---
title: "Structural control on geo-hazards"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:structural-control-hazard"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/structural-control-hazard
site: BCT Engineering
---

# Structural control on geo-hazards

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2079 Chaitra, 3 marks; 2078 Poush (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/structural-control-hazard
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

Describe the various structural controls on geo-hazards (landslides).

## Answer

Structural control means that the **orientation and nature of geological structures** (bedding, foliation, joints, faults, folds) decide whether a slope will fail, and in what manner.

1. **Dip of bedding/foliation versus slope:**
   - *Dip slope* (dip direction same as the slope, dip less than the slope angle): bedding planes daylight, giving planar slides.
   - *Dip into the slope* (reverse): stable against sliding, but toppling is possible.
   - *Dip across the slope:* wedge or lateral movement.
2. **Joint sets:** two intersecting joint sets form a wedge whose line of intersection daylights in the slope face (wedge failure).
3. **Faults, shear zones and thrusts:** crushed, clay-rich weak zones act as slip surfaces and conduits for water; many large landslides lie along the MBT and MCT zones.
4. **Folds:** limbs dipping out of the slope are unstable; fold hinges have fractured rock.
5. **Joint spacing and persistence:** closely spaced, continuous joints lead to rock fall and toppling.
6. **Weak interbeds** (clay, mica schist, shale layers) form planes of sliding.
7. **Discontinuity condition:** smooth, clay-filled, wet joints have low friction.

Conditions for planar failure: the strike of the plane is within about 20 degrees of the slope face, and the dip of the plane is less than the slope angle but more than the friction angle. These are checked by stereographic projection (kinematic analysis).

## Check yourself

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

1. When is a dip slope prone to planar failure?
   A. Bedding is horizontal
   B. Dip is steeper than the slope
   C. Bedding dips the same way as the slope with dip less than slope angle
   D. Bedding dips into the hill
   Answer: C. Bedding planes daylight, so planar slide is possible. (Key point: Dip slope)

2. What causes wedge failure?
   A. A single vertical joint
   B. Horizontal beds
   C. Dip into the slope
   D. Two intersecting joint sets whose intersection line daylights
   Answer: D. Wedge is formed by intersecting joints. (Key point: Wedge failure)

3. Why are fault and shear zones important to landslides?
   A. Crushed clay-rich weak zones act as slip surfaces and water conduits
   B. They are strongest rocks
   C. They block groundwater
   D. They reduce slope angle
   Answer: A. Most large slides are located on MBT and MCT zones. (Key point: Fault zones)

4. What is the usual hazard when beds dip into the slope?
   A. Debris flow
   B. Stable, but toppling is possible
   C. Planar slide
   D. Wedge slide
   Answer: B. Reverse dip is stable, but may topple. (Key point: Dip into slope)

## Related in Engineering Geology II

- [Landslide causes and mitigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Alandslide-causes-mitigation) — Engineering Geology II, chapter 5, asked 6×

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