high-steep slopes is not only conducive to the protection of the surrounding environment, but also conducive to the stability of the slope. Bridges usually choose the form of pile foundation-high pier bridge. This paper uses numerical simulation to study and analyze the bridge pile foundation of the slope section. Relying on actual engineering, use the finite element software ABAQUS6.14 to establish a three-dimensional finite element model to study the bearing mechanism and mechanical characteristics of the pile foundation under vertical load, horizontal load and inclined load, discuss the influence of the nature of the soil around the pile and the stiffness of the pile body on the deformation and internal force of the bridge pile foundation in the slope section. The analysis results show that the horizontal load has a great influence on the horizontal displacement of the pile, but has a small influence on the vertical displacement, and the vertical load is just the opposite. Inclined load has obvious “p-Δ” effect. The increase in soil elastic modulus and pile stiffness will reduce the displacement of the pile foundation, but after reaching a certain range, the displacement of the pile foundation will tend to be stable. Therefore, in actual engineering, if the displacement of the pile foundation fails to meet the requirements, the hardness of the soil and the stiffness of the pile can be appropriately increased, but not blindly.


Introduction
With the rapid economic development, the pace of China's infrastructure construction is getting faster and faster. Especially in recent years, the country is vigorously developing mountainous areas. Most areas of China are mountainous and hilly. Some projects, such as railway and highway projects, must be adopted the structural forms of viaducts or half-roads and half-bridges, as well as projects across rivers and mountains, are found everywhere. Therefore, these situations lead to the need to build some pile foundations on high and steep slopes. The force and bearing characteristics of this type of pile foundation are very complicated. The bridge pile foundation on the slope cannot be simply regarded as the pile foundation on the flat ground, nor can it be regarded as the anti-slide pile.
At present, many scholars at home and abroad have done a lot of work on the bearing characteristics and bearing mechanism of bridge pile foundations on high and steep slopes. Among them, Zhao Minghua's research group has made many valuable research results. Bi Jihong [1] used the finite element software ANSYS to analyze the impact of vehicle braking on the slope, and divided the braking action into different working conditions for simulation, and obtained its impact on the slope pile foundation. Guo Yongjian, Xie Yongli, Jiang Li et al. [2] obtained through laboratory experiments and studies that end-bearing piles and friction piles behave similarly when the lateral load is small, and the vertical load has a greater impact on the friction pile. Lin Pengzhen, Wu Fahui, and Yang Zijiang [3] studied the influence of lateral load on the pile foundation of the slope and the stress characteristics of the slope with multiple bottom layers. Using finite element software to analyze, the results show that the front resistance of the slope has a great influence on the displacement and stress of the pile body. At the boundary of the soil layer, the displacement and stress of the pile foundation will change suddenly. Zhang Yongjie, Li Youjun, Zhao Minghua, etc. [4] proposed a calculation method for bridge pile foundations on high and steep slopes.
This method improves the uneconomical and low safety factor of traditional methods. Feng Zhongju, Wang Hang, Wei Jin et al. [5] analyzed the influence of slope and pile length on the bearing mechanism of the pile foundation on the slope based on the independently developed test platform. Zhao Minghua, Yang Chaowei, Chen Yaohao, et al. [6] conducted field test research to summarize and analyze the field test data, including pile shaft force, pile side resistance and pile bending moment. Lin et al. [7] believed that in the process of studying the impact of scouring on bridge pile foundations, the previous methods ignored the impact of the stress history of the soil before and after scouring on the pile foundation.
In summary, most of the current researches focus on the force and deformation characteristics of pile foundations on flat ground, and there is a lack of systematic theoretical research on the load transfer mechanism of bridge pile foundations on slopes. Therefore, further research on the bridge pile foundation on the slope has great practical engineering value.  Figure 1, the grid division of the model is shown in Figure 2, and the material properties of pile soil are shown in Table 1.

Model Verification
This article uses the model in the literature [6] for further analysis. On the basis of the literature test results, the finite element software ABAQUS6.14 is used for numerical simulation, and the rationality of the model and parameters selected in this paper is verified. The comparison between the calculation results and the test results is shown in Figure 3.

Single Vertical Load Action
Assuming that other loads are zero, and vertical loads V (10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa, 60 MPa) are applied in stages to simulate the deformation and stress of the bridge pile foundation.   mm. This is because of the slope, the length of the rear pile exposed to the soil is longer than that of the front pile, and the exposed part of the pile is not constrained by the rock and soil. The displacement of the rear pile is significantly greater than that of the front pile.

1) Analysis of pile foundation displacement
From the above analysis, it can be seen that the influence of the vertical load on the top of the pile on the vertical displacement is greater than the influence on the horizontal displacement. Therefore, the control of the vertical displacement is particularly important in the actual engineering mainly subject to vertical loads.
2) Analysis of internal force of pile foundation This is because on the high and steep slopes, the soil around the pile is no longer a semi-infinite space body, and the soil layer within the buried depth of 16m does not fully exert the side friction resistance.

Single Horizontal Load Effect
Assuming that other loads are zero, horizontal loads H (330 kPa, 660 kPa, 990 kPa, 1320 kPa, 1650 kPa, 1980 kPa) are applied in stages to simulate the deformation and stress of the bridge pile foundation.   It can be seen from Figure 9 that the bending moment first decreases along the pile body, then increases, then decreases, and finally tends to zero in the hard soil layer.  It can be seen that the horizontal displacement under the inclined load is not a simple superposition of the vertical load and the horizontal displacement under the horizontal load, but has an obvious "p-Δ" effect (Buildings with relatively flexible lateral stiffness will produce a large horizontal displacement Δ under the action of wind load or horizontal earthquake. Because the structure is under the action of vertical load P, the structure will further increase the lateral displacement value and cause the internal components of the structure to generate additional internal force). The existence of the "p-Δ" effect makes the inclined load act The horizontal displacement below is greater than the sum of the horizontal displacements of the two loads acting alone.

Inclined Load Effect
It can be seen from Figure 11 that under the action of inclined load, the vertical and horizontal displacements of the rear pile are larger than those of the front pile. This is because the rear pile has less soil in front of the pile, the length of the free section is longer, and the soil resistance is smaller. The horizontal and vertical displacement is greater. It can be seen that in actual engineering, the influence of the "p-Δ" effect on the pile foundation should be fully considered.
2) Analysis of internal force of pile foundation

The Influence of the Soil Properties around the Pile
In the actual engineering of high-steep slope bridge pile foundation, the geological conditions are very complicated, and the elastic modulus of soil is often very different. Therefore, it is necessary to study the influence of soil performance on pile foundation, keep the other parameters of the model unchanged, take the soil elastic modulus E = 0.25 E, 0.5 E, 1 E and 2 E.
It can be drawn from Figure 14, Figure 15.  In summary, the elastic modulus of soil of bridge pile foundations on high and steep slopes has a great influence on the deformation and stress characteristics of the pile foundation, and its influence on the horizontal displacement of the pile foundation is much greater than the vertical displacement of the pile foundation. The increase of the soil elastic modulus will reduce the pile displacement, but it will eventually stabilize. Therefore, in actual engineering, when the soil elastic modulus is large, the influence of other factors can be appropriately relaxed. On the contrary, if the soil elastic modulus is small, the bearing performance of the soil needs to be improved.

The Influence of Pile Stiffness
In actual engineering, the design of pile foundation stiffness is a very important link, which determines the safety factor of the entire project. Take the pile stiffness EI = 0.25 EI, 0.5 EI, 1 EI and 2 EI, and keep the other parameters unchanged to analyze the influence of the pile stiffness on the pile foundation of the high-steep slope bridge. The pile displacement and bending moment are shown in Figure 16 and Figure 17.
As can be seen from the above picture: 1) In summary, the influence of pile stiffness on the vertical displacement of bridge pile foundations on high and steep slopes is far greater than the horizontal displacement, and as the stiffness of the pile increases, its influence will continue to decrease until the stiffness of the pile is reduced. The influence is negligible. Therefore, in the design of pile foundation engineering, proper pile stiffness should be selected so that the displacement and bending moment of the pile are small, and the stiffness of the pile cannot be increased blindly. This will increase the project budget and cause unnecessary waste.

Conclusions
Based on an actual project, this chapter establishes a slope bridge pile foundation model through the finite element software ABAQUS6.14, and analyzes the force and deformation of the pile foundation under vertical, lateral and inclined loads.
The influence of the properties of the soil around the pile and the stiffness of the pile on the pile foundation is discussed. The main conclusions are as follows: 1) Under the action of a single vertical load, the vertical displacement of the pile body is relatively large, and it is mainly concentrated near the top of the pile, and the horizontal displacement of the pile body is small. As the pile shaft decreases, the pile side resistance increases to a certain range and tends to be stable.
2) Under the action of a single horizontal load, the horizontal displacement of the pile body is relatively large, mainly in the upper part of the pile body, and the vertical displacement of the pile body is small. The impact of horizontal load on the back pile will be slightly greater than that of the front pile. The maximum bending moment of the pile foundation appears at the top of the pile, and the maximum reverse bending moment appears when the pile length is about 6 m.
3) The pile foundation under the inclined load exhibits an obvious "p-Δ" effect. The existence of the "p-Δ" effect makes the horizontal displacement under the inclined load greater than the sum of the horizontal displacements of the two loads acting alone. The axial force and bending moment of the pile body show similar laws as the vertical and horizontal loads alone.
4) The nature of the soil around the pile, the stiffness of the pile, and the depth of rock socketing all have different degrees of influence on the pile foundation.
The elastic modulus of soil and the stiffness of the pile will mostly reduce the displacement of the pile foundation, but after reaching a certain range, the displacement of the pile foundation will tend to be stable. Therefore, in actual engineering, if the displacement of the pile foundation cannot meet the requirements, the hardness of the soil and the rigidity of the pile should be appropriately increased, but not blindly increased.