---
title: "Short note: Underpinning"
subject: "Building Technology"
chapter: 8
key: "semester_6/building_technology:short-underpinning"
source: https://www.bctengineer.com/q/semester_6/building_technology/short-underpinning
site: BCT Engineering
---

# Short note: Underpinning

Building Technology · Chapter 8: Temporary construction

- Asked in: 2066 Jestha (old course), 2.5 marks
- Video: [Underpinning](https://www.youtube.com/watch?v=7mGkZgaN0zM) — Civil Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/short-underpinning
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-8

## Question

Write a short note on underpinning.

## Short answer

Underpinning strengthens an existing foundation by extending it to firmer soil or enlarging the base to safely carry load. It is needed for settled foundations, extra loads, nearby excavation, water table changes or damage. Common methods include the pit method (concrete in alternate sections), piles, pretest and chemical grouting.

## 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.

**Necessity:** weak or settled foundation, extra load on the building, nearby deep excavation, damage by flood or earthquake, or change in the water table.

**Methods:** pit method, pile method, pretest method and chemical grouting.

**Pit method:** pits are dug below the footing in alternate sections about 1-1.5 m wide, filled with concrete to about 75 mm below the old footing, the gap is packed with dry mortar, then the remaining sections are done.

```text
   wall
   |||||| old footing
   ||||||==| new pier
          | 75 mm packing
```

## Check yourself

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

1. Underpinning is best described as
   A. covering walls with finishing material
   B. strengthening or deepening an existing foundation
   C. a temporary support for walls
   D. building a partition
   Answer: B. It makes an existing foundation carry load safely by going to a firmer stratum or widening the base. (Key point: Definition)

2. Which is a cause needing underpinning?
   A. Cracks in plaster only
   B. Fading paint
   C. Damage by flood or earthquake or a change in the water table
   D. A new false ceiling
   Answer: C. Flood, earthquake and water-table changes reduce soil bearing capacity. (Key point: Necessity)

3. Which list gives methods of underpinning?
   A. Flush, keyed, tuck
   B. Raking, flying, dead
   C. Single, double, cantilever
   D. Pit, pile, pretest and chemical grouting
   Answer: D. These four are the underpinning methods named; the others are shoring, pointing and scaffold types. (Key point: Methods)

4. In the pit method, why is a gap of about 75 mm left under the old footing before packing?
   A. To allow dry mortar packing to be rammed in for tight contact
   B. To let water drain
   C. To insert a damp-proof course
   D. To install a pile
   Answer: A. The gap is rammed with dry mortar so the new pier fully bears the old footing. (Key point: Pit method gap)