---
title: "Foundation settlement, effects, black cotton soil"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:settlement-black-cotton-062ba"
source: https://www.bctengineer.com/q/semester_6/building_technology/settlement-black-cotton-062ba
site: BCT Engineering
---

# Foundation settlement, effects, black cotton soil

Building Technology · Chapter 2: Foundations

- Asked in: 2062 Baisakh (old course), 16 marks
- Video: [Construction of Buildings & Roads in Black Cotton Soil:Preventive Measures |Geotechnical Engineering](https://www.youtube.com/watch?v=l33-KuswGQw) — Geotechnical Engineering Consultancy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/settlement-black-cotton-062ba
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

Describe briefly the causes of foundation settlement and also the effects resulted in the structure from the unequal settlements. Explain the reasons for not preferring black cotton soil in the foundation?

## Short answer

Foundation settlement arises from consolidation, weak/filled soil, non-uniform strata, water table changes, overloading, adjacent excavation, tree roots, erosion, voids and earthquakes. Unequal settlement causes cracks, tilting, frame distortion, jammed doors/windows, service damage, dampness and possible collapse. Black cotton soil is avoided because it swells and shrinks with moisture, has low bearing capacity, deep cracks, differential movement, poor drainage and high compressibility.

## Answer

#### Causes of foundation settlement
1. Consolidation of soft clay or silt (water squeezed out slowly under load).
2. Loose, weak or filled soil and poor compaction.
3. Non-uniform soil layers and unequal loads.
4. Fall in water table, or seasonal shrinkage and swelling.
5. Overloading, extra floors, or poor foundation size.
6. Nearby excavations, vibration and dewatering.
7. Tree roots, leaking pipes, erosion and underground voids.
8. Earthquake densification or liquefaction.

#### Effects of unequal settlement on the structure
- **Cracks** in walls, beams, slabs and plaster, usually diagonal and wider at the top or bottom.
- **Tilting** of the building or of walls and columns.
- **Distortion** of frames and extra bending moments and shear in members.
- **Jamming** of doors and windows, and uneven floors.
- **Damage to services**: broken pipes, drains, and electrical lines.
- **Loss of waterproofing**, dampness, and leakage.
- In severe cases, **partial or complete failure** or collapse.

#### Why black cotton soil is not preferred
Black cotton soil is a clayey soil rich in montmorillonite, found in parts of Nepal's Terai and India.
1. **High swelling and shrinkage**: It expands in the rainy season and shrinks in the dry season, a movement that can be 10 cm or more.
2. **Low bearing capacity** (about 50 to 100 kN/m²) and low shear strength when wet.
3. **Large cracks** (up to 1 m deep) in dry weather and extreme softness when wet.
4. **Differential movement** under the building leads to cracks and uplift.
5. **Poor drainage** and high compressibility.
6. **Damage to foundations and floors** because it exerts uplift pressure.

Hence, the foundation should go below the active zone, or the soil should be replaced or treated (under-reamed piles, sand cushion, raft).

## Check yourself

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

1. Black cotton soil is rich in
   A. Quartz only
   B. Montmorillonite
   C. Gypsum
   D. Mica
   Answer: B. Montmorillonite causes swelling. (Key point: Black cotton mineral)

2. Unequal settlement mainly causes
   A. Stronger walls
   B. No effect
   C. Lower loads
   D. Cracks, tilting and distortion of frames
   Answer: D. Frames get extra bending moments and shear. (Key point: Unequal settlement)

3. Why is it not preferred?
   A. High bearing capacity
   B. No volume change
   C. It is free-draining
   D. High swelling and shrinkage with low bearing capacity
   Answer: D. It moves seasonally and is weak when wet. (Key point: Black cotton behavior)

4. A suitable remedy is
   A. Shallow footing in active zone
   B. Ignoring it
   C. Under-reamed piles, sand cushion or raft below the active zone
   D. Adding water
   Answer: C. Go below the active zone or replace/treat the soil. (Key point: Remedy)

## Related in Building Technology

- [Foundations on black cotton soil](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ablack-cotton-foundation) — Building Technology, chapter 2
- [Foundation settlement causes and treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afoundation-settlement-treatment) — Building Technology, chapter 2
- [Foundation settlement causes and underpinning](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asettlement-underpinning-067as) — Building Technology, chapter 2
- [Causes of foundation failure](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afoundation-failures) — Building Technology, chapter 2, asked 2×

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