---
title: "Fold, fault, joint, thrust in tunnelling"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:fold-fault-joint-thrust-in"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fold-fault-joint-thrust-in
site: BCT Engineering
---

# Fold, fault, joint, thrust in tunnelling

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2063 Baisakh (old course), 4+4 marks
- Video: [Overview of Geologic Structures Part 2: Faults and Folds](https://www.youtube.com/watch?v=aodnQWXYkZ0) — Professor Dave Explains
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fold-fault-joint-thrust-in
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Define fold, fault, joint and thrust. Describe the engineering significance of each of the above mentioned geological structures in tunnelling.

## Short answer

Fold: bent rock layers; anticline crests are fractured causing roof falls, syncline troughs have high pressure and water; tunnel should cross fold axis at high angle. Fault: crushed zone causes collapse, squeezing, water inflow; cross at right angle, probe and grout. Joint: cause overbreak, wedge falls, seepage; support with bolts and shotcrete. Thrust: wide shear zone with high squeezing and water inflow, requiring heavy support.

## Answer

#### Definitions
- **Fold**: a bend or wavy curvature of originally flat rock layers caused by compression, such as an anticline or syncline.
- **Fault**: a fracture in the rock along which the two blocks have displaced relative to each other.
- **Joint**: a fracture in the rock along which there has been no (or negligible) displacement.
- **Thrust**: a low-angle (<45°) reverse fault where the hanging wall moves up over the foot wall, bringing older rocks over younger.

#### Significance in tunnelling
| Structure | Significance in tunnelling |
|---|---|
| **Fold** | Crest of an anticline is loosened and fractured (roof falls); the trough of a syncline has high rock pressure and water collection. The tunnel axis should cross the fold axis at a high angle and avoid running along the axis in the crest or trough. Limb dip decides the support type. |
| **Fault** | Crushed zone (breccia, gouge) gives roof collapse, squeezing and heavy support. Faults carry water, giving sudden inflow and flooding. Tunnels should cross faults at a right angle, and the active fault may displace the lining. Probe drilling and grouting are used. |
| **Joint** | Cause overbreak, wedge falls and water seepage. Tunnels parallel to the main joint set are less stable; rock bolts, shotcrete and grouting are used for support. |
| **Thrust** | A wide shear zone of crushed, sheared rock with sheared clay, very high squeezing and high water inflow; for example, tunnels in the Himalaya crossing MCT and MBT face heavy support and delays. |

## Check yourself

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

1. What is a thrust?
   A. A vertical normal fault
   B. A fracture without displacement
   C. A low-angle (<45°) reverse fault where older rocks are carried over younger
   D. A bend in rock layers
   Answer: C. Hanging wall moves up over the foot wall. (Key point: Thrust definition)

2. How should tunnels cross faults?
   A. Along the fault plane
   B. Parallel to the fault zone
   C. Inside the crushed zone
   D. At a right angle
   Answer: D. Crossing at high angle shortens exposure. (Key point: Tunnel crossing)

3. What problems do joints cause in tunnels?
   A. Overbreak, wedge falls and seepage
   B. Higher stand-up time
   C. No problem
   D. Lower permeability
   Answer: A. Joints form wedges and water paths. (Key point: Joint significance)

4. What does a tunnel crossing the MCT or MBT face?
   A. Fresh massive rock needing no support
   B. Heavy support, squeezing, high water inflow and delays
   C. Only dry ground
   D. Light lining only
   Answer: B. Thrusts form wide crushed shear zones. (Key point: Thrust zones in Himalaya)

## Related in Engineering Geology I

- [Engineering significance of joint and fault](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-joint-and) — Engineering Geology I, chapter 5, asked 2×
- [What is joint; engineering significance of joint and fault](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Awhat-is-joint-engineering-significance) — Engineering Geology I, chapter 5, asked 2×
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- [Engineering significance of fold](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-significance-of-fold) — Engineering Geology I, chapter 5
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