Short note: Retaining wall

A retaining wall is a structure built to hold back (retain) earth, water or other fill at a difference in level, preventing it from sliding or collapsing; it is used for hill roads, terraces, basements, bridge abutments and embankments.

Types

• Gravity wall: stone, brick or plain concrete; resists the earth pressure by its own weight.
• Cantilever wall: RCC stem and base slab; the weight of soil on the heel helps stability.
• Counterfort wall: RCC wall with vertical ribs (counterforts) at 2.5-3.5 m spacing behind the stem; for heights above 6 m.
• Buttress wall: like a counterfort wall, but the ribs are in front.

Forces acting on a retaining wall

1. Self weight $W$ of the wall (and the weight of soil on the heel), acting downward at the centroid.
2. Active earth pressure $P_a$ from the retained soil, acting horizontally at $H/3$ from the base:

$$P_a = \tfrac{1}{2} K_a \gamma H^2, \quad K_a = \frac{1-\sin\phi}{1+\sin\phi}$$

3. Surcharge load $q$ on the backfill, which adds a uniform lateral pressure $K_a q$, giving an extra force $K_a q H$ acting at $H/2$.
4. Water pressure if the backfill is saturated (reduced by weep holes and drains).
5. Passive resistance of the soil in front of the toe (generally neglected or reduced).
6. Base reaction (bearing pressure) and friction at the base, which resist sliding.

        backfill
   ---------------+
        |         |  <- surcharge q
        |  wall   |
   P_a->|  (W)    |
        |_________|
      toe  heel
        base reaction

Stability checks

• Overturning: $FS = M_R/M_O \ge 1.5$ (2 for a gravity wall).
• Sliding: $FS = \mu \Sigma W / P_a \ge 1.5$.
• Bearing pressure within the safe bearing capacity, and no tension at the base (resultant within the middle third).

Weep holes (100 mm at 2-3 m c/c) and a granular filter behind the wall drain the backfill.