---
title: "Short note: Retaining wall"
subject: "Building Technology"
chapter: 3
key: "semester_6/building_technology:short-retaining-wall"
source: https://www.bctengineer.com/q/semester_6/building_technology/short-retaining-wall
site: BCT Engineering
---

# Short note: Retaining wall

Building Technology · Chapter 3: Mortars & Masonry works

- Asked in: 2075 Baisakh, 3 marks; 2062 Baisakh (old course), 4 marks (asked 2 times)
- Video: [Retaining Walls Explained | Types, Forces, Failure and Reinforcement](https://www.youtube.com/watch?v=UummY_mo85Y) — Structures Explained
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/short-retaining-wall
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-3

## Question

Write a short note on retaining wall (and forces acting on it).

## Short answer

A retaining wall holds back earth at a change in grade, resisting lateral pressure. Types: gravity, cantilever, counterfort, buttress. Key forces: self-weight, active pressure (Pₐ = ½ Kₐ γ H² at H/3), surcharge, water pressure; stability checked for overturning, sliding, bearing.

## Answer

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.

```text
        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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A retaining wall is built to:
   A. Carry roof
   B. Hold water vapour
   C. Provide thermal insulation
   D. Hold back earth or fill at a level difference
   Answer: D. Used for hill roads, terraces, basements and abutments. (Key point: Purpose)

2. Active earth pressure Pa acts at:
   A. H/2 from the top
   B. At the toe
   C. At the top
   D. H/3 from the base
   Answer: D. Pa = 0.5 Ka gamma H^2 with triangular distribution, acting at H/3. (Key point: Active earth pressure)

3. Counterfort walls are used for:
   A. Dry stone
   B. Heights below 1 m
   C. Only gravity walls
   D. Height above about 6 m
   Answer: D. Ribs at 2.5-3.5 m behind the stem. (Key point: Counterfort)

4. Minimum FS against overturning for a gravity wall is:
   A. 1
   B. 2
   C. 10
   D. 0.5
   Answer: B. FS = MR/MO >= 1.5 (2 for gravity wall). (Key point: Stability FOS)

## Related in Building Technology

- [Parapet wall, gravity and counterfort retaining walls](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aparapet-retaining-counterfort) — Building Technology, chapter 3
- [Short note: Basement and retaining wall](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ashort-basement-retaining-wall) — Building Technology, chapter 3

## Related in other subjects

- [Forces acting on well foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-forces) — Foundation Engineering, chapter 9, asked 2×
- [Forces acting on gravity dam](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agravity-dam-forces-sketch) — Hydro Power, chapter 4
- [Braced cut design versus retaining wall](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abracing-vs-retaining-wall) — Foundation Engineering, chapter 4, asked 2×
- [Passive thrust on retaining wall back](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awall-passive-thrust-two-layer) — Foundation Engineering, chapter 3