# Experimental Study on Characteristics of Pile-Soil Interaction in Screw Piles

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## Abstract

**:**

## 1. Introduction

_{cr}) and the bearing capacity were derived.

## 2. Materials and Methods

#### 2.1. Model Test Object and Similarity Ratio

#### 2.2. Test Apparatus and Material

_{cu,k}= 20 MPa). The mixture ratio was calculated using numerous experiments, and the compressive strength was found to be 21.89 MPa (Table 2). The Young’s modulus of concrete was 21.85 GPa. Meanwhile, no reinforcement was installed in the screw teeth for consistency with the practical condition.

#### 2.3. Loading Procedure

## 3. Results

#### 3.1. Macroscopic Shear Characteristics

_{ult}of the plane shear plate under the overburden pressure of 40 kPa to 160 kPa was 9.00 kN, 13.92 kN, 18.07 kN, and 21.08 kN, respectively. In order to further determine the contact relationship between the pile–soil interface, the Q

_{ult}–p

_{0}variation curve of the plane shear plate is drawn (Figure 8), where δ

_{0}and c

_{0}are the friction angle and cohesion of the pile–soil interface, respectively, and p

_{0}is the overburden pressure. The curve presents a linear change trend, and the intercept is not zero. This indicates that, even for conventional circular piles, the interaction between the pile and soil is not only a frictional contact [9,29,30]. However, the interaction may involve more complex contact relationships, including mechanical occlusion, bonding, and chemical cementation, macroscopically shown as c

_{0}. This problem should be considered when analyzing the pile–soil interface interaction to avoid oversimplifying the theoretical model and actual difference.

_{ult}–b curves were drawn and analyzed. The variation curves of the ultimate bearing capacity of the screw shear plates with different screw pitches are shown in Figure 9.

_{h}, φ, and c are the lateral earth pressure, internal friction angle, and soil cohesion, respectively.

#### 3.2. Mechanical Characteristics of Screw Teeth

#### 3.2.1. Soil Stress

_{cr}) was drawn under pressures of 40 kPa and 120 kPa (Figure 13). The distribution law of soil stress was shown to be similar to that of the 8b screw shear plate. In other words, when the screw pitch was larger than the critical pitch, the final distribution law of soil stress was similar, including the influence range of the screw teeth and the distribution form of soil stress.

#### 3.2.2. Soil Deformation

#### 3.3. Theoretical Analysis

_{t}.

_{0}+ c

_{0}under ultimate load, where δ

_{0}and c

_{0}are the friction angle and cohesion between the pile–soil interface, respectively. The inclination angle of the screw teeth is δ

_{1}, and the angle between p

_{t}and normal stress is δ

_{2}. The calculation method for δ

_{1}and δ

_{2}is as follows:

_{1}and t

_{2}are the bottom and top thicknesses of the screw teeth, respectively (Figure 16a).

_{0}and c

_{0}can be obtained from the relationship curve of Q

_{ult}–p

_{0}of the plane shear plate (Figure 8). The fitting results show that δ

_{0}= 0.50φ and c

_{0}= 0.57c.

_{t}on OA was projected on the virtual surface O’A, which is perpendicular to p

_{t}. The corresponding pressure is p, as shown in Figure 16b. According to the geometrical relationship, the conversion relationship between p

_{t}and p can be obtained as follows:

- (1)
- It was assumed that the vertical pressure on the CD surface is uniformly distributed and that the vertical pressure on the DE surface increases uniformly from p
_{0}to p_{E}, where p_{0}is the overlying soil pressure and p_{E}is the normal stress at point E. - (2)
- The friction force and cohesion on the CE surface were not considered.
- (3)
- The plastic curved surface AB was simplified as a plane. The normal and shear stresses were assumed to increase linearly. The normal and shear stresses at points A and B were p
_{A}and τ_{A}and p_{0}and τ_{0}.

_{f}and tangential stress τ

_{f}on the AE surface were solved. According to the force balance in the vertical and horizontal directions, the following could be obtained:

_{0}is the length of the screw teeth on the screw shear plate.

## 4. Discussion

_{2}, the normal stress σ on the OA surface was assumed to be equal to the normal stress p

_{1}on the BD surface, which is inconsistent with the practical normal stress. Therefore, it needs to be amended. In this paper, the repeated iterations method is used to make the calculated value of δ

_{2}close to the accurate value. The calculated value was considered accurate if the error was less than a certain value.

_{t}on OA can be obtained. Subsequently, the corresponding normal stress σ could be obtained through the following steps:

_{2}could be obtained iteratively according to the following steps:

- (1)
- Assuming σ = p
_{1}, the initial value δ_{20}of the δ_{2}could be calculated. - (2)
- Substituting the value into the equations in Section 3.3 and calculating the normal stress σ on the surface OA again.
- (3)
- Substitute the new σ into Equation (2) to obtain the new value of δ
_{2}, which is δ_{21}. - (4)
- Compare δ
_{21}and δ_{20}; when $\frac{{\delta}_{21}-{\delta}_{20}}{{\delta}_{20}}\le \Delta $, δ_{2}= δ_{21}, where $\Delta $ is the criterion value, and it can be set at 0.1%. - (5)
- When $\frac{{\delta}_{21}-{\delta}_{20}}{{\delta}_{20}}>\Delta $, assign the value of the δ
_{21}to δ_{20}and repeat Equations (2)–(4) until the criterion in Equation (4) is satisfied to obtain the calculated value of δ_{2}.

_{2}are shown in Table 4. Obviously, the calculated value of the δ

_{2}is similar to the practical value after only three iterations.

_{cr}under the test conditions was 4.7b. The screw pitch of the 4b screw shear plate was less than the critical screw pitch. Assuming that the failure surface forms a cladding surface along the outer edge of the screw teeth, the calculation formula for the ultimate bearing capacity is ${Q}_{\mathrm{ult}}={A}_{\mathrm{T}}\left({p}_{0}\mathrm{tan}\phi +c\right)$. The bearing capacity of the 8b and 12b screw shear plates was calculated using Equation (10). Next, the theoretical formula was verified through the test results of the screw shear plates with different screw pitches (Table 5).

_{t}and Q

_{ult}are the theoretical and measured values of the ultimate bearing capacity of the screw shear plates, respectively. η is the deviation rate, which was calculated using the following formula:

## 5. Conclusions

_{cr}) and the bearing capacity were proposed and verified. The following are the major conclusions:

- (1)
- The bearing capacity of the screw shear plate was larger than that of the plane shear plate, indicating that the bearing capacity can be significantly improved by the screw pile compared with the circular pile.
- (2)
- With an increase in the screw pitch, the bearing capacity of the screw shear plate first increased and then decreased. There was an optimal screw pitch, enabling the bearing capacity and the bearing effect of the soil around the screw teeth to reach their maximum. The optimal screw pitch made the screw reach the IBF state, which is the critical screw pitch.
- (3)
- For the screw piles in the IBF state, the main influence range of the screw teeth was about 4–5b along the loading direction and 1–2b along the vertical direction. The corresponding values of the core influence zone were about 1–2b and 1b.
- (4)
- The rationality of the proposed method for calculating the bearing capacity of the screw shear plate and the critical screw pitch in the IBF state was verified by the test results. The calculation equation of s
_{cr}contains the shear strength parameters of the soil and the geometric parameters of the screw teeth, which have better applicability as s_{cr}than a general fixed value.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 4.**Monitoring arrangement of the 8b screw shear plate. (

**a**) Front view, (

**b**) top view of the earth pressure cell of Layer 1, and (

**c**) monitoring network of soil deformation.

**Figure 6.**Q–u curves under different conditions. (

**a**) 40 kPa, (

**b**) 80 kPa, (

**c**) 120 kPa, and (

**d**) 160 kPa.

**Figure 11.**Theoretical assumptions of the failure mode under the screw teeth. (

**a**) Meyerhof, (

**b**) Terzaghi, and (

**c**) arch.

**Figure 12.**Final soil stress distribution of the 8b screw shear plate. (

**a**) 40 kPa, (

**b**) 80 kPa, (

**c**) 120 kPa, and (

**d**) 160 kPa.

**Figure 14.**Variation of soil stress under different pressures. (

**a**) Q = 6 kN, u = 0.21 mm, (

**b**) Q = 18 kN, u = 2.14 mm, (

**c**) Q = 27 kN, u = 12.26 mm, and (

**d**) Q = 48 kN, u = 39.53 mm.

**Figure 15.**Final deformation of 8b screw shear plate. (

**a**) 40 kPa, (

**b**) 80 kPa, (

**c**) 120 kPa, and (

**d**) 160 kPa.

Physical Quantities | Similarity Constant |
---|---|

Geometry | C_{l} = 2 |

Modulus of elasticity | C_{E} = 1 |

Strain | C_{ε} = 1 |

Stress | C_{σ} = C_{E} C_{ε} = 2 |

Poisson ratio | C_{μ} = 1 |

Concentrated load | C_{F} = C_{σ} C_{l}^{2} = 4 |

Linear load | C_{q} = C_{σ} C_{l} = 2 |

Area load | C_{p} = C_{σ} = 1 |

Cement | Water | River Sand | Coarse Aggregate |
---|---|---|---|

1.00 | 0.55 | 1.44 | 2.08 |

Index | E_{s} (MPa) | φ (°) | c (kPa) | ρ (g/cm^{3}) | w (%) | e |
---|---|---|---|---|---|---|

Loess | 7.97 | 28.10 | 26.80 | 1.75 | 14.10 | 0.76 |

Iterations | p_{0}/kPa | tanδ_{2} | δ_{20}/° | σ/kPa | δ_{21}/° | Deviation/% | $\mathbf{if}\mathbf{\Delta}$ | Final δ_{2}/° |
---|---|---|---|---|---|---|---|---|

1 | 40.0 | 0.32 | 17.676 | 353.84 | 16.121 | 8.80 | NO | 16.031 |

2 | 0.29 | 16.121 | 369.26 | 16.035 | 0.53 | NO | ||

3 | 0.29 | 16.035 | 370.13 | 16.031 | 0.02 | YES |

s | p_{0} | 40 kPa | 80 kPa | 120 kPa | 160 kPa |
---|---|---|---|---|---|

4b | Q_{t}/kN | 19.28 | 27.76 | 36.24 | 44.72 |

Q_{ult}/kN | 17.86 | 23.72 | 34.55 | 40.95 | |

η/% | 7.95 | 17.03 | 4.89 | 9.21 | |

8b | Q_{t}/kN | 21.26 | 32.33 | 43.40 | 54.48 |

Q_{ult}/kN | 23.78 | 36.00 | 48.15 | 55.88 | |

η/% | −10.60 | −10.19 | −9.87 | −2.51 | |

12b | Q_{t}/kN | 16.90 | 25.43 | 33.97 | 42.51 |

Q_{ult}/kN | 19.69 | 26.22 | 31.71 | 39.54 | |

η/% | −14.17 | −3.01 | 7.13 | 7.51 |

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## Share and Cite

**MDPI and ACS Style**

Ma, J.; Luo, L.; Mu, T.; Guo, H.; Tang, Y.
Experimental Study on Characteristics of Pile-Soil Interaction in Screw Piles. *Buildings* **2022**, *12*, 2091.
https://doi.org/10.3390/buildings12122091

**AMA Style**

Ma J, Luo L, Mu T, Guo H, Tang Y.
Experimental Study on Characteristics of Pile-Soil Interaction in Screw Piles. *Buildings*. 2022; 12(12):2091.
https://doi.org/10.3390/buildings12122091

**Chicago/Turabian Style**

Ma, Jiakuan, Lijuan Luo, Tong Mu, Hongtao Guo, and Yong Tang.
2022. "Experimental Study on Characteristics of Pile-Soil Interaction in Screw Piles" *Buildings* 12, no. 12: 2091.
https://doi.org/10.3390/buildings12122091