---
title: "Soil exploration, safe bearing capacity, improvement"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:exploration-bc-065s"
source: https://www.bctengineer.com/q/semester_6/building_technology/exploration-bc-065s
site: BCT Engineering
---

# Soil exploration, safe bearing capacity, improvement

Building Technology · Chapter 2: Foundations

- Asked in: 2065 Shrawan (old course), 16 marks
- Video: [How to Increase Bearing Capacity of Soil? || By CivilGuruji](https://www.youtube.com/watch?v=YERCZQs8MLY) — The Institute of Construction Managment 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/exploration-bc-065s
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

What do you know about the methods of soil exploration? Define safe bearing capacity of soil and describe briefly the methods of improving bearing capacity of soil.

## Short answer

Soil exploration uses test pits, augering, wash boring, percussion, rotary drilling, SPT, and geophysical surveys. Safe bearing capacity is the maximum load per unit area safely supported without shear failure or excessive settlement, obtained by dividing ultimate capacity by factor of safety (2.5–3). Methods: compaction, drainage, gravel replacement, larger footing, stone columns, grouting, stabilisation, piles.

## Answer

#### Methods of soil exploration
1. **Test pits (trial pits)**: Pits dug 1.5 to 3 m deep; the soil is inspected and samples are taken. Cheap; for shallow depths and small buildings.
2. **Auger boring**: A hand or power auger drills holes in soft soil up to about 10 m deep; gives disturbed samples.
3. **Wash boring**: A water jet cuts the soil inside a casing; used in sand and clay.
4. **Percussion boring**: A chisel breaks hard soil or rock.
5. **Rotary drilling**: Core drilling for rock and hard strata.
6. **Sampling**: Disturbed and undisturbed samples (thin-wall tube, split spoon).
7. **Field tests**: Standard Penetration Test (SPT), static and dynamic cone penetration tests, vane shear test, plate load test.
8. **Geophysical methods**: Seismic refraction and electrical resistivity, for large areas and depth profiles.
9. **Laboratory tests**: Grain size, Atterberg limits, shear strength, consolidation.

The number and depth of boreholes depend on the size of the building and the variation of the soil; the depth of exploration is at least 1.5 times the width of the footing (the depth of significant stress).

#### Safe bearing capacity
**Safe bearing capacity** is the maximum intensity of load per unit area that the soil can carry safely without shear failure and with permissible settlement:

$$
q_s = \frac{q_u}{FS}
$$

where $q_u$ is the ultimate bearing capacity and $FS$ is the factor of safety (2.5 to 3).

#### Methods of improving bearing capacity
- **Compaction** by rolling and ramming.
- **Drainage** of ground water.
- **Replacement** by a compacted sand or gravel cushion.
- **Increasing the depth** or width of the foundation.
- **Sand piles and stone columns**.
- **Grouting** and chemical injection.
- **Stabilisation** with cement or lime.
- **Confinement** by sheet piles; **preloading**; **geosynthetics**; or **pile foundation**.

## Check yourself

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

1. What is a test pit?
   A. A 50 m borehole
   B. A geophysical survey
   C. A lab test
   D. A trial pit 1.5 to 3 m deep from which soil is inspected
   Answer: D. Test pits are cheap, shallow inspections. (Key point: Test pits)

2. Which method is used for rock and hard strata?
   A. Hand auger
   B. Test pit
   C. Wash boring in clay only
   D. Rotary core drilling
   Answer: D. Rotary drilling cores rock. (Key point: Rotary drilling)

3. Borehole depth should be at least
   A. 1.5 times the foundation width
   B. 0.1 times the width
   C. Equal to 1 m always
   D. Half the width
   Answer: A. Depth is at least 1.5 times the width or the depth of influence. (Key point: Borehole depth)

4. Safe bearing capacity equals
   A. q_u / FS
   B. q_u x FS
   C. q_u + FS
   D. FS / q_u
   Answer: A. q_s = q_u/FS, FS 2.5 to 3. (Key point: Safe BC)

## Related in Building Technology

- [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
- [Safe and ultimate bearing capacity, improvement](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Abc-define-improve-066j) — 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×
- [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
- [Need for soil test / soil exploration](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asoil-test-need) — 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
- [Bearing capacity from plate load curve](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abc-from-load-settlement-sand) — Foundation Engineering, chapter 6