---
title: "Arching in soil and its requirements"
subject: "Foundation Engineering"
chapter: 4
key: "semester_5/foundation_engineering:arching-requirements"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/arching-requirements
site: BCT Engineering
---

# Arching in soil and its requirements

Foundation Engineering · Chapter 4: Arching in Soils and Braced Cuts

- Asked in: 2082 Bhadra, 4 marks; 2076 Chaitra, 1+3 marks; 2078 Bhadra, 2 marks; 2076 Asoj, 1+1 marks; 2078 Kartik, 1 marks; 2080 Baisakh, 1 marks; 2081 Baisakh, 2 marks; 2072 Chaitra, 1 marks (asked 8 times)
- Video: [Arching of soil and Terzaghi trap door experiment](https://www.youtube.com/watch?v=l3bUARajQpg) — Techie BUD
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/arching-requirements
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-4

## Question

What is arching in soil? What are the essential requirements for the arching effect to come into play?

## Short answer

Arching is the transfer of stress from a yielding soil portion to adjacent rigid parts through shear resistance. It requires relative movement of the support, soil shear strength, sufficient depth of cover, limited movement, and stiff neighbouring supports.

## Answer

**Arching** in soil is the transfer of pressure from a yielding part of a soil mass to the adjacent non-yielding (stationary) parts through the shear resistance of the soil. When one part of the support moves away, the soil above tries to move with it; the shear stresses along the boundaries of the moving mass hold it up, so the pressure on the yielding part falls and the pressure on the neighbouring stationary part rises.

```text
   ground
   ~~~~~~~~~~~~~~~~~~~~~~~~
   | stationary |  moving  | stationary |
   |  (stress   | (stress  |  (stress   |
   |  increases)| reduces) |  increases)|
   |____________|__________|____________|
                 yielding support
```

#### Essential requirements
1. **Relative movement:** one part of the soil support must yield (move or deflect) relative to the neighbouring part; arching does not occur in a rigid, unyielding support.
2. **Shear strength in the soil:** the soil must be able to mobilise shear resistance (friction and/or cohesion) along the boundaries of the moving mass; therefore it works best in dense sand, and in clay in the short term.
3. **Sufficient depth of soil cover:** a sufficient height of soil above the yielding part, so that the arch can form (arching fails if the cover is too thin).
4. **Limited movement:** the yielding must be small. Large movement breaks the arch, and the soil then loses its supporting effect.
5. **Stable adjacent parts:** the adjacent soil and supports must be stiff enough to resist the extra load transferred to them.

Examples: pressure on tunnel lining, buried conduits, braced cuts, flexible retaining walls and piles in soil.

## Check yourself

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

1. What is arching in soil?
   A. Transfer of pressure from a yielding part to adjacent stationary parts by shear resistance
   B. Uplift pressure
   C. Soil settlement by consolidation
   D. Capillary rise
   Answer: A. Shear along the boundaries of the yielding mass reduces pressure on it and raises it on neighbours. (Key point: Definition of arching)

2. Which condition is essential?
   A. Rigid unyielding support
   B. Relative movement of one part of the support, small in amount
   C. Very large movement
   D. Zero soil strength
   Answer: B. A part must yield, but only slightly; large movement breaks the arch. (Key point: Need for yielding)

3. Why does arching need shear strength?
   A. Shear along the boundaries carries the load to adjacent parts
   B. To increase pore pressure
   C. To compact soil
   D. To reduce density
   Answer: A. Friction or cohesion mobilised on the boundaries holds up the moving mass. (Key point: Soil strength)

4. What happens to pressure on the yielding part?
   A. It increases
   B. It decreases
   C. It stays equal
   D. It becomes tensile
   Answer: B. The yielding part is partly supported by shear, so pressure on it falls. (Key point: Effect on yielding part)

## Related in Foundation Engineering

- [Earthquake effect and strain for active state](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aearthquake-effect-active-strain) — Foundation Engineering, chapter 3
- [Pile capacity in sand, group effect, load test](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-sand-group-load-test) — Foundation Engineering, chapter 8
- [Trench bottom heave safety factor](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Atrench-heave-rock) — Foundation Engineering, chapter 4
- [Terzaghi's trap door experiment](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aterzaghi-trap-door) — Foundation Engineering, chapter 4, asked 4×

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- [Essential requirements and types of stair by shape](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-requirements-shape-types) — Building Technology, chapter 5
- [Effect of compaction on soil properties](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acompaction-effect-engineering-behavior) — Soil Mechanics, chapter 5, asked 3×
- [Requirements of steam power station design](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-station-design-requirements) — Power Plant Equipment, chapter 7
- [Fuel requirements and structure of petroleum](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Afuel-requirements-petroleum-structure) — Internal Combustion Engines, chapter 3