Retaining Walls Explained

Introduction

• Overview of retaining walls
• Video divided into 4 parts:
  1. General types and components
  2. Types of failures in retaining walls
  3. General forces and behavior of retaining walls
  4. Typical reinforcement in a cantilever retaining wall

What is a Retaining Wall?

• Used to hold earth/material, prevents soil from taking natural position.
• Components:
  • Stem
  • Toe Slab
  • Heel Slab
• Foundation includes Toe and Heel slab, may have a key for sliding prevention.
• Drain holes may be provided for water drainage.
• Waterproofing is typically included on the soil contact portion.

Types of Retaining Walls

1. Gravity Wall

   • Retains soil by its own weight.
   • Usually larger, built in stone masonry or plain concrete.

2. Cantilever Wall

   • Most common, for retention height up to 8 meters (25 feet).
   • Components function as one-way cantilever slabs.
   • Stem acts as vertical cantilever under lateral pressure.
   • Main reinforcement provided in tension regions.

3. Counterfort Wall

   • Features supports (counterforts) connecting stem and heel slabs.
   • Used for retention heights over 7 meters (23 feet).
   • Counterforts create rectangular panels supported on three sides.

4. Buttress Wall

   • Similar to Counterfort Wall but supports are on the toe side, not buried.
   • Counterfort walls are preferred for usability and appearance.

5. Restrained Walls

   • Found in buildings as basement walls or bridge abutments.
   • Stem is supported by the floor slab or deck.
   • Analyzed as a beam fixed at the base and simply supported at the top.

Failure Modes of a Cantilever Retaining Wall

1. Overturning

   • Toe acts as center of rotation; wall deforms under lateral pressures.
   • Weight of the wall and soil on heel act as stabilizing forces.

2. Sliding

   • Lateral forces attempt to slide the wall.
   • Resistance primarily from friction between base slab and soil.
   • Shear key can be introduced to increase resistance.

3. Bearing Capacity Failure

   • Soil below the wall fails due to pressure beyond allowable limits.
   • Base slab width must adequately distribute vertical reaction.

Forces Acting on a Simple Cantilever Retaining Wall

• Active Pressure (Pa):

  • Varies with depth; highest at bottom, zero at top.
  • Calculated using: Pa = Ka * gamma_s * H
  • Coefficient Ka derived from Rankine's theory.

• Passive Pressure (Kp):

  • Acts from toe side, generally not included in design.
  • Coefficient for granular soil with phi of 30 degrees is 3.

• Surcharge:

  • Additional load from traffic/live loads; acts laterally as active pressure.
  • Calculated using: Force = Ka * w * height.

• Water Pressure:

  • Accumulates behind the wall when drainage fails.
  • Pressure from dry soil and submerged soil calculated separately.
  • Proper drainage is essential to avoid large water pressure build-up.

Typical Reinforcement in a Cantilever Retaining Wall

• Main reinforcement in the stem with equal spacing and 90-degree hooks.
• Extended into key if present.
• Compression face reinforcement extends into the base slab for anchorage.
• Base slab has both top and bottom face reinforcement for tension and compression.
• Distribution bars are included to hold main reinforcement in place.

Conclusion

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