8.3 Design of retaining walls (gravity, cantilever, and reinforced earth walls)

Retaining walls are crucial structures in geotechnical engineering, holding back soil and water. This section covers various types, from gravity walls to reinforced earth systems, each suited for different applications and soil conditions.

Understanding lateral earth pressures is key to designing stable retaining walls. We'll explore active, passive, and at-rest pressure states, as well as how factors like surcharge loads and water pressure impact wall design and stability.

Retaining Wall Types and Applications

Gravity and Cantilever Walls

Top images from around the web for Gravity and Cantilever Walls

• 

  Category:Retaining wall diagrams - Wikimedia Commons View original

  Is this image relevant?

• 

  Retaining wall - Wikipedia View original

  Is this image relevant?

• 

  Retaining wall - Wikipedia View original

  Is this image relevant?

• 

  Category:Retaining wall diagrams - Wikimedia Commons View original

  Is this image relevant?

• 

  Retaining wall - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Gravity and Cantilever Walls

• 

  Category:Retaining wall diagrams - Wikimedia Commons View original

  Is this image relevant?

• 

  Retaining wall - Wikipedia View original

  Is this image relevant?

• 

  Retaining wall - Wikipedia View original

  Is this image relevant?

• 

  Category:Retaining wall diagrams - Wikimedia Commons View original

  Is this image relevant?

• 

  Retaining wall - Wikipedia View original

  Is this image relevant?

1 of 3

• Gravity retaining walls use their own weight to resist lateral earth pressures suitable for low to medium height applications
• Cantilever retaining walls leverage backfill soil weight for stability efficient for medium to tall wall heights
  • Use reinforced concrete structure
  • More material-efficient than gravity walls for taller heights
• Counterfort retaining walls incorporate vertical concrete ribs to enhance stability ideal for very tall applications
  • Ribs act as additional support against bending and shear forces
• Buttress retaining walls feature protruding supports on exposed face increasing resistance against overturning for tall structures
  • Buttresses typically spaced at regular intervals along wall length

Reinforced Earth and Sheet Pile Walls

• Reinforced earth walls create composite structure using tensile reinforcement elements within backfill soil suitable for various heights and soil conditions
  • Reinforcement elements include (geotextiles, geogrids, metal strips)
  • Can accommodate significant vertical and lateral loads
• Sheet pile walls consist of slender walls driven into ground commonly used in temporary excavations or waterfront structures
  • Materials include (steel, vinyl, concrete)
  • Effective in areas with high water tables or soft soils
• Soldier pile and lagging walls combine vertical piles with horizontal lagging often employed in urban environments
  • Suitable for both temporary and permanent earth retention
  • Allow for staged construction in confined spaces

Lateral Earth Pressures on Walls

Earth Pressure States and Theories

• Lateral earth pressure exerts horizontal force on wall structure influenced by soil properties, wall movement, and loading conditions
• Active earth pressure develops when wall moves away from soil representing minimum pressure state
  • Typically used in design for yielding walls (cantilever, gravity)
• Passive earth pressure occurs when wall moves towards soil representing maximum pressure state
  • Often utilized in resisting sliding forces at wall base
• At-rest earth pressure exists with no wall movement commonly applied in design of rigid, unyielding structures (basement walls)
• Rankine's theory and Coulomb's theory provide fundamental methods for calculating lateral earth pressures
  • Rankine assumes no wall friction, simplifying calculations
  • Coulomb accounts for wall friction, more accurate for some scenarios

Additional Pressure Considerations

• Surcharge loads contribute additional lateral pressures accounted for in wall design
  • Examples include (adjacent structures, traffic loads, construction equipment)
• Water pressure and seepage forces significantly increase lateral pressures requiring careful consideration
  • Proper drainage design essential to mitigate hydrostatic pressure
  • Seepage analysis may be necessary in areas with high groundwater table

Gravity Wall Design and Stability

Stability Analysis

• Gravity wall design checks for overturning, sliding, and bearing capacity failure modes ensuring overall stability
• Factor of safety against overturning calculated by comparing resisting moment to overturning moment
  • Resisting moment from wall weight
  • Overturning moment from lateral earth pressure
• Sliding stability evaluated by comparing frictional resistance along base to horizontal component of lateral earth pressure
  • May include passive resistance at toe if applicable
• Bearing capacity analysis ensures underlying soil supports combined vertical loads
  • Includes wall weight and vertical component of earth pressure
• Eccentricity of resultant force at base must be within acceptable limits preventing excessive stress concentrations
  • Typically limited to middle third of base width

Design Considerations and Drainage

• Wall proportions iteratively adjusted to meet stability requirements while optimizing material usage
  • Base width typically 0.5 to 0.7 times wall height
  • Stem thickness varies based on height and material (concrete, masonry)
• Proper drainage systems essential to prevent hydrostatic pressure buildup behind wall
  • Weep holes allow water to pass through wall face
  • Drainage layers (gravel, geotextiles) facilitate water movement to base of wall

Cantilever Wall Analysis and Design

Structural Components and Analysis

• Cantilever retaining walls consist of vertical stem and base slab utilizing backfill soil weight over heel for stability
• Stem designed as cantilever beam to resist bending moments and shear forces from lateral earth pressures
  • Moment increases cubically with height
  • Shear force increases quadratically with height
• Base slab divided into toe and heel sections designed for different loading conditions
  • Toe experiences upward soil pressure and downward wall weight
  • Heel subjected to downward soil and wall weight with upward soil reaction

Reinforcement Design

• Flexural reinforcement in stem concentrated on tension (soil) side with distribution steel on opposite face
  • Main reinforcement typically vertical bars
  • Horizontal bars provide temperature and shrinkage control
• Shear reinforcement may be required near stem-base junction where high shear stresses occur
  • Stirrups or bent bars used to resist diagonal tension
• Temperature and shrinkage reinforcement provided to control cracking in both stem and base slab
  • Typically minimum of 0.2% of gross concrete area

Reinforced Earth Walls: Principles and Design

Reinforcement Mechanics and Stability

• Reinforced earth walls incorporate tensile reinforcement elements within backfill soil creating composite, gravity-type structure
• Reinforcement elements transfer tensile stresses from soil to facing elements through friction or mechanical interlock
  • Friction developed along length of reinforcement
  • Mechanical interlock achieved through apertures in geogrids or ribs on strips
• Internal stability analysis checks for reinforcement pullout and tensile rupture failure modes
  • Pullout resistance depends on overburden pressure and soil-reinforcement interaction
  • Tensile rupture considers long-term strength of reinforcement material
• External stability considerations include sliding, overturning, bearing capacity, and global stability similar to conventional retaining walls

Design Elements and Drainage

• Spacing, length, and strength of reinforcement layers designed based on required tensile resistance at different depths
  • Typically denser spacing near top of wall where lateral pressures are highest
  • Length of reinforcement usually 0.7 to 1.0 times wall height
• Facing elements can be precast concrete panels, modular blocks, or wrapped geosynthetics each with specific connection details to reinforcement
  • Concrete panels provide durable, aesthetic finish
  • Modular blocks allow for easy construction and curved wall layouts
• Drainage design crucial in reinforced earth walls to prevent water pressure buildup and ensure long-term performance
  • Granular drainage layers behind and within reinforced soil zone
  • Collector pipes at base of wall to remove water from system