---
title: "Sub-soil exploration, improving bearing capacity"
subject: "Building Technology"
chapter: 2
key: "semester_6/building_technology:subsoil-exploration-improve"
source: https://www.bctengineer.com/q/semester_6/building_technology/subsoil-exploration-improve
site: BCT Engineering
---

# Sub-soil exploration, improving bearing capacity

Building Technology · Chapter 2: Foundations

- Asked in: 2071 Magh, 8 marks
- Video: [||Sub surface Investigation||Methods for improving Bearing Capacity of soils||](https://www.youtube.com/watch?v=xY2wR170B6g) — Mrugesh Rathod
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/subsoil-exploration-improve
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-2

## Question

What is sub soil exploration? Explain a method of improving bearing capacity of soil.

## Short answer

Sub-soil exploration investigates ground layers, strength, water table and bearing capacity by borings, sampling and tests. Bearing capacity can be improved by a sand cushion: excavate weak soil to 1-1.5 m depth and beyond footing width, lay clean sand in 200-300 mm layers, sprinkle water and compact thoroughly.

## Answer

#### Sub-soil exploration
**Sub-soil exploration** (soil exploration) is the investigation of the ground below a site to find the soil layers, their thickness, strength, water table and bearing capacity, so as to choose and design the foundation. It includes reconnaissance, boring or pits, sampling, in-situ tests (SPT, plate load, cone penetration), and lab tests. The result is a soil report with borehole log, soil profile and recommended SBC.

#### Method of improving bearing capacity: Sand cushion (replacement)
This is the method commonly used for light buildings.
1. The weak soil under the footing is excavated to a depth of about 1 to 1.5 m (or until firm soil), and over a width larger than the footing (extending beyond it by the thickness of the cushion).
2. Clean, well graded sand, gravel or murrum is laid in layers of 200 to 300 mm.
3. Each layer is sprinkled with water and compacted thoroughly by rammers or rollers up to the required density.
4. The foundation is then constructed on the cushion.

The sand layer spreads the load over a larger area of the weak soil below, reducing pressure, and also drains the water.

```text
        [ footing ]
     /               \
    /  compacted sand  \
   /     cushion        \
   weak soil below
```

Other methods: compaction, drainage, sand piles, stone columns, grouting, stabilisation, and increasing the footing depth.

## Check yourself

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

1. What does a soil report include?
   A. Paint schedule
   B. Roof plan
   C. Cost of bricks
   D. Borehole log, soil profile and recommended SBC
   Answer: D. The report gives log, profile and SBC. (Key point: Sub-soil exploration)

2. In what layers is sand laid in a sand cushion?
   A. 2 to 3 m
   B. 200 to 300 mm
   C. 5 mm
   D. 1 m
   Answer: B. Layers of 200-300 mm are watered and compacted. (Key point: Cushion layers)

3. How far should the cushion extend beyond the footing?
   A. Not at all
   B. 10 m
   C. By the thickness of the cushion
   D. Half the footing
   Answer: C. Excavation is wider than the footing by the cushion thickness. (Key point: Cushion width)

4. How does the sand cushion help?
   A. Increases footing pressure
   B. Makes soil swell
   C. Removes the footing
   D. Spreads load over a larger area of weak soil and drains water
   Answer: D. Load spreads and water drains. (Key point: Action)

## 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
- [Soil exploration, safe bearing capacity, improvement](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexploration-bc-065s) — 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×
- [Define soil exploration](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adefine-soil-exploration) — Building Technology, chapter 2