# Analysis of Slip Failure Characteristics and Support Deformation Law of Structural Planes and Rock Foundation Pits with Developed Karst Caves

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## Abstract

**:**

## 1. Introduction

## 2. Analysis of the Influence of Rock Slopes of Deep Foundation Pits with Karst Caves and Structural Planes on the Failure Mode

## 3. Numerical Simulation of Pile-Anchor Support Technology for Rock Slopes of Deep Foundation Pits

## 4. Numerical Simulation of Pile-Bracing Support Technology for Rock Slopes of Deep Rock Foundation Pits

## 5. Contrastive Analysis of Different Supporting Schemes of Rock Slopes of Deep Foundation Pits

#### 5.1. Supporting Analysis of the Pile Bracing Structure of the Rock Foundation Pit

#### 5.2. Analysis of the Combined Support of the Pile-Bracing Anchor of the Rock Foundation Pit

## 6. Analysis Based on the Foundation Pit Project of Sanhuan South Road Station of Xuzhou Metro Line 3

## 7. Analysis of Optimizing the Foundation Pit Supporting System Based on the Space Effect

## 8. Conclusions

- (1)
- For bedding rock slopes, the failure on the bedding rock side is prone to be affected by karst caves and thus changes the final sliding path to form a broken line landslide, while arc-shaped toppling failure is prone to be formed on the toppling rock side.
- (2)
- For bedding rock foundation pits, it is recommended to use prestressed anchor cables in conjunction with the anchor cables, which can reduce the maximum displacement by about 12%, while the anchor cable spacing needs to be strictly controlled to maximize the supporting effect of the anchor cables. However, the additional bending load from anchor cables on the pile body on the toppling rock side should be taken into consideration.
- (3)
- For the foundation pit side slope with a long longitudinal distance, the sectionalized support of the retaining pile could effectively restrict the deformation of the retaining structure and the range of rock landslides. Taking three times the spacing of piles as an example, the maximum displacement of the retaining piles in the bedding direction can be reduced by about 13% under the same spacing of piles.
- (4)
- On the basis of spatial effect support, the top of the foundation pit on the bedding rock side can be provided with a tie beam for overall reinforcement to maintain the integrity of the support structure. The tie beam can replace the anchor cable, and its support effect can reduce the maximum horizontal displacement by 13% compared with the pile anchor support in the toppling slope.
- (5)
- For the foundation pit side slope with sectionalized support, if the requirement of final displacement control on the bedding rock side could not be met, prestressed anchor cables could be set above the slip surface, which can further control the deformation of the pile body, while the toppling rock side slope can basically meet the maximum displacement requirements by adopting the sectionalized support design. At the same time, the spacing of piles in the bedding rock side can be extended by 0.5 times as that of the toppling side.
- (6)
- When using anti-sliding piles to divide the foundation pit section, it is necessary to calculate the stability against sliding of the anti-sliding piles so as to ensure that the integral damage will not occur.
- (7)
- From the results of monitoring and numerical simulation, the foundation pit of Sanhuan South Road Station is in a safe state. If the traditional pile-anchor support method is adopted, the spacing between piles can be enlarged by three times or the local stability can be improved by using the sectional support method.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**Failure process of a 4 m-wide rock mass with circular karst caves penetrating into the foundation pit at 60° structural plane dip angle. (

**a**) 5000 steps; (

**b**) 8500 steps.

**Figure 3.**Cross-sectional view of failure mode of rock mass within 50 m length of foundation pit under 60° dip angle of structural plane. (

**a**) failure mode of the foundation pit on the bedding rock side; (

**b**) failure mode of the foundation pit on the toppling rock side; (

**c**) section view of failure mode of the foundation pit.

**Figure 4.**Pile diameter of 0.8 m, 1.5 times pile spacing, cantilever pile support (three retaining piles).

**Figure 5.**Pile diameter of 0.8 m, 2.0 times pile spacing, cantilever pile support (three retaining piles).

**Figure 6.**Displacement curve of cantilever piles with different pile diameters at 1.5 times pile spacing (three piles).

**Figure 8.**Displacement curve of pile body with 3.0 times pile spacing (2 piles). (

**a**) The diameter of the cantilever pile is 0.8 m. (

**b**) The pile diameter is 0.8 m and the crown beam is set. (

**c**) The pile diameter is 0.8 m, and the crown beam and anchor cable are set.

**Figure 9.**Displacement curve of pile body with 4.0 times pile spacing (2 piles). (

**a**) The diameter of the cantilever pile is 0.8 m. (

**b**) The pile diameter is 0.8 m, and the crown beam and anchor cable are set.

**Figure 10.**Numerical analysis of pile-bracing support with 1.5 times pile spacing. (

**a**) schematic diagram for setting analysis points of retaining piles (unit: m, scale: 1:100); (

**b**) failure and deformation cloud of bedding rock side; (

**c**) failure and deformation cloud of toppling rock side.

**Figure 11.**Stress analysis of pile-bracing support with 1.5 times pile spacing (pile body analysis). (

**a**) displacement diagram of the pile; (

**b**) bending moment diagram of the pile.

**Figure 12.**Analysis on the stress of pile-bracing support with 1.5 times pile spacing (analysis on the support beam). (

**a**) displacement of support beam; (

**b**) bending moment of support beam.

**Figure 13.**Analysis on the stress of pile-bracing support system. (

**a**) displacement of retaining piles with a pile spacing of 2.0 times; (

**b**) displacement of retaining piles with a pile spacing of 3.0 times; (

**c**) displacement of retaining piles with a pile spacing of 4.0 times.

**Figure 14.**Analysis on the stress of combined support system of pile-bracing-anchor with 4.0 times pile spacing. (

**a**) layout of anchor cables (unit: m); (

**b**) displacement cloud on bedding rock side; (

**c**) displacement of pile.

**Figure 17.**Monitoring point of foundation pit. (

**a**) monitoring point for inclination (unit: mm, scale: 1:200); (

**b**) monitoring points for stress and strain meters of retaining piles (unit: mm, scale: 1:50); (

**c**) monitoring points for stress and strain meters of support beam (unit: mm, scale: 1:200).

**Figure 20.**Reinforcement axial force curve of the top beam. (

**a**) side slopes on the toppling rock side; (

**b**) side slopes on the bedding rock side.

**Figure 21.**Strain curve of steel waist beam. (

**a**) side slopes on the toppling rock side; (

**b**) side slopes on the bedding rock side.

**Figure 23.**Stress analysis of sectionalized support system (design of sparse-dense pile). (

**a**) schematic diagram of sectionalized sparse-dense pile support model (Unit: m); (

**b**) displacement of retaining pile; (

**c**) displacement of pile body at reinforced point (bedding rock side); (

**d**) displacement of pile body at the reinforced point (toppling rock side).

**Figure 24.**Stress analysis of the sectionalized support system (arrangement of sparse-dense pile and tie-beam). (

**a**) schematic diagram of the support model of sparse-dense pile and tie beam (Unit: m); (

**b**) displacement of retaining pile body; (

**c**) displacement of pile body at reinforced point (bedding rock side); (

**d**) displacement of pile body at the reinforced point (toppling rock side).

Description | Density (kg/m³) | Young’s Modulus (Pa) | Poisson’s Ratio | Cohesion (Pa) | Internal Friction Angle (°) | Compressive Strength (Pa) |
---|---|---|---|---|---|---|

Limestone | 2300 | 21.0 × 10^{9} | 0.22 | 3.0 × 10^{6} | 30 | 1.0 × 10^{6} |

Description | Normal Stiffness (Pa) | Shear Stiffness (Pa) | Cohesion (Pa) | Angle of Internal Friction (°) | Compressive Strength (Pa) |
---|---|---|---|---|---|

Structural plane IV | 2.0 × 10^{9} | 1.0 × 10^{9} | 2.0 × 10^{5} | 15 | 1.0 × 10^{5} |

Description | Density (kg/m³) | Young’s Modulus (Pa) | Poisson’s Ratio | Cohesion (Pa) | Internal Friction Angle (°) | Compressive Strength (Pa) |
---|---|---|---|---|---|---|

Limestone | 2680 | 21.18 × 10^{9} | 0.17 | 3.0 × 10^{6} | 34 | 1.0 × 10^{6} |

The Type of Support | The Basic Parameters | Basic Law of Displacement Increase/Decrease after Pile Spacing Change in Bedding Slope (on the Basis of the Previous Level) | ||
---|---|---|---|---|

The Pile Diameter (m) | Pile Spacing (Multiple of Pile Diameter) | Longitudinal Length of Model (m) | ||

Pile-bracing | 1.0 | 1.5, 2.0, 3.0, 4.0 | 50 | 1.5 times to 2 times displacement increase by about 12.5%. 2 times to 3 times displacement increase by about 40%. 3 times to 4 times displacement increase by about 18%. |

Pile-brace-anchor combination | 1.0 | 4.0 | 50 | Displacement is reduced by about 12% compared to 4.0 times of the pile-bracing structure. |

Sectionalized support | 1.0 | 3.0 | 50 | Sparse-dense pile: displacement is reduced by about 14% compared with 3.0 times of pile-bracing. Sparse-dense pile and tie beam: displacement is reduced by 13% compared with 3.0 times of pile-bracing. |

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**MDPI and ACS Style**

Xu, J.; Wang, Y.
Analysis of Slip Failure Characteristics and Support Deformation Law of Structural Planes and Rock Foundation Pits with Developed Karst Caves. *Appl. Sci.* **2022**, *12*, 4076.
https://doi.org/10.3390/app12084076

**AMA Style**

Xu J, Wang Y.
Analysis of Slip Failure Characteristics and Support Deformation Law of Structural Planes and Rock Foundation Pits with Developed Karst Caves. *Applied Sciences*. 2022; 12(8):4076.
https://doi.org/10.3390/app12084076

**Chicago/Turabian Style**

Xu, Jin, and Yansen Wang.
2022. "Analysis of Slip Failure Characteristics and Support Deformation Law of Structural Planes and Rock Foundation Pits with Developed Karst Caves" *Applied Sciences* 12, no. 8: 4076.
https://doi.org/10.3390/app12084076