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Article

Research on the Soil-Plugging Effect on Small-Diameter Jacked Piles through In Situ Testing and DEM Simulation

1
School of Urban Planning and Municipal Engineering, **’an Polytechnic University, **’an 710048, China
2
School of Civil Engineering, **’an University of Architecture & Technology, **’an 710055, China
3
Shaanxi Key Lab of Geotechnical and Underground Space Engineering, **’an 710055, China
4
**’an Traffic Engineering Institute, **’an 710300, China
*
Author to whom correspondence should be addressed.
Buildings 2022, 12(11), 2022; https://doi.org/10.3390/buildings12112022
Submission received: 26 August 2022 / Revised: 3 November 2022 / Accepted: 14 November 2022 / Published: 18 November 2022
(This article belongs to the Collection Innovation of Materials and Technologies in Civil Construction)

Abstract

:
Small-diameter jacked piles are widely used in civil engineering. The formation and development of the soil-plugging effect and surface frictional behavior of jacked piles have a high impact on the construction process and pile quality. Clarifying the developmental pattern of the soil-plugging effect and the change law of frictional force forms the premise of scientific construction and construction quality. Firstly, we carried out two groups of in situ tests on the small-diameter jacked piles, recording the relationship between penetration depth and resistance force. Then, the discrete element method (DEM) was used to analyze the mechanical behavior of the small-diameter jacked piles during the construction process. The particle flow code (PFC) 2D was used to carry out the DEM simulation. The research results show that pile resistance exhibited an irregular development trend as the construction process proceeded. There is a sudden change in pile resistance when the pile tip reaches the interface of certain soil layers. Both tests revealed the same phenomenon, yet both occurred at different depths. The DEM analysis showed that plug sliding was the main reason for the above phenomenon. The difference in strength and stiffness of adjacent soil layers causes the soil plug to slide, leading to a sudden change in pile resistance. When the upper layer is soft and the layer below is hard, this phenomenon is especially obvious. This also leads to a difference in the location of the sudden change in pile resistance between the two groups of tests. The research results of this paper can be helpful for revealing the relationship between the soil-plugging effect of small-diameter jacked piles and the development of pile resistance and also provides a reference for relevant engineering construction and design.

1. Introduction

Small-diameter jacked piles are widely used in urban foundation engineering because of their high bearing capacity, low cost, and low construction noise. Strengthening the uneven settlement of the existing building foundation through the use of jacked piles can not only raise the reinforcement quality of the foundation but also ensure building safety and environmental protection [1]. It can be applied to various soils and has become the most preferred scheme of foundation treatment. In order to understand its construction properties, researchers have used model tests, in situ tests, and numerical analysis to study the penetration characteristics and pile resistance of jacked piles.
In terms of tests, Lehane and Gavin [2] studied the relationship between the foundation stiffness and the bearing capacity of soil plugs through indoor model tests. Vesic [3] analyzed the change in pile tip resistance and proposed the critical depth of the pile according to the results of the field tests. Randolf et al. [4] studied the influence of foundation consolidation history on pile stress through parameterization analysis. In the process of pile penetration via jacked piles, the soil-squeezing effect and soil-plugging effect coexist. The existence of the soil plug has a great influence on the characteristics of pile penetration and the development of the bearing capacity, which makes the engineering characteristics of jacked piles very different from other kinds. Murthy et al. [5] studied the formation characteristics of soil plugs during pile construction using laboratory model tests. They also obtained the influence of pile diameter, pile pressure, and penetration depth on soil plugs based on data analysis. Hailei et al. [6] conducted experimental fluctuant research on the mechanical performance of model piles under different penetration rates and obtained the relationship between the pile penetration rates and the size of the soil plug. Based on the cavity expansion theory, Li et al. [7] proposed a calculation formula for pile resistance considering the influence range of pile tip resistance during the penetration process in soft soil layers. Lehane and Gill [8] and Cao et al. [9] obtained the displacement field of the soil around jacked piles and the deformation law inside the soil during pile penetration based on transparent soil material; the result is interesting, but transparent soil is still separate from natural sand, which should be further studied. Liu et al. [10] and Sang et al. [11] used the micro FBG-MEMS pressure-sensing technique to monitor the stress of the pile during the penetration process. They concluded that the pile resistance force (when constructing in a homogeneous soil layer) also increased in waves, and side resistance degenerated at the same depth. Kodsy and Iskander [12] analyzed the soil plug effect of 74 groups of jacked tubular piles with different sizes. With the help of fiber Bragg grating sensor technology, Kou et al. [13] analyzed the interaction feature of the frictional force between the inner wall and outer wall of the jacked tubular pile influenced by the soil plug. Han et al. [14] conducted field tests using both static and dynamic loads on jacked piles. They also compared the resistance force measured by static load tests with the resistance force obtained by static cone penetration tests.
On the other hand, various numerical simulation methods were established and used in the analysis of jacked piles, such as the conventional finite element method [15,16], arbitrary Lagrangian-Eulerian (ALE) method [17,18,19,20], coupled Eulerian-Lagrangian (CEL) method [21], material point method (MPM) method [22,23,24], smooth particle hydrodynamics (SPH) method [25,26], and the discrete element method (DEM) [27,28]. Hong and Xueyi [29] used FEM simulation to analyze the soil-squeezing effect of static pressure piles based on spherical cavity expansion theory. Lorenzo et al. [23] used the material point method to simulate the penetration process of jacked piles, proving the feasibility of the material point method. Ko et al. [21] simulated the driving process of open-ended piles in sand using the coupled Eulerian-Lagrangian (CEL) approach; the parametric analysis revealed that the plugging effect was mostly influenced by driving energy, followed by pile diameter and pile embedment depth. Hu et al. [30] used the Voigt model to treat the soil plug as an additional mass attached to the pile body and considered the compactness of the soil plug in their study. Their research shows that the thickness of the pile influences the expansion pressure as well as the dynamic effect of the soil plug on the pile. Liu et al. [31] established a discrete element model of the penetration response of jacked piles considering different diameters, which qualitatively reflects the changing trends in soil flow, soil plugging, pile resistance, and the stress distribution of the soil, but it is hard to directly use in practice. Among all numerical methods, DEM is particularly suitable for the mechanism analysis of soil materials because it provides a convenient and comprehensive alternative to investigate both the macroscopic and microscopic behavior of granular soil [28,32].
Most of the above studies focus on homogeneous soil layers. Few studies involve in situ testing or analyzing the formation and development mechanism of the penetration process, plugging effect, soil-arching effect, and other characteristics in detail. In this paper, the penetration process of a small-diameter jacked pile is analyzed through a set of in situ tests in the soft foundation of a garbage backfill engineering site. The discrete element method was used to simulate the penetration process of the in situ testing. The pattern of change for soil plug thickness and pile resistance are analyzed based on the in situ testing data and the result of the DEM simulation. The research results of this paper are expected to provide a reference for similar projects and provide a theoretical basis for the design and construction of small-diameter jacked piles.

2. In Situ Tests of Small-Diameter Jacked Pile

2.1. Engineering Background of the In Situ Test

The in situ testing in this paper was conducted during a strengthening engineering project in ** the soil plug. As a result, the bottom soil plug in a pile begins to bear the soil pressure at the pile tip, so the pile penetration resistance increases rapidly.
In Figure 9, the length of the soil plug began to increase when the penetration depth exceeded 2 m. This is because the pile tip entered the middle sand layer from the filling sand layer. As the compression modulus and strength of the middle sand layer are significantly higher than that of the filling sand layer, the soil arch temporarily failed. The friction status between the soil plug and the pile changed from static to dynamic, and the effect of the soil plug was ineffective at this moment. Therefore, the pile penetration resistance plummeted. During the 2–4 m stage, the length of the soil plug increased gradually, and the pile penetration resistance basically remained unchanged. After about 4 m, the soil plug formed again. It can be seen from Figure 2 and Figure 9 that the length of the soil plug no longer increased, and the pile penetration resistance began to rise rapidly again at the same depth.
Similarly, Figure 3 and Figure 10 show the relationship between penetration resistance and soil plug length for condition b. It can be seen that the development curve of soil plug length with penetration depth can be divided into three stages: 0–2 m, 2–4 m, and 4–6 m. The length of the soil plug increases linearly before the pile penetration depth reaches 2 m, which is in accordance with the slow linear increase stage of pile penetration resistance in the early period in Figure 10. This is because the surface soil in test condition b is composed of the plain fill layer, the compression modulus and strength of which are too low to form a soil plug. When the pile tip enters the medium sand layer, the soil plug forms slowly. At this time, the length of the soil plug no longer changes within the 2–4 m stage, and the pile tip resistance of the soil plug begins to work, meaning the pile penetration resistance rises sharply.
Similar to condition a, when the pile penetration depth reached 4 m, the pile touched the coarse sand layer, and the strength and stiffness of the coarse sand layer were greater than that of the medium sand layer. For the same reason, the effect of soil arching on the soil plug temporarily fails, with the soil plug length then gradually increasing, while the pile penetration resistance remains almost unchanged.

4.3.2. Lateral Resistance

In order to further understand the composition and change mechanism of the penetration resistance of the test piles, the frictional resistance between the soil particles and both the inner and outer surface of the piles was obtained by calculating the resultant force in the Y-direction, as shown in Figure 11 and Figure 12. It can be seen that the total friction is different in value from that in Figure 2 and Figure 3, but the overall trend is consistent, and it can also correspond to the changing trend in soil plug thickness.
By comparing and analyzing the changing pattern of the inner and outer frictional resistance, we concluded that the outer frictional resistance basically maintains linear growth with penetration depth and shows different slopes in different soil layers. This is because the outer frictional resistance is only related to the earth pressure acting on the pile and the interface property between soil and pile, so it is relatively stable.
On the other hand, the inner frictional resistance is influenced by the soil plugging-effect and soil-arching effect. It can be seen that the variation trend of the total frictional resistance is basically dominated by the inner frictional resistance. In Figure 11 and Figure 12, the inner and total frictional resistance changed dramatically at 1.5 m, 3 m, and 4 m, which is related to the sudden change in the soil plug length in the previous section. When the soil plug length is stable, the soil-plugging effect is obvious, and the inner frictional resistance is large. On the other hand, when the soil plug length increases, evidently, denoting that the soil plug is in the formation or reformation stage, the soil plug and the pile slide during this kind of situation; the static friction changes into sliding friction, and the soil-arching effect fails, meaning the friction is low.

5. Conclusions

By combining in situ test analysis and DEM simulation, this paper discusses the development mechanism of the soil-plugging effect on surface resistance during the penetration process of small-diameter jacked piles. Through the analysis of the test results and the comparison between the in-situ tests and the simulation results, it was found that the changing curve of pile penetration resistance of the two groups from the in situ tests showed a similar development pattern. Our detailed conclusion is as follows.
For the first test condition, penetration resistance increases with pile penetration depth, then decreases or remains unchanged after reaching a certain depth. Then, there is a second stage of continuous growth. For the second condition, the first stage was not observed. For both of the test conditions, the sudden change in pile penetration resistance occurred when the pile tip moved through the interface of two layers of soil, yet the depth of this sudden change was different for the two conditions.
The DEM analysis results show that the soil plug slides with the inner surface of the pile due to the difference in strength and stiffness between the different soil layers, which is the reason for the sudden change in the pile penetration resistance. When the soil layer is soft and hard, this phenomenon is relatively strong, which also explains the missing part of the first stage for condition b, as the first layer in the second condition is too soft for the soil plug to form quickly. The analysis results of this paper can be helpful for revealing the relationship between the soil-plugging effect of small-diameter jacked piles and the development of pile resistance and can also provide a reference for relevant engineering construction and design.
Future work includes the following aspects:
  • The influence of the simulation dimension of the DEM should be further discussed;
  • The friction characteristics between the soil and pile and the corresponding micromechanism should be studied;
  • A more accurate simulation method should be developed for the prediction of penetration resistance.

Author Contributions

X.W.: Data curation, Formal analysis, Writing—original draft. Writing—review & editing. Y.M.: Conceptualization, Funding acquisition, Methodology, Supervision. Y.Y.: Resources, Validation. R.W.: Visualization, Resources. D.Z.: Investigation, Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

Funding

The research described in this paper was financially supported by the National Natural Science Foundation of China (Grant No. 52178302), the Key R & D Projects in Shaanxi Province (No. 2020SF-373, 2021SF-523), and the Natural Science Basic Research Program of Shaanxi [grant number 2022JQ-375].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. In situ test of small-diameter jacked pile.
Figure 1. In situ test of small-diameter jacked pile.
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Figure 2. Penetration resistance curve for condition a.
Figure 2. Penetration resistance curve for condition a.
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Figure 3. Penetration resistance curve for condition b.
Figure 3. Penetration resistance curve for condition b.
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Figure 4. Soil container and pile model from the DEM model.
Figure 4. Soil container and pile model from the DEM model.
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Figure 5. Influence of loading rate on shear strength.
Figure 5. Influence of loading rate on shear strength.
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Figure 6. Modeling of soil particles.
Figure 6. Modeling of soil particles.
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Figure 7. Simulation results of condition a.
Figure 7. Simulation results of condition a.
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Figure 8. Simulation results of condition b.
Figure 8. Simulation results of condition b.
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Figure 9. Soil plug length curve for condition a.
Figure 9. Soil plug length curve for condition a.
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Figure 10. Soil plug length curve for condition b.
Figure 10. Soil plug length curve for condition b.
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Figure 11. Penetration resistance curve for condition a.
Figure 11. Penetration resistance curve for condition a.
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Figure 12. Penetration resistance curve for condition b.
Figure 12. Penetration resistance curve for condition b.
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Table 1. Soil layer distribution and engineering properties.
Table 1. Soil layer distribution and engineering properties.
LabelLayer NameThickness
(m)
Bearing
Capacity
(kPa)
Bulk Density/γ
(KN/m3)
Cohesive/ϲ
(kPa)
Angle of Friction/ϕ
(º)
Modulus of
Compressibility/Es
(MPa)
Artificial plain filling 0.5~3.98515.210104.8
Filling sand 0.3~2.415017.651812.5
Medium sand 0.7~5.420019/2820.3
Silty clay 0~0.516017.930225.1
Coarse sand 1.1~3.925019.5/3025.7
Table 2. Calibration results.
Table 2. Calibration results.
LabelLayer NameFriccb_tens(cb_shears)
102 kPa
KratioEmod/
104 kPa
Artificial plain filling 0.572.182.622.42
Filling sand 0.911.523.088.21
Medium sand 1.32/1.7115.34
Silty clay 1.5610.693.533.72
Coarse sand 2.21/3.1620.18
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Wang, X.; Mei, Y.; Yuan, Y.; Wang, R.; Zhou, D. Research on the Soil-Plugging Effect on Small-Diameter Jacked Piles through In Situ Testing and DEM Simulation. Buildings 2022, 12, 2022. https://doi.org/10.3390/buildings12112022

AMA Style

Wang X, Mei Y, Yuan Y, Wang R, Zhou D. Research on the Soil-Plugging Effect on Small-Diameter Jacked Piles through In Situ Testing and DEM Simulation. Buildings. 2022; 12(11):2022. https://doi.org/10.3390/buildings12112022

Chicago/Turabian Style

Wang, Xueyan, Yuan Mei, Yili Yuan, Rong Wang, and Dongbo Zhou. 2022. "Research on the Soil-Plugging Effect on Small-Diameter Jacked Piles through In Situ Testing and DEM Simulation" Buildings 12, no. 11: 2022. https://doi.org/10.3390/buildings12112022

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