8.4 Stability analysis of retaining walls

Retaining walls are crucial structures that hold back soil and prevent landslides. They face three main failure modes: overturning, sliding, and bearing capacity failure. Understanding these risks is key to designing safe and stable walls.

Stability analysis involves calculating safety factors for each failure mode. This process considers wall geometry, soil properties, and loading conditions. By evaluating these factors, engineers can ensure retaining walls will stand strong against the forces trying to topple them.

Retaining Wall Stability Assessment

Failure Modes and Analysis

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• Retaining walls resist lateral earth pressures and prevent soil movement
• Three primary failure modes require separate analysis
  • Overturning stability compares resisting moment to overturning moment
  • Sliding stability evaluates horizontal forces and frictional resistance
  • Bearing capacity failure occurs when soil cannot support applied loads
• Vertical stress distribution beneath wall foundation impacts bearing capacity and eccentricity
• Stability analysis calculates safety factors for each failure mode considering various loading conditions and soil properties

Overturning Stability Analysis

• Resisting moment stems from wall weight and stabilizing forces
• Overturning moment caused by lateral earth pressures
• Analysis compares these moments about the wall toe
• Factors influencing overturning stability
  • Wall geometry (height, width, batter)
  • Soil properties (unit weight, friction angle)
  • Backfill configuration
• Example calculation: For a 5m high concrete retaining wall, resisting moment = 450 kN-m, overturning moment = 300 kN-m

Sliding and Bearing Capacity Assessments

• Sliding analysis evaluates horizontal forces and base friction
• Bearing capacity considers soil strength beneath foundation
• Factors affecting sliding and bearing capacity
  • Soil-foundation interface properties
  • Groundwater conditions
  • Applied surcharge loads
• Example: Clay foundation with cohesion = 50 kPa, friction angle = 25°, wall base width = 3m

Factors of Safety for Retaining Walls

Safety Factor Calculations

• Factor of safety (FOS) ratio of resisting forces to driving forces
• Values greater than 1.0 indicate stability
• Overturning FOS = resisting moment / overturning moment
• Sliding FOS = available sliding resistance / total horizontal driving force
• Bearing capacity FOS = ultimate bearing capacity / maximum applied pressure
• Typical minimum acceptable factors of safety
  • Overturning: 1.5
  • Sliding: 1.5
  • Bearing capacity: 3.0
• Critical failure mode identified by lowest factor of safety

Interpreting and Applying Safety Factors

• FOS values vary based on local codes and project requirements
• Higher FOS needed for critical structures or uncertain soil conditions
• Probabilistic methods account for uncertainties in soil properties and loading
• Reliability-based factors of safety consider probability of failure
• Example: Cantilever retaining wall with calculated FOS values
  • Overturning: 1.8
  • Sliding: 1.6
  • Bearing capacity: 3.5

Surcharge, Groundwater, and Seismic Effects

Surcharge and Groundwater Impacts

• Surcharge loads (traffic, adjacent structures) increase lateral earth pressure
• Groundwater behind wall increases hydrostatic pressures
• Seepage forces reduce effective stresses and frictional resistance
• Effects on stability modes
  • Overturning: Increased lateral forces
  • Sliding: Reduced friction, increased driving forces
  • Bearing capacity: Increased vertical loads, reduced soil strength
• Example: Parking lot surcharge of 10 kPa increases lateral pressure by 30%

Seismic Considerations

• Seismic forces introduce dynamic lateral earth pressures and inertial forces
• Analysis methods
  • Pseudo-static approach
  • Dynamic analysis (time-history, response spectrum)
• Mononobe-Okabe method estimates seismic earth pressures
• Liquefaction potential assessment crucial for foundation soils
• Combined effects of surcharge, groundwater, and seismic forces often require iterative analysis
• Example: Design horizontal acceleration of 0.2g increases lateral earth pressure by 50%

Enhancing Retaining Wall Stability

Geometric and Structural Modifications

• Increase wall base width or use counterfort design for high lateral pressures
• Incorporate shear key or toe extension to enhance sliding resistance
• Strategies for different soil types
  • Cohesionless soils: Wider base, shear key
  • Cohesive soils: Deeper embedment, drainage systems
• Example: Adding a 0.5m deep shear key increases sliding resistance by 40%

Drainage and Reinforcement Techniques

• Implement proper drainage systems
  • Weep holes
  • Drainage blankets
  • Chimney drains
• Ground improvement techniques provide additional lateral support
  • Soil nailing
  • Rock anchors
  • Tie-backs
• Geosynthetic reinforcement (geogrids in MSE walls) distributes loads
• Example: Installing a drainage blanket reduces hydrostatic pressure by 70%

Seismic and Foundation Enhancements

• Increase wall flexibility for seismic performance
• Use isolation systems in earthquake-prone regions
• Design for ductile failure modes
• Address poor foundation conditions
  • Deep foundations (piles, caissons)
  • Ground improvement methods (soil mixing, grouting)
• Example: Soil-cement mixing increases bearing capacity from 200 kPa to 500 kPa