Mass movement is the downslope movement of rock and soil under gravity; it includes slow creep, slides, slumps, flows and rockfalls; failure is triggered when the driving force (gravity, increased by steep slopes, heavy rain, loading and undercutting) exceeds the resisting force (friction and cohesion, reduced by water and weak or weathered rock); the risk is reduced by improving drainage, reducing slope angle, building retaining structures and avoiding building on unstable ground.

What this dot point is asking

Eduqas wants you to explain mass movement, the downslope movement of rock and soil under gravity, to name its types (creep, slides, slumps, flows and rockfalls), and to explain why slopes fail using the balance between the driving force (gravity, raised by steep slopes, heavy rain, loading and undercutting) and the resisting force (friction and cohesion, lowered by water and weak rock). You also need to explain how risk is reduced (drainage, reducing slope angle, retaining structures, and avoiding unstable ground). The high-mark questions ask you to apply the force balance, especially the role of water.

The answer

What mass movement is

Mass movement is the movement of rock and soil downslope under gravity. It ranges from imperceptibly slow to catastrophically fast:

• Creep. Very slow, continuous movement of soil downhill (millimetres a year), shown by tilted fences and curved tree trunks.
• Slides. A mass moves down a flat or planar surface, keeping much of its shape (a landslide).
• Slumps. A mass rotates along a curved surface, so the top tilts back (common in clay cliffs).
• Flows. Saturated material flows like a fluid (mudflows and earthflows).
• Rockfalls. Blocks detach from a cliff and fall or bounce down.

Why slopes fail: the force balance

Whether a slope holds or fails comes down to two opposing forces:

• The driving force is the downslope pull of gravity. It is larger on steeper slopes, when extra weight is added (rain, buildings, spoil), and when the base is undercut.
• The resisting force is the friction and cohesion holding the material in place. It is smaller in weak, weathered or clay-rich rock and when water is present.

A slope is stable while the resisting force is greater than the driving force, and it fails when the driving force exceeds the resisting force.

The triggers (especially water)

A slope near the balance can be tipped into failure by a trigger:

• Heavy rain. Water does two things at once: it adds weight (raising the driving force) and it lubricates the grains and raises pore-water pressure (pushing grains apart and lowering friction and cohesion, so the resisting force falls). This is why so many landslides follow heavy rain.
• Steepening or undercutting. Waves at a cliff base, or a road cut, remove support and steepen the slope, raising the driving force.
• Loading. Buildings or spoil tips add weight to the top of a slope.
• Earthquakes. Shaking can trigger slides and rockfalls directly.
• Weak or weathered rock. Clay and heavily weathered rock have low strength, so the resisting force is low to begin with.

Reducing the risk

Engineering measures work by lowering the driving force or raising the resisting force:

• Improve drainage. Drains and pipes remove water, cutting the weight and restoring friction and cohesion. Often the most effective single measure, because water is the usual culprit.
• Reduce the slope angle (regrade). Cutting the slope back, or terracing it, lowers the downslope component of gravity.
• Retaining structures. Walls, rock bolts, gabions and netting physically hold material back, adding to the resisting force.
• Vegetation. Plant roots bind soil and take up water.
• Avoid building on unstable ground. Hazard mapping identifies slopes likely to fail so development can be sited elsewhere.

Examples in context

Example 1. Coastal clay cliffs. Soft clay cliffs slump repeatedly because clay is weak when wet and waves keep undercutting the base. Drainage and sea defences at the toe are the usual response.

Example 2. Creep on a hillside. Slow soil creep is harmless to people but tilts fence posts and walls and curves tree trunks over years, a reminder that gravity acts on every slope, not just the steep ones.

Try this

Q1. Name the force that drives mass movement. [1 mark]

• Cue. Gravity (the downslope component of gravity, the driving force).

Q2. Explain why heavy rain makes a slope more likely to fail. [2 marks]

• Cue. Water adds weight (increasing the driving force) and lubricates the grains and raises pore-water pressure (reducing friction and cohesion, the resisting force).

Q3. Give one engineering measure that reduces the risk of mass movement and how it works. [2 marks]

• Cue. For example improving drainage (removes water, lowering weight and raising friction), or reducing the slope angle (lowers the downslope pull of gravity).