---
title: "Underpinning: necessity, procedure, methods"
subject: "Building Technology"
chapter: 8
key: "semester_6/building_technology:underpinning-necessity-methods"
source: https://www.bctengineer.com/q/semester_6/building_technology/underpinning-necessity-methods
site: BCT Engineering
---

# Underpinning: necessity, procedure, methods

Building Technology · Chapter 8: Temporary construction

- Asked in: 2071 Magh, 8 marks; 2065 Shrawan (old course), 16 marks; 2062 Baisakh (old course), 16 marks (asked 3 times)
- Video: [Under Pinning of Foundation | Methods of under pinning of foundation | Prashant YT | Building const|](https://www.youtube.com/watch?v=9ehqwwQWD1w) — Prashant YT
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/underpinning-necessity-methods
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-8

## Question

Define underpinning. Describe its necessity and the methods (types and procedure) of underpinning.

## Short answer

Underpinning strengthens or deepens an existing foundation. It is needed when the foundation has settled, loads increase, adjacent excavation occurs, or soil is weakened. The pit method (most common) excavates alternate 1-1.5 m sections, fills with concrete to required depth, and packs the 75 mm gap with dry mortar; other methods include piling, pretest, grouting, and needle beams.

## Answer

#### Definition
**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
- Foundation has settled or is weak due to poor soil or an overload.
- A building is to be raised (extra storeys) and loads increase.
- Adjacent deep excavation or new basement lowers the support (e.g. neighbouring building).
- A change in water table, scouring or earthquake damage reduces bearing capacity.
- Faulty or shallow original foundation, or damage by roots, mining, or subsidence.

#### Methods of underpinning
1. **Pit method (pier method):** The wall is divided into 1-1.5 m sections. Pits are dug below the old footing in alternate sections (the others stay in place), the soil is excavated to the required depth and filled with concrete or masonry, and a gap of about 75 mm is left under the old foundation, then packed with dry mortar or wedges. This is repeated with the next sections.
2. **Pile method:** Piles (bored or driven) are installed on both sides of the wall and connected by cross beams (needles) below the wall. Useful when good soil is deep.
3. **Pretest method:** Hydraulic jacks apply a test load to piles, then wedges are inserted to avoid later settlement.
4. **Chemical (grouting) method:** Cement or chemical grout is injected into weak soil to increase bearing capacity.
5. **Cantilever/needle beam method:** Needle beams carry the wall's load so the footing can be rebuilt below.
6. **Jet grouting / micropile method** for tight spaces.

#### Procedure of the pit method (most common)
1. Study the soil and the structure, and support the building by shoring if necessary.
2. Dig the first pit below the footing, in sections about 1-1.5 m wide, leaving alternate sections untouched.
3. Dig to the design depth, with timbering support if needed.
4. Fill with PCC or masonry in cement mortar (1:4) up to about 75 mm below the old foundation.
5. Pack the gap with dry cement-sand mortar (1:1) or wedges and leave to set for 3-4 days.
6. Repeat for the intermediate sections, in a planned order.
7. Backfill and compact.

```text
   wall
   ||||||  old footing
   ||||||====|  pit (section 1)
          |  new masonry/concrete
          |  packing (75 mm gap)
          |__ firm stratum
```

## Check yourself

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

1. Underpinning is:
   A. Temporary propping of a wall with rakers
   B. Strengthening or deepening the foundation of an existing building by extending it to a firmer or deeper stratum or enlarging the base
   C. Lowering the water table
   D. Plastering the lower part of a wall
   Answer: B. Underpinning improves the foundation of an existing building so that it can carry its load safely. (Key point: Definition)

2. Which situation calls for underpinning?
   A. Painting the building
   B. A building is to be raised by extra storeys, increasing the load on the foundation
   C. Replacing floor tiles
   D. Fixing window frames
   Answer: B. Added storeys, settlement, adjacent deep excavation and reduced bearing capacity are reasons to underpin. (Key point: Necessity)

3. In the pit method, why are pits dug only in alternate 1-1.5 m sections?
   A. To save concrete
   B. So the untouched sections keep supporting the wall while others are underpinned
   C. To make the wall lighter
   D. To let water drain
   Answer: B. Leaving alternate sections in place keeps the wall supported; the next sections are done after the first set. (Key point: Pit method principle)

4. What is done with the gap of about 75 mm left under the old foundation?
   A. Left empty
   B. Filled with sand only
   C. Packed with dry cement-sand mortar (1:1) or wedges and left to set for 3-4 days
   D. Filled with water
   Answer: C. Filling to about 75 mm below the old footing and packing the gap with dry mortar or wedges transfers the load to the new work. (Key point: Pit method gap packing)

## Related in Building Technology

- [Necessity of underpinning work](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aunderpinning-necessity-072m) — Building Technology, chapter 8