2.2.1 mass movement

What is mass movement and when does it occur?

Mass movement = downslope movement of sediment, soil and rock material as a single unit under the driving force of gravity.

Occurs when:

• Shear stress > shear strength

• → slope failure

• → mass movement

What is the angle of repose?

• The steepest angle at which a sloping surface of weathered/eroded material remains stable without collapsing.

  • At equilibrium: shear stress = shear strength

  • Slope exceeds angle of repose → shear stress > shear strength → mass movement

  Typical angles:

  • Clay/moist sand: 20–35°

  • Dry sand: 20–30°

  • Moist sand: 30–45°

  • Wet sand: 20–30°

What are shear stress and shear strength?

• Shear stress

  • Gravitational force attempting to pull material downslope.

  • Slope angle ↑ → shear stress ↑

  Shear strength

  • Internal resistance of material to downslope movement.

  • ↑ with friction + cohesion + vegetation/root binding

What is Factor of Safety (FS)?

FS = shear strength ÷ shear stress

• FS > 1 → slope stable

• FS ≤ 1 → slope failure

• Shear stress ↑ and/or shear strength ↓ → FS ↓ → mass movement risk ↑

What is pore water pressure and how does it affect slope stability?

Pore water pressure = pressure of fluid within pore spaces of soil/rock.

Excess water:

• Pore spaces fill → pore water pressure ↑

• Particles pushed apart

• Friction/cohesion ↓

• Shear strength ↓

Water also:

• Adds weight → shear stress ↑

• Acts as lubricant → friction ↓

• → mass movement risk ↑

How did Carson & Kirby classify mass movements?

Based on:

• Speed of movement

• Moisture content

Heave → slide → flow

What are falls and what are their main types?

Fall = material breaks away and falls from near-vertical/vertical slopes, often falling → rolling → bouncing.

Rock fall

• Sudden vertical movement of rock blocks/fragments.

• Common in arid + polar environments.

Debris fall

• Mixture of rock + weathered regolith.

• Common in seasonally humid/humid tropics.

Evidence:

• Recession of steep rock wall

• Talus accumulation at base

Causes:

• Well-jointed/bedded rocks weakened by weathering

• Pressure release

• Tectonic/seismic activity

• River undercutting of slope base

What is a flow and what are its 3 main types?

Flow = mass is internally deformed through liquefaction and moves downslope like a viscous fluid.

• Movement is generally continuous

• Velocity is fastest near surface and ↓ with depth due to friction

Types:

• Earth flow

• Mud flow

• Debris flow

What is an earth flow and what evidence does it produce?

Earth flow = saturated soil, clay-rich rock/regolith or loose sediment flowing downslope as a viscous mass.

• Usually relatively slow

• Triggered by heavy rainfall, undercutting or existing springs

Evidence:

• Steepened scarp at top

• Convex longitudinal profile

• Toe/tongue-shaped deposit at bottom

What is a mud flow?

Mud flow = saturated fine-textured sediment, e.g. clay/silt, flowing downslope.

• Water may form up to 60% of its weight

• Faster and may carry more material than earth flows

• Heavy rainfall → runoff collects fine sediment → mud concentration ↑ → viscous flow

Volcanic mudflow = lahar

• Snowmelt/heavy rain + volcanic debris → mudflow

More dangerous

• Move faster

• Carry more material

• May discharge suddenly across piedmont zones, where settlement/agriculture may occur.

What is a debris flow and what evidence does it produce?

Debris flow = rapid movement of relatively coarse material, especially in mountainous areas.

• Often triggered by extremely heavy rainfall → oversaturation

• Can carry large boulders

• Highly destructive

Other triggers:

• Rapid snowmelt

• Exceptional rainfall

• Collapse of volcanic crater lakes

Evidence:

• Tongue-like front

• Ridges and furrows

• Irregular surface

• Alluvial fans at valley mouths

What is soil creep and how does it occur?

Soil creep = slow downslope movement of soil particles due to net displacement from repeated expansion + contraction.

Main processes:

• Wetting/drying

• Freeze-thaw

• Rainsplash

• Biological activity

Mechanism:

• Particle expands/lifts perpendicular to slope

• Contracts/falls vertically under gravity

• → net downslope displacement

Rates:

• Humid tropics: ~3–6 mm/year

• Semi-arid areas with cold winters: up to ~10 mm/year

Evidence:

• Terracettes

• Bent tree trunks

• Leaning poles/fences

• Downslope accumulation against obstacles

What is a slide and what are its 2 main types?

Slide = failure along a shear plane, where rock/regolith moves downslope as a coherent mass (en masse).

Translational / planar slide

• Moves along a straight/inclined plane of weakness

Rotational slide / slump

• Moves along a curved concave shear plane

What conditions favour translational slides?

• Steep slopes

• Joints/bedding planes dipping downslope

• Weak clay/silt layers beneath heavier rocks

• Earthquakes

• Undercutting/oversteepening

• Water → pore water pressure ↑ → links between particles weakened

What are the 3 types of translational slides?

Landslide

• Mixture of regolith

• Common in humid climates with moderate–large amounts of moisture

• Movement is uniform with no rotation

• Water → pore water pressure ↑ → links between particles weaken → sliding

Rockslide

• Broken bits/fragments of rock

• Usually associated with dry climates that facilitate physical weathering

• Sudden downslope movement of individual masses of resistant rock

• Bedding planes + joints are crucial controls

Mudslide

• Mainly clay-sized material

• Water is usually present

What evidence indicates a translational/planar slide?

• Material moves along a relatively straight slide plane

• Fragments may break up during movement

• → piles of talus at the bottom of the slope

How does a rotational slide/slump occur and what evidence does it produce?

Sudden movement of cohesive unsaturated regolith/bedrock along a curved concave shear plane.

Often:

• Clay/silt layers present

• Water percolates into layer

• Lubrication ↑ + shear strength ↓

• → rotational failure

Evidence:

• Crescent-shaped scarp at top

• Upper surface of slump block relatively undisturbed

• Lobe/tongue-shaped deposit downslope

What are the 6 natural factors influencing mass movement?

• Climate & water content

• Vegetation

• Weathering

• Tectonic activity

• Relief

• Geology

How does tectonic activity trigger mass movement?

Earthquakes

• Vibrations loosen/displace material

• Friction/cohesion ↓

• → shear strength ↓ → slope failure

Volcanic activity

• Unconsolidated volcanic material → unstable

• Eruption may melt snow/ice

• Meltwater + volcanic debris → lahars/mudflows

How does relief affect mass movement?

Generally:

Slope angle ↑
→ downslope gravitational force ↑
→ shear stress ↑ → mass movement risk ↑

However:

• Most slides occur at intermediate slopes of ~31–39°

• Extremely steep slopes may lack enough material for slides

Gentle slopes:

• Lower shear stress

• But high infiltration → saturation/pore water pressure ↑ → failure still possible

How does geology affect mass movement?

Geology controls:

• Rock/material strength

• Permeability/porosity

• Weathering

• Water infiltration/percolation

• Planes of weakness

Important situations:

• Joints/faults/bedding planes → weaknesses

• Permeable rock → infiltration/saturation ↑

• Impermeable clay beneath permeable rock → water accumulates → boundary lubricated → shear strength ↓

• Bedding planes dipping downslope → potential slip plane

What are the 4 main human factors influencing mass movement?

• Deforestation

• Urbanisation

• Construction

• Mining

Deforestation case study.

Colombia, 2017

• ~100,000 ha land/forest cleared annually

• 130 mm rainfall in one night

• Landslides killed >330 people in Mocoa and Manizales

How do urbanisation and construction increase mass movement?

Urbanisation:

• Development expands onto steep/marginal slopes

• Hong Kong: landslides have killed nearly 500 people since 1948

Construction:

• Vegetation/soil removal → cohesion ↓

• Machinery vibrations → particles loosen

• Excavation → weaker material exposed

• Buildings/roads add weight → shear stress ↑

• Undercutting/oversteepening → instability ↑

CS: Clementi, Singapore (2022)

• Landslide after heavy rain near Clementi NorthArc BTO

• Occurred while a retaining wall was being constructed

How does mining increase mass movement?

Surface mining

• Soil loosened

• Erosion/topsoil loss ↑

• Drainage disrupted → water pooling/saturation ↑

• → slope stability ↓

Underground mining

• Underground voids/goafs created

• Support removed

• Stress redistributed

• → collapse/subsidence + mass movement

CS: Indonesia, 2019

• Illegal gold mine in North Sulawesi collapsed

• >40 people buried and killed

What happened in the 1970 Yungay, Peru debris flow?

• Earthquake dislodged an ~800 m mass of ice + rock from Huascarán

• Fell ~1,000 m

• Generated enormous debris flow

• Reached Yungay in 157 seconds at ~480 km/h

• ~10,000 people killed

Shows: tectonic activity → debris flow

How does the 1985 Armero tragedy show volcanic mass movement?

• Nevado del Ruiz erupted after 69 years of dormancy

• Snow/ice melted

• → water + volcanic debris → lahar/mudflow

• Travelled >30 m/s

• >20,000 people killed

• ~85% of Armero covered in mud

How does the 2021 Pasir Ris landslide show the role of rainfall?

• Rainfall was within the top 1% of maximum daily rainfall records in 39 years

• 184.4 mm fell from 12am–12pm on 2 Jan 2021

• Heavy rainfall → saturation ↑

• Pore water pressure ↑

• Shear strength ↓

• → slope failure