Comprehensive Guide to Anti-Slide Piles: Design, Types, and Applications

Introduction to Anti-Slide Piles

Anti-slide piles are structural elements inserted into stable strata below potential sliding surfaces to counteract landslide thrust through anchorage effects. Widely used in slope reinforcement, landslide stabilization, bridge abutments, and tunnel reinforcement projects, these piles offer significant advantages in geotechnical engineering.

Key Advantages:

1. High anti-sliding capacity with excellent retaining effects

2. Minimal disturbance to slope stability during construction

3. Flexible positioning options

4. Rapid stabilization capability for unstable slopes

5. Preventive installation potential (piles can be installed before excavation)

6. Dual-purpose functionality – pile holes can serve as exploration shafts to verify slip surface location and direction

Types of Anti-Slide Piles

The selection of pile type depends on multiple factors:

• Landslide characteristics and scale

• Geological conditions

• Properties of sliding bed materials

• Construction constraints and timeline requirements

Common Failure Modes

Understanding failure mechanisms is crucial for proper design:

Special Note for Flow-Plastic Soils:
Low friction between the pile and the soil matrix necessitates:

• Connection plates/beams between piles

• Smaller cross-sections with reduced spacing

• Careful excavation practices to prevent slope acceleration

Design Fundamentals

Core Requirements

As passive stabilization structures, anti-slide piles require some slope deformation before becoming fully effective. Ideal for:

• Slopes with clearly defined potential slip surfaces

• Soil, soil-rock composite, and fractured rock slopes

• Projects with moderate deformation tolerance

Optimal Placement:

• Lower slope sections with gentle slip surfaces

• Multiple rows in a staggered (quincunx) pattern for long slip surfaces

Design Criteria:
✔ Transfer landslide thrust to stable strata, achieving the required safety factors
✔ Ensure slope material neither over-tops piles nor extrudes between them
✔ Maintain adequate pile stability through proper dimensions and spacing
✔ Provide sufficient structural strength with optimized reinforcement
✔ Balance safety, constructability, and cost-effectiveness

Design Process Flow

1. Investigate landslide causes, properties, and stability status

2. Calculate landslide thrust using geological profiles and shear strength parameters

3. Determine pile locations based on terrain and construction factors

4. Preliminary sizing: length, anchorage depth, cross-section, spacing

5. Establish calculation width and foundation coefficients

6. Compute deformation coefficients to determine rigid/elastic pile classification

7. Calculate displacements, internal forces, and lateral stresses

8. Verify stratum capacity; adjust parameters as needed

9. Generate shear and moment diagrams

10. Conduct reinforcement design for RC piles

Force Systems

Primary external forces include:

• Landslide thrust

• Up-slope resistance (above the slip surface)

• Anchorage zone resistance (below the slip surface)

• Side friction/adhesion

• Base reaction forces

Landslide Thrust Calculation

Thrust distribution depends on:

• Landslide characteristics

• Slip surface geometry

• Pile location and spacing

Calculation Assumptions:

1. Divide the slope into vertical slices along the main sliding axis

2. Parallel thrust direction to the local slip surface

3. Unit width calculation (ignore side friction)

4. Consider the slope as a continuous, non-compressible medium

Force Components:

• Basic: Self-weight (Wi), residual thrust (Ei-1), support force (Ei), bed reaction (Ni), shear resistance (Ti)

• Special: External loads (Pi), hydrodynamic pressure (Di), buoyancy (Si, Si’), seismic forces (Esi)

Recommended Method: Reduced shear resistance calculation

Thrust Distribution Patterns

Distribution varies with slope type and ground conditions:

Pile-Soil Interaction

Resistance Mechanisms

1. “K” Method: Constant foundation coefficient (intact rock/hard clay)

2. “m” Method: Linearly increasing coefficient (dense soils/fractured rock)

Elastic Resistance:
σ_y = C * y_x
Where:
C = Foundation coefficient
y_x = Displacement at depth y

Up-Slope Resistance

Factors affecting resistance:

• Slope volume and shear strength

• Slip surface roughness

• Presence of multiple slip surfaces

Determination Methods:

1. Landslide thrust curve (limit equilibrium)

2. Passive earth pressure (Rankine theory)

3. Foundation coefficient method (ignoring slip surface)

Note: Resistance becomes zero if up-slope material is excavated

Anchorage Zone Resistance

Two calculation approaches:

1. Intact Rock: Treat as semi-infinite elastic space (chain method)

2. Fractured Rock/Soil: Elastic medium approach (foundation coefficient method)

Critical Design Parameters

1. Layout and Spacing

Optimal Location: Lower slope sections with:

• Gentler slip surfaces

• Smaller thrust forces

• Easier construction

Spacing Guidelines:

• Typically 6-10m center-to-center

• Minimum 2.5 × short side diameter

2. Cross-Section Design

• Rectangular: Long side parallel to sliding direction (clear movement)

• Circular: Unclear movement direction

• Minimum Dimension: 1.25m width

3. Anchorage Depth

• Initial estimate: 1/4-1/3 of total length

• Final determination via calculation

4. Support Conditions

5. Rigid vs. Elastic Classification

• Rigid Pile: βh₂ ≤ 1.0 or αh₂ ≤ 2.5

• Elastic Pile: All other cases

Structural Design

• Designed as flexural members (deformation checks typically unnecessary)

• Materials:

  • Concrete: Grade C30

  • Main Reinforcement: HRB 400 steel

  • Stirrups: HRB 335/400 steel

Analysis Methods

International Practice

• Above slip surface: Cantilever beam analysis

• Below the slip surface: Finite difference method

Chinese Methods

1. Cantilever Method: Conservative approach

2. Foundation Coefficient Method:

   • “K” method (constant coefficient)

   • “m” method (linear variation)

Note: China’s Railway Subgrade Retaining Structure Design Code (TB 10025) recommends the cantilever method

Anchored Anti-Slide Piles

Advantages over Conventional Piles:

1. Reduced bending moments → smaller sections

2. Active stabilization through pre-tensioning

3. Better displacement control

4. Faster stabilization

Design Assumptions:

1. Model as laterally constrained elastic foundation beams

2. Anchor displacement = pile displacement at connection

3. Each anchor carries thrust from adjacent piles

Conclusion

Proper anti-slide pile design requires a comprehensive understanding of geotechnical conditions, precise thrust calculations, and appropriate selection of pile parameters. Engineers must balance structural requirements with practical construction considerations to develop effective, economical slope stabilization solutions.