# Numerical Study on the Coupling Effect on a Retaining Structure of a Complex Deep Foundation Pit Group Excavation in a Soft-Soil Area

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## Abstract

**:**

## 1. Introduction

## 2. Project Overview

#### 2.1. Project Description

#### 2.2. Site Description

#### 2.3. Retaining Structure and Construction Process

## 3. Numerical Model

#### 3.1. Finite Element Model

#### 3.2. Model Parameters

#### 3.3. Construction Simulation

## 4. Results and Discussion

#### 4.1. Lateral Wall Deflection

#### 4.2. Internal Support Axial Force

## 5. Influence of Construction Sequence

#### 5.1. Influence on Lateral Wall Deflection

#### 5.2. Influence on Internal Support Axial Force

## 6. Conclusions

- In the original construction scenario (Scenario 1), the lateral deformation of the diaphragm walls at A-2 and C-4 showed the obvious coupling effect from the surrounding excavation, and the maximum change rates were 28.2% and 20.8%, respectively. The coupling effect on the lateral deformation of the diaphragm wall at A-1 and C-3 was not apparent, and the maximum change rates were 3.3% and 4%, respectively. However, the lateral deformation of the diaphragm wall at B-5 or B-6 was hardly affected by the excavation of other pits.
- In Scenario 1, the maximum axial force in Pit A was far greater than that in other pits, and it increased by 89.6% after the construction of Stage 2, indicating a strong coupling effect. In contrast, the construction of Stage 3 had little impact on the maximum axial force in Pit A and a certain degree of coupling effect on the maximum axial force in Pit C.
- After changing the construction sequence, the coupling effects on the lateral deformation of the diaphragm walls at A-1, A-2, C-4, and E-10 were obviously different. Among the six construction scenarios, the maximum change rates of maximum lateral deformation at the above positions were 15.9%, 7.5%, 15.4%, and 10.7%, respectively. However, the maximum change rates of maximum lateral deformation at other positions did not exceed 3%. The maximum change rates of the maximum axial force in Pit A and Pit F were 89.5% and 193.6%, respectively, and that in other pits was little affected by the construction sequence.
- The coupling effect of foundation pit group excavation on the lateral displacement of the diaphragm wall near the existing building was more significant, and the deformation development should be focused on. The constraint of the concrete cushion can effectively reduce the lateral displacement of the diaphragm wall. In addition, the maximum axial force of the support in the pit adjacent to the existing building was obviously affected by the coupling effect of the adjacent excavations.
- Due to the quantity and the irregular shape of the foundation pits in the foundation pit group, many different construction scenarios can be formulated. However, considering the impact of construction on traffic flow and the safety of the foundation pit group, the six scenarios studied in this paper are representative and reflect the coupling effects between each foundation pit during the construction of the foundation pit group, which can be used as a reference. The practical construction scheme must also be formulated by considering the on-site environment and various other factors.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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Soil/Rock | Thickness (m) | Unit Weight (kN/m^{3}) | Compressive Modulus (MPa) | Poisson’s Ratio | Cohesion (kPa) | Friction Angle (°) | Permeability Coefficient (cm/s) |
---|---|---|---|---|---|---|---|

Miscellaneous fill | 3 | 1990 | 4.1 | 0.30 | 15 | 12 | 2 × 10^{–3} |

Pain fill | 3 | 1790 | 3.77 | 0.35 | 19 | 16 | 2.5 × 10^{–5} |

Mucky silty clay | 25 | 1760 | 3.57 | 0.30 | 10.6 | 18.2 | 5 × 10^{–6} |

Clayey silt | 13 | 1770 | 4.2 | 0.35 | 10.8 | 19.8 | 4 × 10^{–6} |

Fine silt sand | 11 | 1840 | 11.2 | 0.28 | 2.3 | 33.9 | 1.63 × 10^{–3} |

Limestone | 25 | 2750 | - | 0.36 | 800 | 40 | - |

Soil/Rock | Compression Index λ | Swelling Index κ | Critical State Parameter M | Initial Void Ratio e_{0} |
---|---|---|---|---|

Miscellaneous fill | 0.095 | 0.006 | 0.543 | 0.98 |

Pain fill | 0.083 | 0.009 | 0.607 | 0.99 |

Mucky silty clay | 0.127 | 0.008 | 0.673 | 1.06 |

Clayey silt | 0.081 | 0.005 | 0.735 | 0.90 |

Fine silt sand | 0.054 | 0.003 | 1.418 | 0.72 |

Structure Type | Weight (kN/m^{3}) | Elastic Modulus (MPa) | Poisson’s Ratio |
---|---|---|---|

Diaphragm wall | 25 | 3.15 × 10^{4} | 0.2 |

Concrete support | 25 | 3.25 × 10^{4} | 0.2 |

Steel pipe support | 78 | 2.1 × 10^{5} | 0.3 |

Concrete cushion | 25 | 2.55 × 10^{4} | 0.2 |

Retaining pile (equivalent wall) | 16.36 | 1.54 × 10^{4} | 0.2 |

Calculation Step | Construction Stage | Construction Process |
---|---|---|

0 | Initial stress equilibrium | |

1 | Construction of surrounding buildings | |

2 | Stage 1 | Construction of diaphragm walls |

3 | Dewatering (level 1) | |

4 | Excavation (level 1) | |

5 | Casting the struts (level 1) | |

6 | Dewatering (level 2) | |

… | … | |

29 | Stage 2 | Construction of diaphragm walls |

30 | Dewatering (level 1) | |

31 | Excavation (level 1) | |

32 | Casting the struts (level 1) | |

33 | Dewatering (level 2) | |

… | … | |

48 | Stage 3 | Construction of diaphragm walls |

49 | Dewatering (level 1) | |

50 | Excavation (level 1) | |

51 | Casting the struts (level 1) | |

52 | Dewatering (level 2) | |

… | … |

Construction Stage | Excavation Depth |
---|---|

Stage 1–1 | Pit A is excavated to −15 m. |

Stage 1–2 | Pit A is excavated to −29 m. |

Stage 2–1 | Pit B and Pit C are excavated to −15 m. |

Stage 2–2 | Pit B is excavated to −22 m and pit-in-pit is excavated to −29 m. Pit C is excavated to −23.8 m and pit-in-pit is excavated to −27.7 m. |

Stage 3–1 | Pit D and Pit E are excavated to −15 m. |

Stage 3–2 | Pit D is excavated to −22 m. Pit E is excavated to −23 m. Pit F is excavated to −4.5 m. |

Scenario No. | Construction Sequence | ||
---|---|---|---|

Stage 1 | Stage 2 | Stage 3 | |

Scenario 1 (original) | Pit A | Pit B, Pit C | Pit D, Pit E, Pit F |

Scenario 2 | Pit A | Pit D, Pit E, Pit F | Pit B, Pit C |

Scenario 3 | Pit B, Pit C | Pit A | Pit D, Pit E, Pit F |

Scenario 4 | Pit B, Pit C | Pit D, Pit E, Pit F | Pit A |

Scenario 5 | Pit D, Pit E, Pit F | Pit A | Pit B, Pit C |

Scenario 6 | Pit D, Pit E, Pit F | Pit B, Pit C | Pit A |

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**MDPI and ACS Style**

You, X.; Zhou, Q.; Xiao, Y.; Tong, L.; Yang, Q.
Numerical Study on the Coupling Effect on a Retaining Structure of a Complex Deep Foundation Pit Group Excavation in a Soft-Soil Area. *Appl. Sci.* **2023**, *13*, 3263.
https://doi.org/10.3390/app13053263

**AMA Style**

You X, Zhou Q, Xiao Y, Tong L, Yang Q.
Numerical Study on the Coupling Effect on a Retaining Structure of a Complex Deep Foundation Pit Group Excavation in a Soft-Soil Area. *Applied Sciences*. 2023; 13(5):3263.
https://doi.org/10.3390/app13053263

**Chicago/Turabian Style**

You, Xinyu, Qiulong Zhou, Yu Xiao, Liyuan Tong, and Qiang Yang.
2023. "Numerical Study on the Coupling Effect on a Retaining Structure of a Complex Deep Foundation Pit Group Excavation in a Soft-Soil Area" *Applied Sciences* 13, no. 5: 3263.
https://doi.org/10.3390/app13053263