Retaining walls Flashcards
(24 cards)
What is a retaining wall?
A retaining wall is a relatively rigid structure built to hold back soil or earth at different levels on its two sides.
Why are retaining walls needed?
To prevent soil movement or erosion, especially on slopes or areas with elevation changes.
Name different types of retaining walls.
Gabion walls
In-situ reinforced concrete walls
Revetment works (sloped walls)
Masonry retaining walls (stone, brick, or block)
Precast concrete systems
What are gabion retaining walls?
Gabions are wire cages filled with rocks. They are flexible and allow water to drain through, reducing pressure on the wall.
What is in-situ reinforced concrete?
Concrete poured and set on-site, reinforced with steel to provide strength for holding large amounts of soil.
What is a revetment wall?
A sloped retaining wall often used along embankments to protect against erosion and provide support.
What are masonry retaining walls made from?
Built using stone, brick, or block units. They rely on their weight to resist the pressure of soil.
What are precast concrete systems?
Retaining wall sections manufactured off-site and assembled on location for quicker installation.
Why is drainage behind retaining walls important?
To prevent water buildup behind the wall, which can increase pressure and cause wall failure.
What is a common drainage method used behind retaining walls?
Using porous backfill (e.g., gravel) and perforated drainage pipes to direct water away from the wall.
What is a “toe and heel” in a retaining wall?
Parts of the wall’s base:
Toe: Front portion (exposed side)
Heel: Rear portion (under the soil)
They provide stability by balancing pressure.
What is a counterfort retaining wall?
A high reinforced concrete wall with diagonal supports (counterforts) on the back side to reduce wall thickness and increase strength.
What is the basic procedure to size a retaining wall?
H – Determine wall height
B – Estimate base width
Toe – Calculate as B/3
Wall thickness – Approx. 0.1 × H
Draw the wall
Check stability with calculations
What forces act on a retaining wall?
Vertical forces from the wall’s own weight and any load on top
Horizontal forces from the soil pressure behind the wall and water pressure if drainage is poor
What are the three main failure modes of retaining walls?
Sliding
Overturning
Soil Bearing Failure
What is sliding failure in a retaining wall?
When the wall moves horizontally due to insufficient frictional resistance at the base.
Equation for Sliding Safety Factor (SFₛ):
SF sliding = ResistingForces/DrivingForces = Fr +c.A/Pa >1.5
* W⋅tan(ϕ) (frictional resistance,
* 𝑊 = vertical load)
* c⋅A = cohesion at base (optional)
* 𝑃a = horizontal active earth pressure
* Typical required SF: > 1.5
What is overturning failure in a retaining wall?
When the wall rotates about its toe due to unbalanced moments caused by soil pressure.
Equation for Overturning Safety Factor (SFₒ):
SF overturning = ∑M resisting/∑M overturning > 2
+M (clockwise): Righting moments from the weight of the wall and soil on heel
−M (counterclockwise): Overturning moments from active earth pressure
Typical required SF: > 2
What is soil bearing failure?
When soil beneath the wall cannot safely support the loads, causing tilting or sinking.
Conditions to avoid bearing failure:
qₘᵢₙ > 0 → No uplift or tension under base
qₘₐₓ < qₐₗₗ → Maximum pressure must be less than allowable bearing pressure
Equations for soil pressure under footing:
q max/min = W/B (1± 6e/B)
Where:
W = total vertical load
B = base (footing) widt
e = eccentricity =
∑𝑀 aboutcenter/W
What are the vertical forces acting on a retaining wall?
Self-weight of wall
Weight of soil above heel
Any surcharge loads on the backfill
These forces act downwards and contribute to resisting sliding and overturning.
What are the horizontal forces acting on a retaining wall?
Active earth pressure from the retained soil
Water pressure (if not drained)
Surcharge pressure (if applicable)
These forces act laterally and try to push or overturn the wall.
