---
title: "Formwork and timbering for trench excavation"
subject: "Building Technology"
chapter: 8
key: "semester_6/building_technology:trench-formwork"
source: https://www.bctengineer.com/q/semester_6/building_technology/trench-formwork
site: BCT Engineering
---

# Formwork and timbering for trench excavation

Building Technology · Chapter 8: Temporary construction

- Asked in: 2081 Chaitra, 6 marks; 2072 Asoj, 6 marks; 2070 Chaitra (old course), 4 marks (asked 3 times)
- Video: [Explain four method of formwork for Excavation in Trenches with figure | Building Construction |](https://www.youtube.com/watch?v=CbNpIwzhioo) — 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/trench-formwork
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-8

## Question

Explain formwork (timbering) for excavation of trenches.

## Short answer

Timbering is required when the trench exceeds 1.5 m depth or soil is loose. In firm soil, poling boards with struts at 2-3 m intervals support the sides. In loose soil, close sheeting, walings and struts form a box; runners or sheet piling are used in very soft ground.

## Answer

Timbering (formwork) of trenches is the temporary support of the sides of an excavation with timber or steel, to prevent the earth from collapsing and to protect the workers. It is needed when the trench is deeper than about 1.5 m or the soil is loose, wet or surcharged.

#### Timbering in firm soil
- **Stay bracing:** vertical boards (poling boards, about 200 x 40-50 mm) are placed in pairs on opposite sides of the trench at about 2-3 m intervals and held apart by one or two horizontal **struts**. Used in firm soil for depths up to about 2 m.

#### Timbering in loose soil
- **Box sheeting:** vertical (or horizontal) sheeting boards are placed close together and held by horizontal **walings** and cross **struts**, forming a box. Used in loose soil for depths up to about 4 m.
- **Vertical sheeting:** used for deeper trenches (up to about 10 m) in soft soil. The trench is dug in stages of 2-3 m, each stage having its own closely placed vertical boards, walings and struts, with an offset of 250-500 mm at each stage.
- **Runners:** long, thick boards are driven about 300 mm ahead of the excavation in very soft or running soil.
- **Sheet piling:** timber or steel sheet piles are driven for wide or deep excavations in soft, wet soil.

```text
   ground level
  |<--- trench width --->|
  |poling   strut   poling|
  |board  ====o====  board|
  |   |  waling      |    |
  |   |========o=====|    |
  |   |              |    |
  |___|______________|____|
```
Components: poling boards, struts (props, adjustable with wedges), walings, runners, and cleats.
Precautions: inspect daily, keep spoil heaps at least 1 m from the edge, fix struts tightly, and provide ladders and drainage.

## Check yourself

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

1. Why is timbering of trenches provided, and when is it generally needed?
   A. To increase trench width; for any depth
   B. To support the sides and protect workers; when deeper than about 1.5 m or soil is loose, wet or surcharged
   C. To drain water; only in rock
   D. To support the foundation concrete; only after casting
   Answer: B. Timbering prevents collapse of the sides and protects workers, and is needed for depths over about 1.5 m or in poor soil. (Key point: Purpose and need of timbering)

2. Stay bracing is used in which situation?
   A. Running soil deeper than 6 m
   B. Fairly firm soil up to about 2 m depth, with poling boards at 2-3 m intervals held by struts
   C. Rock excavation
   D. Only on one side of the trench
   Answer: B. Stay bracing uses vertical poling boards at intervals on both sides held apart by struts, for fairly firm soil up to about 2 m. (Key point: Stay bracing)

3. Which method suits very soft or running soil and depths over 3 m?
   A. Stay bracing
   B. Box sheeting
   C. Runners and sheet piling driven ahead of excavation
   D. Open cut with no support
   Answer: C. Runners and sheet piling are driven ahead of the excavation for very soft or running soil and depths over 3 m. (Key point: Timbering for loose soil)

4. Which precaution is recommended for trench timbering?
   A. Pile the spoil right at the trench edge
   B. Inspect daily and keep spoil heaps 1 m or more from the edge
   C. Leave struts loose to allow movement
   D. Avoid ladders to save space
   Answer: B. Daily inspection, spoil kept at least 1 m away, tight struts, ladders and drainage are the listed precautions. (Key point: Precautions)

## Related in Building Technology

- [Formwork for excavation, timbering, RCC formwork](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aformwork-timbering-062p) — Building Technology, chapter 8
- [Factors in construction stage of trench excavation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atrench-excavation-factors) — Building Technology, chapter 2