---
title: "Safe and ultimate bearing capacity, improvement"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:bc-define-improve-066j"
source: https://www.bctengineer.com/q/semester_6/building_technology/bc-define-improve-066j
site: BCT Engineering
---

# Safe and ultimate bearing capacity, improvement

Building Technology · Chapter 2: Foundations

- Asked in: 2066 Jestha (old course), 3+7 marks
- Video: [7 Methods to Improve the Bearing Capacity of Soil](https://www.youtube.com/watch?v=OOFyd6ZcnBg) — Engineering learners
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/bc-define-improve-066j
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

Define the safe bearing capacity and ultimate bearing capacity. Explain the methods to improve bearing capacity of soil.

## Short answer

Ultimate bearing capacity is the maximum pressure causing shear failure; safe bearing capacity is that divided by a factor of safety (2–3) to limit shear failure and settlement. Methods: compaction, drainage, replacement with compacted fill, confining soil, increasing footing size, sand piles, grouting, chemical stabilisation, preloading, geotextiles, and piles.

## Answer

#### Ultimate bearing capacity
**Ultimate bearing capacity** ($q_u$) is the maximum pressure per unit area that the soil can bear at the foundation level, at which it fails in shear.

#### Safe bearing capacity
**Safe bearing capacity** ($q_s$) is the maximum pressure that soil can carry safely without shear failure and with settlement within the permissible limit.

$$
q_s = \frac{q_u - \gamma D_f}{FS} + \gamma D_f
$$

or simply $q_s = q_u/FS$ with a factor of safety of 2 to 3. The **net safe bearing capacity** is the net ultimate capacity divided by FS.

#### Methods to improve bearing capacity
1. **Compaction**: The soil is compacted in layers using rollers, rammers and vibrators to raise density; good for granular soils.
2. **Drainage**: Lowering the water table by sumps, well points and sub-soil drains gives higher effective stress.
3. **Replacement**: Removing weak soil and replacing it by sand, gravel or murrum compacted in layers (sand cushion).
4. **Confining soil**: Using sheet piles or retaining walls to prevent sideways flow.
5. **Increasing depth and width** of footing.
6. **Sand piles and stone columns**.
7. **Chemical treatment and grouting**: Cement, lime or silicate grout filling pores.
8. **Soil stabilisation**: Mixing cement, lime, fly ash or bitumen.
9. **Preloading with surcharge** to consolidate soft clay, and **geotextiles** for reinforcement.
10. **Pile foundation** if the above are not enough.

```text
 footing
 [=======]
 |sand cushion|   <- replacement
 weak clay
```

## Check yourself

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

1. What is ultimate bearing capacity?
   A. Pressure with no settlement
   B. Weight of the footing
   C. Pressure at which the soil fails in shear
   D. q_u divided by FS
   Answer: C. q_u is the maximum pressure the soil can bear before shear failure. (Key point: Ultimate BC)

2. What is safe bearing capacity?
   A. Pressure at failure
   B. Pressure at zero load
   C. Maximum pressure carried safely without shear failure and with permissible settlement
   D. Pressure from the soil weight
   Answer: C. Safe BC keeps shear and settlement within limits. (Key point: Safe BC)

3. What FS range is given?
   A. 0.5 to 1
   B. 10 to 20
   C. 2 to 3
   D. Exactly 1
   Answer: C. q_s = q_u/FS with FS of 2 to 3. (Key point: Factor of safety)

4. Compaction in layers is most suited for
   A. Granular soils
   B. Rock
   C. Water
   D. Peat only
   Answer: A. Rollers, rammers and vibrators densify granular soils. (Key point: Compaction)

## Related in Building Technology

- [Soil exploration, safe bearing capacity, improvement](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexploration-bc-065s) — Building Technology, chapter 2
- [Soil exploration and improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asoil-exploration-improving-bc-nepal) — Building Technology, chapter 2
- [Sub-soil exploration, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asubsoil-exploration-improve) — Building Technology, chapter 2
- [Factors affecting foundation design, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afactors-design-improve-bc) — Building Technology, chapter 2
- [Methods of improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aimproving-bc) — Building Technology, chapter 2, asked 2×
- [Methods of determining bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Abc-determination-methods) — Building Technology, chapter 2, asked 2×

## Related in other subjects

- [Skempton's findings on clay bearing capacity](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Askempton-net-safe-bc) — Foundation Engineering, chapter 6
- [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5×
- [Mat bearing capacity: cohesive vs cohesionless](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Amat-bc-cohesive-vs-cohesionless) — Foundation Engineering, chapter 7, asked 2×
- [Methods of soil improvement](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-methods-list) — Foundation Engineering, chapter 10, asked 6×