Keywords

1 Introduction

With the further expansion of the modern city, the engineering geological conditions are becoming more and more complex, and the search for a safe and stable foundation has become the primary goal of the project builders. Among many foundations, pile foundation can be widely used due to its advantages of safety and strong adaptability. In the construction of pile foundation, there are three main modes: hammer method, static pressure method and vibration method [1]. Among them, the hammer method has the advantages of fast construction speed, small power needed, small space required for operation, good mobility and so on, and has become the most commonly used pile driving method.

At the same time, the hammer method also has many adverse effects. In the process of pile driving, it will cause the soil vibration around the pile. A project in Jiangan District, Wuhan city, Hubei Province, caused the vibration caused by the construction of pile foundation, which caused dispute cases and brought economic losses to the project. Secondly, in the process of piling, the continuous penetration of the pile makes the soil around the pile open around, thus producing the soil squeezing effect [2], Bring adverse effects to the project. Due to the crowding effect of a project in Zhaotong City, Yunnan Province, the vertical displacement of the soil is serious, which causes problems such as surface uplift. At the same time, the process of piling will also have a certain impact on the surrounding environment, resulting in noise, dust and other environmental pollution problems.

In order to ensure the safety and stability of pile foundation, many scholars at home and abroad have done a lot of research on the penetration process of pile body. The new method was applied to the calculation of the stress and strain at the pile end during the deep foundation construction [3]. Somebody proposed the problem of large prediction error of bearing capacity and optimized the pile driving process [4]. The severity of the vibration influence caused by the construction site is analyzed [5]. Two different piling methods, hammer and static pressure, were used to compare their ability to penetrate coarse soil layers of different thickness [6]. Somebody compared the penetration resistance and bearing capacity of model piles by pushing, impact and vibration piling in dry sand foundation and saturated sand foundation [7]. Somebody studied the penetration mechanism of sand pipe pile in the model test, and the results showed that at a certain depth, with the continuous increase of h/D [8].

Among the many research methods of penetration mechanism, numerical analysis method is a method of using simulation software to simulate the piling process, which has the advantages of high efficiency, short period and small cost, and has been widely used. Somebody used ANSYS/LS-DYNA software to simulate the penetration process of the pile body, and analyzed the displacement and stress changes of the soil during the penetration process [9]. Somebody used the finite element software to establish the model and study the vibration attenuation law caused by the piling process [10]. Somebody used ABAQUS to establish a three-dimensional finite element model to study the vibration influence of hammer strike pile driving [11]. By comparison with the measured data in the engineering, the vibration attenuation law of the soil in the process of hammer driving is summarized. Somebody used ANSYS/LS-DYNA to analyze the penetration mechanism of the pile body and the stress change process inside the pile body by using the hammer driving method [12]. Somebody compared the model test and numerical simulation of vibration pile installation in saturated sand to verify the existing simulation techniques [13]. Somebody used ABAQUS finite element software to establish a model of the vibration penetration process of steel pipe pile, and analyzed the impact of the vibration penetration rate, penetration resistance and the ratio of static load and dynamic load force [14].

In this paper, numerical simulation is used to simulate the piling process with ABAQUS finite element software, and to study the deformation behavior of soil, which provides some reference value for practical engineering.

2 Materials and Methods

2.1 Model Parameter

Since the elastic modulus of the pile and the difference of the soil, the deformation of the pile can be ignored, and the pile can be regarded as a rigid body part. In order to simplify the model, the pile is 20 m long, the diameter is 2 m, the pile end is 60° cone Angle, and the penetration depth is 15 m (7.5 times the pile diameter). Since the pile adopts analytically rigid body simulation, it is not necessary to set the material parameters of the pile.

The soil is modeled with an axially symmetric shell, 150 m (7.5 times pile length) wide and 200 m deep (10 times pile length), so as to reduce the influence of boundary conditions on the simulation results. Mohr–Coulomb model is used for the properties of soil, which is simple and practical, has few parameters and can well reflect the actual situation. It is widely used in the field of geotechnical technology. Model parameters of soil are shown in Table 1.

Table 1 Type styles
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2.2 Finite Element Model

Theoretically speaking, the static pressure pile will deform the surrounding soil in the process of penetration, and will bring the generation and dissipation of excess pore pressure. However, the law of soil stress distribution and the formation and dissipation of excess pore pressure is very complex, which is difficult to solve well by general theoretical methods. Therefore, because this paper studies the influence of penetration rate on piling, so the pore water pressure can be ignored.

The interaction between piles and soil during simulated pile penetration is a highly nonlinear deformation behavior, so defining the primary and secondary contact surface is adopted during simulated contact. Using the meshing technique of pile penetration with large deformation, Fig. 1. Different penetration rates were set at 7.5, 5, 2, 1, 0.2 cm/s to study the effect on pile end stress.

Fig. 1
A 3-D finite element model of the soil profile divided into segments is displayed.

Soil mesh division

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3 Results and Discussion

3.1 Stress Distribution Law in the Soil During Pile Penetration

Figure 2a shows the horizontal stress of the soil on the pile side (S11); Fig. 2b shows the stress in the vertical direction of the soil (S22).

Fig. 2
Two spectral images, a and b, display the distribution of stress in the soil along the pile, which is indicated by different shades.

Cloud diagram of stress distribution in soil side of pile when passing into 15 m

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According to the figure, the radial stress is the largest at the pile end and gradually decreases along the horizontal direction, the stress distribution is hemispherical, the range is 6 times the pile diameter, the radial stress below the pile end decreases rapidly; the maximum vertical stress is within 2 times the pile diameter range below the pile end. Compared with the law of radial stress distribution, the stress bubble of the vertical stress is small horizontal but large in the vertical.

3.2 The Crowding Effect of Pile Penetration

Figure 3 shows the cloud displacement of soil on the pile side when the pile body passes into 15 m.

Fig. 3
Two spectral images, a and b, present the distribution of stress in the soil along the pile side, which is indicated by various shades.

Horizontal and vertical displacement of the soil on the pile side when passing into 15 m

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During piling, the soil structure around the pile is disturbed, which changes the stress state of the soil and produces the squeezing effect. If not handled properly, it will cause damage to the surrounding road surface and buildings, so that the surrounding excavation pit will collapse or increase.

It can be seen from the figure, during the process of piling, the soil below the pile end is under pressure first, which causes the soil to move below. As the pile continues to penetrate, the soil began to move oblique, at the same time, the contact between the pile and the soil will produce the pile side of the resistance, making a certain vertical displacement, when the soil into a certain depth, the displacement of the soil above the pile end is very small, that is, when the penetration of the pile is not deep, the soil is loaded; when the pile continues to pass downward, the soil is unloaded.

Figure 4 shows the vertical settlement of the surface near the pile side; the vertical displacement of the soil is maximum near the pile wall and then decreases rapidly along the radial direction; at 2 times the pile diameter from the pile axis, indicating that the vertical displacement on the soil surface affects 2 times the pile diameter during the piling. In addition, with the increasing penetration depth of the pile body, the maximum uplift displacement of the surface on the pile side is small.

Fig. 4
A line graph plots the vertical displacement of soil versus distance from the axis of the pile. The line plotted for vertical displacement of soil depicts a steep downward trend initially, which flattens out with increasing distance from the axis of the pile.

Vertical displacement of ground surface near pile side

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3.3 The Influence of the Pile Penetration Rate

The cloud map of stress in the horizontal direction at different penetration rates 7.5, 5, 2, 1, 0.2 cm/s is shown in Fig. 5. As can be seen from Fig. 5, with the increase of the penetration rate, the distribution shape of soil resistance in the radial direction is roughly the same, that is, the penetration rate has little influence on the stress field distribution, but the stress conditions in different depths have certain changes, and it changes with the penetration depth as shown in Table 2. The graph of the change of pile end resistance with penetration depth and penetration rate is shown in Fig. 6.

Fig. 5
Five spectral images, a to e, display the distribution of radial stress in the soil along the pile, which is indicated by various shades.

Radial stress cloud diagram of 15 m at different penetration rates

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Table 2 Changes of pile end resistance with penetration rate
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Fig. 6
A line graph plots the pile tip resistance versus the depth of penetration. The lines are plotted for 0.2, 1, 2, 5, and 7.5 centimeters per second. All the lines depict an increasing trend.

Change diagram of pile end resistance with penetration depth

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It can be seen from Fig. 6 that as the penetration depth increases, the pile end resistance is also increasing, but the penetration rate is very little affected. For the soil of the same depth, increasing the penetration rate increases the stress weakly.

4 Conclusion

Through the above sorting and analysis of the integrated simulation results, the main conclusions can be summarized as follows:

The stress distribution below the pile end is hemispherical, the stress field diffuses from the pile end to the surrounding, and there is stress concentration at the pile end.

With the increase of penetration depth, the soil first produces downward compression deformation, then oblique extrusion, and in addition, the uplift displacement decreases with the increase of the distance from the pile axis, and becomes flat at 2 times the pile diameter.

With the increase of the penetration depth, the pile end resistance is also increasing, but its penetration rate is very little affected. For the soil of the same depth, increasing the penetration rate increases the stress weakly.