# Study on Deformation Characteristics of Retaining Structures under Coupled Effects of Deep Excavation and Groundwater Lowering in the Affected Area of Fault Zones

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{max}/Δ) of the retaining pile decreases approximately exponentially with the increase of the structural stiffness parameters (E and I) and the embedment ratio within a certain range. The sensitivity analysis of the lateral displacement of the retaining pile to different geological parameters is conducted, and the sensitivity factors of the geological parameters to the deformation of the retaining structure are obtained, namely the maximum internal friction angle, followed by the cohesion, and the elastic modulus is the smallest. Based on the original design plan, an optimization of the excavation design is proposed by reducing the stiffness of the support structure. Therefore, the research findings in this paper have significant theoretical and practical implications for the engineering design of excavation projects located in fault zones. By optimizing the excavation support system, not only can standardized construction procedures be achieved, but also investment costs can be reduced, and construction time shortened, which fully aligns with the current safety, economic, and sustainable design principles of excavation projects aiming to conserve resources.

## 1. Introduction

## 2. Background

#### 2.1. Project Overview

^{2}, a circumference of approximately 380 m and a maximum excavation depth of 16.8 m. The support structure is composed of piles and internal supports, with two internal supports set at an interval of 8.5 m. The pile diameter of the retaining piles and the column piles is 1.2 m, the pile length of the retaining piles is 25.8 m and the pile length of the column piles is 20.0 m. The section sizes of the crown beam, waist beam, and support beam are all 1.0 m × 1.2 m. The concrete strength level of the retaining piles, support beams, connecting beams, and column piles is C30. The cross-sectional dimensions and physical-mechanical property indicators of the support structure are shown in Table 2. The plan position of the excavation engineering and the excavation cross-section diagram are shown in Figure 1 and Figure 2.

#### 2.2. Geological Condition

## 3. Calculation of Deformation of Retaining Piles under the Coupled Effect of Excavation and Dewatering in Foundation Pit

_{a}and E

_{p}are the active and passive soil pressures, respectively, and E

_{0}is the active soil pressure at the bottom of the excavation.

_{i}is the axial force of the internal support i, s

_{i}is the deformation of the wall at the support i, x

_{i}and y

_{i}are the positions of the internal support, and θ

_{i}is the angle between the normal to the excavation side and the internal support i; n is the number of internal supports.

## 4. Numerical Simulation Calculation

#### 4.1. Three-Dimensional Model Establishment

#### 4.2. Boundary Conditions and Calculation Conditions

#### 4.3. Analysis of Numerical Simulation Results

## 5. Comparison and Analysis of Numerical, Theoretical, and Field Measurement Results

## 6. Analysis of the Influence of Structural Design Parameters and Geological Parameters

#### 6.1. Analysis of the Influence of Structural Design Parameters

#### 6.1.1. Analysis of the Influence of Elastic Modulus

_{max}/Δ) of the retaining piles is introduced. The growth rate of the maximum lateral displacement of the retaining piles decreases approximately logarithmically with the increase of elastic modulus. When the concrete strength grade is low, the maximum deformation of the retaining piles will be large and significantly affected by the elastic modulus. When the concrete grade is C20, the elastic modulus is 26 GPa, and the maximum lateral displacement value of the retaining piles is 9.85 mm, showing an obvious logarithmic increase. Although it does not exceed the deformation limit value, the maximum deformation value of the retaining piles increases exponentially. When the actual construction quality is poor, the deformation value of the retaining piles will change significantly, and even lead to excessive damage, posing great hidden dangers to the stability and safety of the foundation pit support. Therefore, from this perspective, to ensure that the retaining piles have sufficient stiffness, the concrete strength of the retaining piles in this project should not be lower than C25.

#### 6.1.2. Analysis of the Influence of Pile Diameter

_{max}/Δ) of the maximum lateral displacement of the retaining piles is introduced. The growth rate decreases approximately in a logarithmic function with the increase of the diameter. When the diameter is 0.8 m, the maximum lateral displacement of the retaining piles is 7.62 mm. When the diameter is further reduced, the increasing trend of the maximum lateral displacement approximately follows a logarithmic function, and the deformation of the retaining piles increases sharply, exceeding the deformation control range, and resulting in engineering quality and safety problems caused by the too-small stiffness of the retaining piles. From this point of view, it is considered that the diameter of the retaining pile affects its stiffness, and the diameter should not be less than the limit of 0.8 m to ensure the safety stiffness reserve of the retaining pile.

#### 6.1.3. Analysis of the Influence of Embedment Ratio

#### 6.2. Analysis of the Influence of Geological Parameters

#### 6.2.1. Analysis of the Influence of Internal Friction Angle

#### 6.2.2. Analysis of the Influence of Cohesion

#### 6.2.3. Analysis of the Influence of Elastic Modulus

#### 6.2.4. Sensitivity Analysis

_{1}, α

_{2}, α

_{3}, …, α

_{n}). The relationship expression between variable γ and parameter α is γ = f (α

_{1}, α

_{2}, α

_{3}, …, α

_{n}). Assuming that under a certain condition, the parameters are set to baseline parameters, the system baseline variable γ’ is obtained as f (α

_{1}’, α

_{2}’, α

_{3}’, …, α

_{n}’). By adjusting the changes in different influencing parameters on the system variable, the trend and degree of deviation of the system variable γ from the baseline variable γ’ are analyzed, which is called sensitivity analysis [30] of the parameters.

_{i}are defined as the ratio of the relative error, denoted as the sensitivity function S

_{i}(α

_{i}), with an approximate expression as shown in Equation (11):

_{i}’ into Equation (11), the sensitivity factor S

_{n}for α

_{i}’ is obtained. Under the baseline state, the sensitivity of γ to αi increases with the increase of S

_{n}. In comparison with the size of the sensitivity factor S

_{n}, the sensitivity is stronger when the sensitivity factor is larger and weaker when it is smaller. This paper conducts sensitivity analysis on geological parameters, namely, internal friction angle φ, cohesion C, and elastic modulus E.

## 7. Structural Design Optimization

## 8. Conclusions

- (1)
- Based on the small deformation theory, it is proposed to consider the coupled effect of excavation and dewatering during foundation pit construction and use the energy method of elasticity theory to solve the analytical solution for the deformation of retaining piles, which yields more desirable calculation results.
- (2)
- By comparing and analyzing the results of the retaining wall deformation theory calculation, finite element calculation, and field measurement data, the numerical rules are basically consistent. Simplified calculation only considers rotational deformation and ignores the translational deformation of the wall, resulting in a large deviation between the theoretical calculation results of the wall bottom deformation and the measured values. To reduce the deviation between numerical results and measured values, strict measures such as “zone excavation, avoiding peripheral loading, and optimizing construction deployment” should be taken in pit construction, strengthen construction monitoring, and reduce the impact on retaining wall deformation.
- (3)
- The maximum deformation growth rate k (ΔS
_{max}/Δ) of the retaining wall decreases exponentially with the increase of the structural stiffness parameters (E and I) and the embedment ratio in a certain range. To ensure the safety of retaining wall deformation, the retaining wall design must have a certain reserve of stiffness and embedment ratio. - (4)
- With the increase of geological parameters, the lateral displacement of the retaining wall gradually decreases, and the decreasing rate gradually decreases within a certain range, that is, the maximum deformation of the retaining wall has a nonlinear relationship with the geological parameters. A quadratic function is used to fit the sensitivity function S(x) of the maximum lateral displacement of the retaining structure to the changes in various geological parameters, and the sensitivity analysis of geological parameters is carried out. It is found that the internal friction angle is the most sensitive factor, followed by cohesion, and the elastic modulus is the smallest.
- (5)
- The structural optimization plan includes replacing the original concrete struts with steel struts, adjusting the diameter of the perimeter piles from 1.2 m to 1.0 m, and increasing the maximum deformation of the perimeter piles from 5.21 mm to 13.63 mm to meet the specification’s (0.3%H) limit. The optimization of the excavation support system not only enables compliance with the standard construction procedures but also reduces investment and shortens construction time, fully aligning with the current design principles of safety, economy, and sustainable development.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 8.**Comparison of numerical calculation results of horizontal displacement of retaining piles.

**Figure 20.**Comparison of sensitivity of geological parameters to maximum deformation of retaining pile.

Stratigraphic (Genetic) | Natural Weight (kN/m^{3}) | Tri-Axial Test Secant Modulus/MPa | Secant Modulus of Elasticity/MPa | Unloading Modulus of Elasticity/MPa | Poisson’s Ratio | Cohesion (kPa) | Internal Friction Angle (°) | Permeability Coefficient (m/d) |
---|---|---|---|---|---|---|---|---|

① Uncompacted fill soil | 19.0 | 2 | 2 | 6 | 0.35 | 14 | 15 | 5 |

② Clay | 18.5 | 2.8 | 2.8 | 8.4 | 0.33 | 25 | 18 | 0.1 |

③ Angular gravel | 19.5 | 6 | 6 | 18 | 0.28 | 0 | 36 | 25 |

④ Sandy clay | 21.0 | 8 | 8 | 24 | 0.26 | 25 | 22 | 5 |

⑤ Completely weathered rock | 20.0 | 16 | 16 | 48 | 0.24 | 23 | 28 | 0.1 |

⑥ Highly weathered rock | 20.5 | 25 | 25 | 75 | 0.23 | 20 | 32 | 0.5 |

Name | Material | Section Dimensions/mm | Unit Weight /(kg/m³) | Elastic Modulus/GPa | Internal Friction Angle/(°) | Poisson’s Ratio |
---|---|---|---|---|---|---|

Retaining pile (Diaphragm wall) | C30 | Thickness 880 | 2400 | 30 | 26 | 0.2 |

Column pile | C30 | Ø1200 | 2400 | 30 | 26 | 0.2 |

Crown beam | C30 | 1000 × 1200 | 2400 | 30 | 26 | 0.2 |

Support beam | C30 | 1000 × 1200 | 2400 | 30 | 26 | 0.2 |

Waist beam | C30 | 1000 × 1200 | 2400 | 30 | 26 | 0.2 |

Construction Stage | Construction Sequence | Working Conditions | Simulation Content |
---|---|---|---|

Prior to excavation of foundation pit | 1 | Initial seepage analysis | Seepage analysis before excavation of foundation pit, displacement reset. |

2 | Initial stress analysis | Stress analysis before excavation of foundation pit, displacement reset | |

During excavation of foundation pit | 3 | Diaphragm wall construction | Activate foundation pit retaining piles, column piles, water cutoff curtains, and other constraints. |

4 | Step 1 excavation | Dewatering the foundation pit to a depth of −9 m (initial water level −3 m). | |

5 | First dewatering | Excavate to a depth of −8.5 m underground, activate the second inner support, and waist beam. | |

6 | Step 2 excavation | Excavate to a depth of −8.5 m underground, activate the second inner support, and waist beam. | |

7 | Second dewatering | Dewatering to a depth of −18 m underground. | |

8 | Step 3 excavation | Excavate to a bottom depth of −16.8 m and activate the bottom plate. |

Indicators | Sensitive Factor S_{n} |
---|---|

Internal Friction Angle φ | 0.760 |

Cohesion C | 0.604 |

Elasticity Modulus E | 0.422 |

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**MDPI and ACS Style**

Niu, Y.; Zou, L.; Wang, Q.; Ma, F.
Study on Deformation Characteristics of Retaining Structures under Coupled Effects of Deep Excavation and Groundwater Lowering in the Affected Area of Fault Zones. *Sustainability* **2023**, *15*, 8060.
https://doi.org/10.3390/su15108060

**AMA Style**

Niu Y, Zou L, Wang Q, Ma F.
Study on Deformation Characteristics of Retaining Structures under Coupled Effects of Deep Excavation and Groundwater Lowering in the Affected Area of Fault Zones. *Sustainability*. 2023; 15(10):8060.
https://doi.org/10.3390/su15108060

**Chicago/Turabian Style**

Niu, Yungang, Liang Zou, Qiongyi Wang, and Fenghai Ma.
2023. "Study on Deformation Characteristics of Retaining Structures under Coupled Effects of Deep Excavation and Groundwater Lowering in the Affected Area of Fault Zones" *Sustainability* 15, no. 10: 8060.
https://doi.org/10.3390/su15108060