---
title: "Overbreak and its importance"
subject: "Engineering Geology II"
chapter: 4
key: "semester_4/engineering_geology_ii:overbreak-factor"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/overbreak-factor
site: BCT Engineering
---

# Overbreak and its importance

Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation

- Asked in: 2080 Chaitra, 3 marks; 2078 Baisakh, 1 marks; 2077 Chaitra, 1 marks; 2076 Bhadra, 1 marks; 2071 Bhadra, 1 marks; 2072 Asoj, 3 marks; 2079 Jestha, 2 marks; 2068 Bhadra, 1 marks (asked 8 times)
- Video: [Overbreak during Tunnelling](https://www.youtube.com/watch?v=aCHqL3c9DPw) — Pallavi Amit Padalkar
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/overbreak-factor
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4

## Question

What is overbreak? Explain its importance and the factors of overbreak in tunnel excavation.

## Short answer

Overbreak is excess excavation beyond the neat tunnel line, expressed as percent of design area. It increases costs and support needs; key factors are rock structure (joints, bedding, faults), weathering, groundwater, and blasting method (drilling precision, charge, delay in support).

## Answer

**Overbreak** is the excavation of rock beyond the designed (neat) tunnel profile. It is the volume or thickness of rock removed outside the theoretical excavation line, usually expressed as a percentage of the design area or as extra thickness in cm.

$$
\text{Overbreak} (\%) = \frac{A_{actual}-A_{design}}{A_{design}}\times 100
$$

#### Importance
- Increases excavation, mucking and disposal volume.
- Needs extra shotcrete/concrete lining to fill the void, so cost increases.
- Reduces stability: loosened rock above the crown can fall.
- Delays the construction schedule and endangers workers.
- Shows the quality of the rock mass and of blasting practice, so it is a performance indicator in the contract (payment is normally only for the neat profile plus a permitted tolerance).

#### Factors of overbreak
**Geological:**
1. Joint spacing and orientation (joints dipping to the crown or parallel to the axis).
2. Bedding, foliation, faults and shear zones.
3. Weak or weathered rock, low rock strength.
4. Groundwater flow and weathering.
5. High in-situ stress (spalling).

**Construction:**
1. Excavation method (drilling and blasting vs. TBM).
2. Blast design: drilling accuracy (look-out angle), hole spacing, explosive type and charge per hole.
3. Time lag before support is installed.
4. Tunnel size and shape.

Good practice: controlled blasting (smooth blasting, presplitting), systematic mapping and prompt support.

## Check yourself

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

1. What is overbreak?
   A. Rock left inside the design profile
   B. The rock removed by TBM only
   C. Rock removed outside the designed (neat) tunnel profile
   D. The extra support installed
   Answer: C. Overbreak = (A actual - A design)/A design x 100. (Key point: Overbreak definition)

2. Why is overbreak undesirable?
   A. It reduces the volume of muck
   B. It makes the crown more stable
   C. It is paid for fully by the contract
   D. It raises excavation, mucking and lining cost and reduces stability
   Answer: D. Extra rock must be removed and voids filled with shotcrete or concrete, while loosened rock may fall. (Key point: Importance)

3. Which geological condition increases overbreak most?
   A. Joints dipping toward the crown or parallel to the axis in weak rock
   B. Massive unjointed strong rock
   C. Joints perpendicular to the tunnel with tight spacing absent
   D. Dry rock with low in-situ stress
   Answer: A. Unfavourable joint orientation and weak rock release blocks beyond the neat line. (Key point: Geological factors)

4. Which practice reduces overbreak?
   A. Heavier charge per hole
   B. Controlled blasting such as smooth blasting or presplitting
   C. Delaying support installation
   D. Using no mapping
   Answer: B. Controlled blasting, systematic mapping and prompt support limit damage beyond the design line. (Key point: Control)

## Related in Engineering Geology II

- [Overbreak and tunnel geological survey](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aoverbreak-tunnel-survey) — Engineering Geology II, chapter 4
- [Geology in tunnel site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-site-selection) — Engineering Geology II, chapter 4, asked 4×

## Related in other subjects

- [Methods of tunnel excavation](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atunnel-excavation-methods) — Construction Management, chapter 4
- [Short note: Tunnel excavation by blasting](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atunnel-blasting-note) — Construction Management, chapter 4
- [Tunnel ventilation methods; bridge site selection factors](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-ventilation-methods-bridge-site-selection) — Transportation Engineering II, chapter 5, asked 2×
- [Tunnel ventilation methods; bridge site factors](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-ventilation-methods-bridge-site-factors) — Transportation Engineering II, chapter 5, asked 2×