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The combined retaining structure has gradually received considerable attention in the slope engineering, due to its good reinforcement effects. However, most of the published research studies were focused on the seismic responses of the single-formal supporting structure only. The investigations of dynamic responses of the combined retaining structures are scarce, and the current seismic design is conducted mainly based on experiences. In this work, a series of large-scale shaking table tests were conducted to investigate the seismic responses of the combined retaining structures (i.e., prestressed anchor cables and double-row antisliding piles) and the reinforced slope under seismic excitations, including amplification effect of internal and surface acceleration of the reinforced slope, distribution and change of prestress of the anchor cable, dynamic response of soil pressure behind the antislide pile, and horizontal displacement of the reinforced slope surface. Test results show that, supported by the reinforcement of composite support system, the slope with the multilayer weak sliding surface can experience strong ground motion of 0.9 g. The load of the antisliding pile has reached 80% of its bearing capacity, and the load of the anchor cable has reached 75.0% of its bearing capacity. When the seismic intensity reaches 0.5 g, the slope surface has an obvious downward trend, which will make the corresponding soil pressure suddenly increase after the antislide pile. At the potential sliding zone, the axial force of the anchor cable will increase suddenly under the action of earthquake; after the earthquake, the initial prestress of the anchor cable will be lost, with the loss range of 17.0%∼23.0%. These test results would provide an important reference for the further study of the seismic performance of such composite support structure.

There are many mountains in Southwest China, so there are many slopes. Particularly, most slope projects in Sichuan Province are located in the areas with high seismic intensity. When strong earthquakes occur, these supporting structures (such as anchor cables and antisliding piles) are often damaged [

As an important mean to study the seismic performance of supporting structures, the shaking table test has been developing rapidly in the past decade. Lai et al. [

The above studies mainly focus on the seismic response of the single-formal support structure. However, the dynamic response of the slope reinforced by the composite support structure under earthquake action is very limited. Moreover, investigations related to the seismic responses of the slope reinforced by prestressed anchor cables and double-row antisliding piles are rather scarce, and the corresponding design method for such combined retaining structure is still unclear. Therefore, further in-depth study on the dynamic responses of prestressed anchor cables and double-row antisliding piles under the earthquake loadings must be available to improve the current seismic design.

To address these issues, a series of large-scale shaking table tests were conducted to investigate the seismic responses of a slope reinforced by prestressed anchor cables and double-row antisliding piles. Some meaningful conclusions and recommendations are obtained based on the analysis of test results.

Figure

Overlook of the prototype slope.

The typical section of the prototype slope.

Material parameters for the prototype slope.

Components | Density ^{3}) | Elastic modulus | Cohesion | Friction angle (°) | Poisson’s ratio |
---|---|---|---|---|---|

Slide bed | 2478.00 | 968.00 | 58.60 | 40.00 | 0.24 |

Sliding mass II | 2458.00 | 965.00 | 58.10 | 38.00 | 0.25 |

Sliding mass I | 1982.00 | 43.00 | 53.70 | 15.00 | 0.32 |

Potential sliding zone | 1820.00 | 1.28 | 8.20 | 12.00 | 0.26 |

The shaking table facility used for the tests allows input of three directions of earthquake records with independent control. The shaking table has 6 degrees of freedom, including 3 degrees of translation and 3 degrees of rotation, and the dimensions of which are 6.0 m by 6.0 m. At full load, the maximum acceleration could reach 1.0 g in the horizontal direction and 0.8 g in the vertical direction. The maximum displacements of the shaking table in the horizontal and vertical direction are ±150.0 mm and ±100.0 mm, respectively, and the loading frequency range of which is 0.1 Hz∼80.0 Hz. Additionally, a data acquisition system with 128 channels is adopted, which the maximum error can be controlled within 5.0%.

According to the scaling laws, three controlling parameters were selected, which are the dimension _{L}), density (_{ρ}), and acceleration (_{a}) were set to be _{L} = 100.0, _{ρ} = 1.0, and _{a} = 1.0 for this shaking table test, leading to the model slope height of 200.0 cm. Based on the Buckingham

Similarity ratios of the shaking table test.

Parameters | Dimensions | Similarity ratio |
---|---|---|

Physical dimension ( | [L] | _{L} = 100 |

Density ( | [M][L]^{−3} | _{ρ} = 1 |

Acceleration ( | [L][T]^{−2} | _{a} = 1 |

Elasticity modulus ( | [M][L]^{−1}[T]^{−2} | _{E} = 100 |

Stress ( | [M][L]^{−1}[T]^{−2} | _{σ} = 100 |

Strain ( | 1 | _{ε} = 1 |

Force ( | [M][L][T]^{−2} | _{F} = 1000000 |

Velocity ( | [L][T]^{−1} | |

Time ( | [T] | _{t} = 10 |

Displacement ( | [L] | _{u} = 100 |

Angular displacement ( | 1 | _{θ} = 1 |

Frequency ( | [T]^{−1} | _{ω} = 0.1 |

Damping ratio ( | 1 | _{λ} = 1 |

Internal friction angle ( | 1 | _{φ} = 1 |

The slope model was placed in a rigid box container with waterproof treatment which fixed on the shaking table, and the dimensions of the box are 325.0 cm × 150.0 cm × 250.0 cm (length × width × height), as shown in Figure

The rigid box container for the shaking table test.

Material parameters for the test model.

Material | Density ^{3}) | Elastic modulus | Friction angle (°) | Cohesion ^{−4} MPa) | Poisson’s ratio | |
---|---|---|---|---|---|---|

Slope | M 1 | 2.50 | 9.80 | 40.00 | 6.00 | 0.25 |

M 2 | 1.95 | 0.40 | 15.00 | 5.50 | 0.30 | |

Rock base | 2.70 | 10.10 | 42.00 | 7.40 | 0.25 | |

Potential sliding zone | 1.80 | 1.20 × 10^{−2} | 12.00 | 0.75 | 0.25 | |

Antisliding pile | 2.70 | 301.00 | Elastic material | 0.20 | ||

Anchorage segment | 2.50 | 290.00 | 0.20 |

After the model slope was completed, the prestressed anchor cables and double-row antisliding piles were used to reinforce the slope through the reserved holes. Considered to be rigid, the antisliding piles were made of concrete with a section of 2.0 cm by 3.0 cm, and the bending deformation of which were ignored in this work. As shown in Figure

Layout of sensors and detailed views of the antisliding pile and prestressed anchor cable.

For the prestressed anchor cables, as presented in Figure

The model slope and the retaining structures. (a) Schematic diagram of the model slope. (b) Prestressed anchor cable. (c) Antisliding pile. (d) Compaction test for the anchor cable.

As shown in Figure

All the abovementioned sensors are new, and calibration of which was conducted before the shaking table test. Moreover, to attenuate the boundary effect on the test results, all the earth pressure cells and axial force sensors were installed on the middle column of antisliding piles and prestressed anchor cables, and all the accelerometers and laser displacement meters were also installed in the middle section.

The seismic loading used in this shaking table test was the El Centro earthquake record which has been widely used in the earthquake engineering. Two simultaneous loading directions of seismic excitations were applied in this shaking table test, namely, the

The input El Centro earthquake motions. (a) The horizontal seismic excitation. (b) The vertical seismic excitation.

Loading sequence for the shaking table test.

No. | Seismic input | Amplitude (g) |
---|---|---|

1 | White noise | 0.05 |

2 | El Centro wave | 0.15 |

3 | El Centro wave | 0.30 |

4 | El Centro wave | 0.40 |

5 | El Centro wave | 0.50 |

6 | El Centro wave | 0.70 |

7 | El Centro wave | 0.90 |

The slope would have obvious nonlinear responses under strong seismic motions [

In this section, the ratio of the peak horizontal acceleration obtained on the slope surface or inside the slope to that collected by A14 is defined as the amplification factor. Figure

Amplification factors of the horizontal acceleration. (a) At the slope surface. (b) Inside the slope.

To research the seismic responses of prestressed anchor cables, the axial force of each anchor cable was measured, and the initial values of which before each excitation are listed in Table

Initial prestress values of the prestressed anchor cables before each seismic excitation (Unit: N).

No. | Amplitude | |||||
---|---|---|---|---|---|---|

0.15 g | 0.30 g | 0.40 g | 0.50 g | 0.70 g | 0.90 g | |

1 | 35.20 | 30.60 | 30.90 | 30.80 | 27.50 | 24.10 |

2 | 26.50 | 21.40 | 22.50 | 22.40 | 20.70 | 17.80 |

3 | 25.40 | 19.20 | 20.70 | 20.90 | 20.40 | 18.60 |

4 | 22.70 | 18.50 | 19.00 | 21.10 | 21.60 | 19.80 |

5 | 16.10 | 10.20 | 10.60 | 12.00 | 13.50 | 14.30 |

6 | 23.50 | 18.40 | 17.60 | 18.40 | 18.60 | 18.30 |

7 | 17.60 | 17.00 | 18.60 | 18.20 | 20.10 | 22.00 |

Time histories of 2# anchor cable under seismic motions with the amplitude of (a) 0.15 g; (b) 0.30 g; (c) 0.40 g; (d) 0.50 g; (e) 0.70 g; (f) 0.90 g.

Figure

Distribution of the peak axial force for all seven anchor cables under seismic motions with different amplitudes.

To further reveal the relationship between the initial axial force and the variation of axial force during shaking, the notation _{1} is the initial axial force of the anchor cable and _{2} denotes the peak value of axial force during earthquake loadings.

The increase rates of the axial force of anchor cables under the El Centro earthquake motions with different amplitudes are depicted in Figure

Variations of the increase rate of axial force under seismic motions with different amplitudes.

The occurrence time of the peak axial force for seven prestressed anchor cables under different excitations is shown in Figure

The occurrence time of the peak axial force of all seven anchor cables.

For the seismic design of the prestressed anchor cable, the prestress loss and the residual value of axial force after earthquake are of great significance. In this work, the notation _{1} is the initial axial force of the anchor cable and _{3} denotes the residual value of axial force after each seismic excitation.

The changing rates of axial force for each anchor cable under seismic excitations are presented in Figure

Variations of the changing rate of axial force under seismic motions with different amplitudes.

The test results show that the axial forces of the prestressed anchor cables in different slope areas are significantly different. It indicates that, for the current seismic design method, all the prestressed anchor cables are assumed to sustain the same load is inaccurate and uneconomic. In practice, the failure of one anchor cable can cause the failures of adjacent ones because of the chain reaction, which could lead to the slope failure. Therefore, the seismic response differences between anchor cables located in different areas should be fully taken into consideration in the seismic design. Additionally, to ensure the reliability of the prestressed anchor cables and the slope stability, specific design considerations should be adopted in the areas with different geological conditions.

The lateral earth pressures acting on the back of the antisliding Pile A and B under the excitations of El Centro earthquakes are shown in Figure

Lateral earth pressure acting on the back of (a) the pile B and (b) Pile A under the El Centro earthquake excitations.

The load-sharing ratios between the pile B and A under El Centro earthquake motions.

For the seismic responses of double-row antisliding piles, few studies were related to the load-sharing ratio. The ratios between the peak lateral earth pressure acting on the back of the Pile B and A are depicted in Figure

The horizontal displacements on the slope surface were measured by the laser displacement meters located at different locations throughout the slope height. In this work, the horizontal displacement towards the slope is defined as negative and that away from the slope is defined as positive. In Figure

Horizontal displacements on the slope surface under the excitations of El Centro earthquake motion. (a) Peak displacement, (b) Permanent displacement.

According to the test results, several conclusions can be drawn:

Comparing with the unreinforced part of the slope, the value and the increase rate of the acceleration amplification factor can be effectively controlled by the reinforcements of prestressed anchor cables and double-row antisliding piles, especially for the slope mass between the Pile A and B.

The maximum of prestress loss is 23.00%. When subjected 0.30 g∼0.90 g excitations, the maximum increment of axial force is 15.00%. It can be highlighted that the initial prestress of the anchor cable is suggested to be raised by 1.20∼1.30 times in the seismic design for the slope with high requirements of deformation control.

The lateral earth pressures acting on the back of the Pile A and B increase with the increasing amplitude of the input seismic motions. Comparing with the Pile B located at the slope toe, the earthquake loading undertaken by the Pile A located at the slope waist is obviously smaller, and the load-sharing ratios between the Pile A and B mainly changed in the range of 2.0∼5.0.

Under the seismic excitations, especially the input amplitude not larger than 0.5 g, the lateral displacements on the slope surface can be controlled by the combined retaining structures well. It can be concluded that, reinforced by prestressed anchor cables and double-row antisliding piles, the slope would have a good overall stability.

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that they have no conflicts of interest.

This work was supported by the National Natural Science Foundation of China (Grant no. 51808466) and Young Talents’ Science and Technology Innovation Project of Hainan Association for Science and Technology (QCXM201807).