6.1 Factors That Control Slope Stability

GEOL1340

Mass Wasting (Mass Movement)

The downslope movement of rock, regolith, and soil driven directly by the force of gravity without requiring a fluid transport medium (like air or water).

Geological Role of Mass Wasting

Acts as the first step in sediment transport within the rock cycle and serves as the most rapid mechanism for modifying slope shapes and reshaping landscapes.

Three Baseline Requirements for Mass Wasting

1. Fracturing and weathering (to weaken intact material); 2. An unstable slope (near its stability threshold); 3. An initiating event or trigger.

Shear Force (Fd)

The gravitational component pushing material directly down a slope. Increases as slope angle steepens.

Shear Strength (Fr / Friction)

The resistance force opposing downslope movement, dictated by material cohesion, internal friction, and structural integrity.

Condition for Slope Failure

Slope failure occurs when the shear force (Fd) exceeds the shear strength (Fr) ($F_d > F_r$).

Angle of Repose

The steepest angle at which loose, unconsolidated sediment remains stable without sliding downslope.

Angle of Repose for Fine Sand

Approximately 35 degrees.

Angle of Repose for Coarse Sand

Approximately 40 degrees.

Angle of Repose for Angular Pebbles/Gravel

Approximately 45 degrees (steeper due to interlocking grain friction).

Effect of Small Amounts of Water on Sediments

Increases shear strength and cohesion through surface tension at grain boundaries, binding grains together (e.g., damp sandcastles).

Effect of Water Oversaturation on Sediments

Destroys cohesion, increases pore water pressure, forces grains apart, reduces internal friction, and adds ~13% more weight (at 25% porosity), drastically destabilizing slopes.

Pore Water Pressure

Pressure exerted by water in pore spaces that pushes mineral grains or rock joints apart, reducing normal friction and causing slope failure.

Orientation of Weakness Planes (Bedding / Foliation)

Unstable when planes dip parallel to the slope face (acting as slip planes); stable when planes dip perpendicular to the slope face.

Role of Vegetation Cover in Slope Stability

Roots anchor soil and stabilize potential failure surfaces, while leaves intercept rainfall and reduce soil saturation through evapotranspiration.

Impact of Stripping Vegetation

Wildfires, land clearing, or deforestation remove root support and increase exposure to water saturation, elevating landslide risk.

Four Primary Structural Preconditions for Slope Instability

1. Fracturing and weathering; 2. Unstable slope geometry (exceeding angle of repose); 3. Unfavorable weakness plane orientation; 4. Sparse vegetation cover.

Primary Mass-Wasting Triggers

1. Oversteepening (river undercutting, road cuts); 2. Saturation (heavy rain, rapid snowmelt); 3. Adding weight atop slopes; 4. Ground shaking (earthquakes, traffic, blasting).

Golden Rule of Landslide Prediction

"Landslides occur where they have occurred before." Past scars and hummocky terrain are the primary indicators of future risk.

Why BC and California are Landslide Hotspots

1. Young, active mountain belts undergoing rapid weathering; 2. Steep slope relief; 3. Frequent seismic activity combined with heavy seasonal rainfall.

Shear Strength of Crystalline Rocks

High (e.g., granite, basalt, gneiss are very strong).

Shear Strength of Metamorphic Schist

Moderately strong to weak; often contains sheet silicates (mica/chlorite) that act as internal weakness planes.

Shear Strength of Sedimentary Rocks

Variable; dolostone/limestone are strong, sandstone is moderate to weak, and mudstone is very weak.

Cohesion in Unconsolidated Sediments

Unconsolidated sediments lack mineral cementation and compression, making them weaker than sedimentary rocks. Sand and silt are particularly weak.

Glacial Till Strength

Very strong (can form near-vertical cliffs) because it has been compacted under tens to thousands of meters of glacial ice.

Smectite (Bentonite) Clays

Expand significantly when absorbing water at a molecular level, becoming extremely slippery with nearly zero shear strength.

Hope Slide (1965) Key Factors

Chlorite schist with clay-altered feldspar sills dipping parallel to a steep slope; trapped spring water froze at the toe, causing catastrophic pore water pressure buildup.

Point Grey Cliffs (Vancouver) Material Setup

Upper layer of weak, un-cemented sand resting on lower, denser layers of saturated sand, silt, and clay.

Sapporo, Japan (2018) Compound Event

Drenched by rainfall from Typhoon Jebi (Sept 4), then shaken by an M6.6 earthquake (Sept 6), triggering thousands of debris flows in saturated volcanic material.

Laguna Beach Landslide (2005) Cause

Occurred in Blue Bird Canyon on an old 1978 slide plane, triggered by extreme rainfall and altered surface water drainage patterns.