# Settlement and Stress Characteristics of the Ground in the Project of a Double-Line Tunnel Undercrossing an Airport Runway in a Sandy Cobble Region

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## Abstract

**:**

## 1. Introduction

## 2. Background

## 3. Three-Dimensional Numerical Model

#### 3.1. FEM Model

#### 3.2. Material Model and Parameters

#### 3.3. Simulation of the Excavation Process

## 4. Settlement Caused by Shield Tunnel Excavation

#### 4.1. Ground Surface Settlement Trough

#### 4.2. Settlement of the Characteristic Point

^{−3}mm. Similarly, after excavation step 60, there was a very small decrease of the settlement and the rising value basically remained at 0.5 × 10

^{−3}mm. The magnitude of the increase was very small. Basically, it can be considered that the settlement is stable and the small amplitude increase of this settlement is probably caused by the formation of the shield shell and lining segment.

#### 4.3. Stratum Displacement at Different Depths

#### 4.4. Ground Surface Settlement for Different Burial Depths

#### 4.5. Pavement Structure Stress for Different Tunnel Burial Depths

_{R}of the asphalt concrete pavement was defined as:

_{R}= σ

_{SP}/K

_{s}

_{SP}is the splitting strength (MPa) of the asphalt concrete and K

_{S}is the tensile strength coefficient, Ks = 0.09 N

_{e}

^{0.22}/A

_{c}. Where A

_{c}is the road grade coefficient, 1.0 for expressway and Class I highway, and N

_{e}is the cumulative equivalent axle time in a lane within the design life; 1.8 × 10

^{7}was taken in this study. The allowable tensile stress of the pavement structure layer was calculated is 0.225 MPa. Considering the requirements for allowable tensile stress of the surface layer, the relationship between the tunnel buried depth and the pavement maximum tensile stress, the tensile stress area width was analysed, as shown in Figure 20. It can be concluded that there is a good linear relationship between the maximum tensile stress, the tensile stress area width and the tunnel buried depth; the judgment coefficients, R

^{2}, were 0.9279 and 0.9852, respectively. With decreasing tunnel burial depth, the tensile stress area width and the maximum tensile stress value of pavement structure increased. Even if the tunnel buried depth was very shallow (15.0 m), the maximum tensile stress of the asphalt pavement did not exceed the allowable tensile stress value; however, considering the influence of aircraft fatigue load after runway operation, the safety detection of the tensile area should be considered to prevent the occurrence of runway cracks and other hidden dangers.

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

FP | First pavement |

SP | Second pavement |

LT | Left tunnel excavation |

RT | Right tunnel excavation |

γ | Unit weight |

c | Cohesion |

ϕ | Internal friction angle |

v | Poisson’s ratio |

RQD | Rock quality designation |

Φ | Diameter |

L1 | Sections of runway midline |

L2 | Sections of sward midline |

RL1′ | Sections of runway midline when right-line tunnel excavation was finished |

RL2′ | Sections of sward midline when right-line tunnel excavation was finished |

LL1′ | Sections of runway midline when left-line tunnel excavation was finished |

LL2′ | Sections of sward midline when left-line tunnel excavation was finished |

L | Tunnel spacing |

H | Tunnel burial depth |

R | Tunnel radius |

C1 | Sward characteristic monitoring point |

D1 | Runway pavement characteristic monitoring point |

σ_{R} | Allowable tensile stress of asphalt concrete |

σ_{SP} | Splitting strength |

K_{S} | Tensile strength coefficient |

N_{e} | Cumulative equivalent axle time |

A_{c} | Road grade coefficient |

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**Figure 3.**Illustration of excavated tunnels undercrossing airport runway: (

**a**) plan view; (

**b**) typical cross-section A-A’.

**Figure 9.**Distribution layout of monitoring points: (

**a**) distribution of monitoring points; (

**b**) site map of settlement monitoring points; (

**c**) schematic diagram of monitoring points by hole-drilling method.

**Figure 13.**Settlement of soil layers at different depths under different conditions (

**a**) condition 1, (

**b**) condition 2 and (

**c**) condition 3.

**Figure 14.**Influence of buried depth on ground surface settlement. (

**a**) settlement trough. (

**b**) the relationship between maximum settlement and tunnel burial depth. (

**c**) settlement development.

**Figure 20.**Influence of buried depth on ground surface settlement. (

**a**) maximum stress vs. tunnel burial depth; (

**b**) tensile stress area width vs. tunnel burial depth.

Soil Layer Number | Unit Weight γ (kN/m^{3}) | Cohesion c (KPa) | Internal Friction Angle ϕ (°) | E_{s0.1–0.2}(MPa) | Poisson’s Ratio v |
---|---|---|---|---|---|

Miscellaneous fill | 19.0 | 10.0 | 8 | / | 0.32 |

Silty clay | 20.0 | 40.9 | 19 | 5.7 | 0.30 |

Pebble soil | 22.0 | 20.0 | 40 | 20 | / |

Weathered mudstone | 23.5 | 300.0 | 35 | 23 | 0.18 |

Test | Routine physical test | Direct shear test | Compression test |

Materials | Density (g/cm^{3}) | Elastic Module (MPa) | Poisson Ratio |
---|---|---|---|

Pavement | 2.5 | 10,000 | 0.25 |

Shield shell | 2.5 | 30,000 | 0.25 |

Initial grouting | 2.1 | 3 | 0.30 |

Hardening grouting | 2.1 | 20 | 0.30 |

Lining | 14.2 | 20,000 | 0.30 |

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**MDPI and ACS Style**

Zhao, X.; Li, J.; Liu, W.; Qiu, W.
Settlement and Stress Characteristics of the Ground in the Project of a Double-Line Tunnel Undercrossing an Airport Runway in a Sandy Cobble Region. *Appl. Sci.* **2022**, *12*, 12498.
https://doi.org/10.3390/app122312498

**AMA Style**

Zhao X, Li J, Liu W, Qiu W.
Settlement and Stress Characteristics of the Ground in the Project of a Double-Line Tunnel Undercrossing an Airport Runway in a Sandy Cobble Region. *Applied Sciences*. 2022; 12(23):12498.
https://doi.org/10.3390/app122312498

**Chicago/Turabian Style**

Zhao, Xuwei, Jia Li, Wei Liu, and Wenge Qiu.
2022. "Settlement and Stress Characteristics of the Ground in the Project of a Double-Line Tunnel Undercrossing an Airport Runway in a Sandy Cobble Region" *Applied Sciences* 12, no. 23: 12498.
https://doi.org/10.3390/app122312498