Questions: Mass Wasting: Types, Triggers, and Hazard Assessment

Question 1

A hillside of saturated clay has been stable for 30 years but fails after a heavy rainstorm, with no change in slope angle. Which explanation best accounts for this in terms of the factor of safety?

• A. The rainstorm added significant weight to the slope, increasing the gravitational driving force beyond what friction could resist
• B. The rainstorm increased pore water pressure in the clay, reducing effective friction along the failure surface and lowering the factor of safety below 1
• C. The rainstorm caused undercutting erosion at the slope base, creating a free face that triggered failure
• D. The rainstorm triggered a minor seismic event, providing the impulsive force needed to overcome static friction

Answer: (B)

While rainfall adds weight (option A), the more critical mechanism in saturated clay is pore water pressure. Water filling pore spaces pushes grains apart, reducing grain-to-grain contact and therefore frictional resistance along the potential failure surface. The factor of safety (FS = resisting forces / driving forces) drops below 1 primarily because the resisting force decreased, not because the driving force increased substantially. A slope stable at FS = 1.05 can fail rapidly if rainfall reduces shear strength enough to push FS below 1 — the slope was not 'triggered,' it was weakened to the point of failure.

Question 2

A fast-moving slurry of water-saturated soil, rock fragments, and vegetation flowing down a valley channel is best classified as a:

• A. Rockfall, because the material includes rock fragments moving rapidly under gravity
• B. Creep, because the material moves continuously in a defined channel
• C. Debris flow, because it combines high water content with chaotic solid material flowing rapidly
• D. Landslide, because it involves movement of soil and rock downslope

Answer: (C)

A debris flow is specifically characterized by high water content (a saturated slurry), rapid movement, a chaotic mixture of water, rock, soil, and organic material, and typically channel-confined flow. It differs from a landslide (coherent movement of a mass along a defined failure surface), a rockfall (free-falling individual blocks from a cliff), and creep (imperceptibly slow, continuous movement without a discrete flow). Debris flows are among the most dangerous mass wasting events because they combine the speed of a flood with the destructive mass of solid material.

Question 3

Pore water pressure from groundwater or heavy rainfall reduces frictional resistance along a potential failure surface, which can cause a previously stable slope to fail even if the slope angle remains unchanged.

• T. True
• F. False

Answer: True

Pore water pressure is one of the most important and counterintuitive factors in slope stability. Water filling pore spaces exerts pressure that partially supports the overlying grains, reducing grain-to-grain contact stress and therefore frictional resistance. The effective normal stress — and thus friction — decreases as pore pressure increases. This is why slopes fail during or after heavy rain even though the slope angle (the gravitational driving force) has not changed: the resisting force was reduced, lowering the factor of safety below 1.

Question 4

Mass wasting events can seldom occur without a discrete external trigger such as an earthquake or rainfall — slopes will remain indefinitely stable until one of these triggers occurs.

• T. True
• F. False

Answer: False

Slopes can fail without a discrete external trigger. Creep occurs continuously on many slopes as a background process without any identifiable event. Progressive weakening from weathering can gradually reduce the factor of safety below 1 over years or decades — the 'trigger' is simply cumulative deterioration. Slow processes like freeze-thaw cycles, root growth and decay, and gradual water table rise can incrementally lower shear strength until failure occurs. The absence of a dramatic external event does not guarantee stability; it means only that no sudden trigger occurred, not that the slope is inherently safe.

Question 5

Why does removing vegetation from a hillside increase the risk of mass wasting, even if rainfall patterns and slope angle remain unchanged?

Model answer: Vegetation contributes to slope stability through two mechanisms. First, plant roots physically reinforce the soil by binding particles together and extending into deeper layers, increasing cohesion and shear strength along potential failure surfaces — essentially acting as a biological reinforcement system. Second, plants intercept rainfall and transpire significant amounts of water, reducing infiltration into the soil and limiting the buildup of pore water pressure. Removing vegetation simultaneously reduces shear strength (loss of root reinforcement) and increases water infiltration (less interception and transpiration), both of which lower the factor of safety. In steep terrain, deforestation can convert a slope with comfortable stability margins to one on the verge of failure without any change in rainfall or slope geometry.

This question targets mechanism-level understanding of vegetation's role, which goes beyond 'plants hold soil.' The two mechanisms — root reinforcement (mechanical) and interception/transpiration (hydrological) — both operate through the factor of safety framework and are independently significant. Understanding both is necessary for hazard assessment in deforested landscapes.