---
title: "Mass movement and its mechanism"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:mass-movement-mechanism"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/mass-movement-mechanism
site: BCT Engineering
---

# Mass movement and its mechanism

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2079 Asoj, 2 marks; 2078 Chaitra, 1 marks; 2075 Bhadra, 1 marks; 2075 Baisakh, 3 marks; 2073 Bhadra, 2 marks; 2071 Magh, 2 marks; 2071 Bhadra, 1 marks; 2070 Bhadra, 2 marks; 2070 Magh, 2 marks; 2068 Bhadra, 1 marks; 2069 Bhadra, 1 marks (asked 11 times)
- Video: [Mass Wasting Mechanisms lecture](https://www.youtube.com/watch?v=Y-ekFgmFRgU) — Casey Johnson
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/mass-movement-mechanism
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

Define mass movement. Describe the mechanism of mass movement (how does mass movement occur?).

## Short answer

Mass movement is downslope movement under gravity. Slope fails when driving shear stress exceeds shear strength; factor of safety F = (cA + W cosβ tanφ)/(W sinβ) <1. Failure is triggered by load increase, steepening, pore-water pressure rise, or strength loss from weathering.

## Answer

**Mass movement (mass wasting)** is the downslope movement of rock, debris or soil under the influence of gravity, with or without water as a helping agent. Landslides, rock falls, creep, debris flows and subsidence are all forms of it.

#### Mechanism
A slope fails when the **driving (shear) forces exceed the resisting forces (shear strength)**. For a mass of weight $W$ on a slope of angle $\beta$:

$$
\text{Factor of safety } F = \frac{\text{Resisting force}}{\text{Driving force}} = \frac{c A + W\cos\beta\tan\phi}{W\sin\beta}
$$

Failure begins when $F<1$.

- **Increase of driving force:** extra load (buildings, fill, rainwater), steepening by river cutting or road excavation, removal of toe support, earthquake shaking, vibration.
- **Decrease of resisting force:** rise in pore water pressure (rain, snow melt, reservoir drawdown), weathering, loss of cohesion, removal of vegetation, clay softening, joint opening.

#### Steps
1. The slope is made unstable by preparatory factors (weak rock, steep slope, adverse structure).
2. A trigger (heavy rain, earthquake, cutting) acts.
3. Tension cracks form at the crown.
4. A slip surface develops and the mass moves downward by sliding, falling, toppling or flowing.
5. The material comes to rest on a gentler slope and forms a deposit.

## Check yourself

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

1. What is mass movement?
   A. Wind transport of dust only
   B. Vertical rise of land by uplift
   C. Downslope movement of rock, debris or soil under gravity
   D. Movement of plates only
   Answer: C. It occurs under gravity with or without water as a helping agent. (Key point: Definition)

2. When does a slope begin to fail?
   A. When F is greater than 1
   B. When resisting force is doubled
   C. When slope angle is zero
   D. When factor of safety F is less than 1
   Answer: D. F = resisting/driving force; driving force exceeding resistance means F<1. (Key point: Failure condition)

3. Which action lowers the resisting force of a slope?
   A. Rise in pore water pressure
   B. Planting vegetation
   C. Adding weight at the toe
   D. Building retaining walls
   Answer: A. Pore pressure, weathering and loss of cohesion reduce shear strength. (Key point: Decrease in resistance)

## Related in Engineering Geology II

- [Factors of mass movement](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Afactors-of-movement) — Engineering Geology II, chapter 5

## Related in other subjects

- [Define mass movement and its classification](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adefine-mass-movement-and-its) — Engineering Geology I, chapter 6, asked 5×
- [All mass movements are landslides](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aall-mass-movements-are-landslides) — Engineering Geology I, chapter 6
- [Balancing one mass using two planes](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Asingle-mass-two-plane-balance-derivation) — Theory of Mechanism and Machine II, chapter 5
- [Three-mass chain frequencies and modes](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Athree-dof-eigenvalues-mode-shapes) — Theory of Mechanism and Machine II, chapter 8