Retaining Walls Explained

Overview

• Video divided into 4 parts:
  1. General types of retaining walls and components.
  2. Types of failures in a retaining wall.
  3. General forces and behavior of retaining walls.
  4. Typical reinforcement in a cantilever retaining wall.
• Purpose of retaining walls: Hold earth/material, prevent soil from taking natural position, make area usable.

Components of Retaining Walls

• Three Main Parts:
  • Stem
  • Toe Slab
  • Heel Slab
• Foundation:
  • Toe and Heel slabs form foundation.
  • Key may be provided in footing to prevent sliding.
  • Drain holes may be included for water drainage.
  • Waterproofing often applied to the soil-contacting portion.

Types of Retaining Walls

1. Gravity Wall:

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

2. Cantilever Wall:

   • Most common type, used for retention height up to 8 meters (25 feet).
   • Comprises stem, toe, and heel acting as one-way cantilever slabs.
   • Stem resists lateral earth pressure, heel resists net weight of retained earth, toe resists net soil pressure.

3. Counterfort Wall:

   • Has supports (counterforts) connecting stem and heel slabs, concealed in retained earth.
   • Used when retention height exceeds 7 meters (23 feet).
   • Panels created by counterforts are supported on three sides.

4. Buttress Wall:

   • Similar to counterfort wall but supports on the toe side, not buried in earth.
   • Counterfort preferred for aesthetics and usable space.
   • Buttress wall suited for efficiency and economy.

5. Restrained Walls:

   • Found in buildings (basement walls) and bridges (abutments), stem supported by floor slab or deck.
   • Analyzed as a beam fixed at base and simply supported or partially restrained at top.

Failure Modes of Cantilever Retaining Walls

1. Overturning:

   • Toe acts as center of rotation.
   • Lateral pressures act as overturning forces, wall weight and soil on heel act as stabilizing forces.

2. Sliding:

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

3. Bearing Failure:

   • Soil below wall fails due to excessive bearing pressure.
   • Width of base slab must distribute vertical reaction adequately.

Forces Acting on a Cantilever Retaining Wall

• Active Pressure (Pa):

  • Varies with depth, maximum at bottom, no pressure at top.
  • Calculated using:
    Pa = Ka * gamma_s * H, where:
    • Ka: active pressure coefficient (based on Rankine's theory)
    • gamma_s: unit weight of soil
    • H: total height of backfill.
  • Acts at centroid of pressure triangle at distance H/3 from base.

• Passive Pressure (Kp):

  • Acts from toe side, generally not included in design calculations for conservativeness.

• Surcharge Load:

  • Comes from live loads (e.g., vehicular traffic).
  • Calculated pressure: Ka * w, where w is intensity of surcharge.

• Water Pressure:

  • Accumulates when no escape route, applies pressure on wall.
  • Analyze pressures from dry and submerged soil separately.
  • Proper drainage facilities required to avoid excessive water pressure.*

Typical Reinforcement in a Cantilever Retaining Wall

• Main reinforcement distributed along stem length, with lapped bars having 90-degree hooks.
• If a key is present, reinforcement extends into the key.
• Compression face reinforcement extended into the base slab for anchorage.
• Base slab reinforcement on both top and bottom faces to resist tension and compression forces.
• Distribution bars spread throughout to hold main reinforcement in place.