---
title: "Define underpinning; one method"
subject: "Building Technology"
chapter: 8
key: "semester_6/building_technology:underpinning-one-method"
source: https://www.bctengineer.com/q/semester_6/building_technology/underpinning-one-method
site: BCT Engineering
---

# Define underpinning; one method

Building Technology · Chapter 8: Temporary construction

- Asked in: 2078 Poush, 3 marks
- Video: [SCAFFOLDING , SHORING & UNDERPINNING - BUILDING CONSTRUCTION](https://www.youtube.com/watch?v=_-XPbRNAEr8) — HomeNEXT DESIGN STUDIO 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/underpinning-one-method
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-8

## Question

Define underpinning and explain any one method of it.

## Short answer

Underpinning strengthens or deepens an existing foundation to a firmer stratum or enlarges its base for safe load carrying. The pit method divides the wall into 1-1.5 m sections, excavates alternate pits below the old footing to firm soil, fills with concrete or masonry leaving a 75 mm gap packed with dry mortar, then repeats for remaining sections.

## Answer

**Underpinning** is the process of strengthening or deepening the foundation of an existing building by extending it to a firmer or deeper stratum, or enlarging the base, so that it can carry the load safely.

#### Pit method (pier method)
1. The wall is divided into sections of 1-1.5 m width.
2. A pit is dug under the old foundation in alternate sections, and the wall is supported if needed.
3. The pit is dug down to a firm stratum and filled with PCC or masonry in cement mortar (1:4) up to about 75 mm below the old footing.
4. The gap is packed with dry cement-sand mortar (1:1) or slate pieces, rammed, and left 3-4 days to set.
5. The same is repeated for the intermediate sections, then the pits are backfilled.

```text
   wall
   |||||| old footing
   ||||||===| pit
          | new concrete
          | 75 mm packing
          |_ firm soil
```

## Check yourself

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

1. What is underpinning?
   A. Temporary support of a leaning wall
   B. Fixing a false ceiling
   C. Strengthening or deepening an existing foundation to a firmer stratum or enlarging its base
   D. Covering a wall with mortar
   Answer: C. Underpinning makes an existing foundation able to carry load safely by deepening or enlarging it. (Key point: Underpinning definition)

2. In the pit method, how are pits dug under the wall?
   A. Only at the corners
   B. In sections of 5 m width
   C. All along the wall at once
   D. In alternate sections of 1-1.5 m width, leaving the others in place
   Answer: D. Alternate sections keep the wall supported while each pit is dug and filled. (Key point: Pit method sections)

3. What is done with the gap of about 75 mm left below the old footing?
   A. It is packed with dry cement-sand mortar (1:1) or slate, rammed, and left 3-4 days to set
   B. It is left empty for drainage
   C. It is filled with wet mortar and loaded at once
   D. It is filled with loose sand
   Answer: A. Dry packing makes tight contact between new concrete and old footing so settlement does not occur. (Key point: 75 mm gap)