What causes slopes to fail, and how can landslide hazards be reduced?
Mass movement and landslide hazards: the types of mass movement (rockfall, slide, slump, flow and creep); the factors controlling slope stability (slope angle, rock and soil strength, water, bedding orientation, vegetation and undercutting); the triggers of slope failure; and the engineering and planning methods used to reduce landslide hazards.
A focused answer to the Eduqas Geology geohazards statement on mass movement. Covers the types of mass movement, the factors controlling slope stability, the triggers of slope failure, and the engineering and planning methods used to reduce landslide hazards.
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What this dot point is asking
Part of the Geohazards theme in Component 3 (and overlapping engineering geology). Eduqas wants you to know the types of mass movement (rockfall, slide, slump, flow, creep), the factors controlling slope stability (slope angle, rock and soil strength, water, bedding orientation, vegetation, undercutting), the triggers of failure, and the engineering and planning methods used to reduce landslide hazards. It is the balance between the forces driving a slope down and those resisting it.
The answer
Types of mass movement
Mass movement is the downslope movement of rock and soil under gravity. The main types, by mechanism and speed:
- Rockfall: detached blocks fall, bounce or roll freely from a steep cliff (fast).
- Slide (translational): a mass moves along a roughly planar surface, such as a bedding plane or joint dipping out of the slope.
- Slump (rotational slide): a mass rotates along a curved (concave) failure surface, leaving a back-tilted block and a steep scar (common in weak clays).
- Flow: saturated material (mudflow, debris flow, earthflow) moves as a fluid; the fastest and most mobile, able to travel far.
- Creep: the very slow (millimetres per year) downslope movement of soil, shown by tilted posts and curved tree trunks.
Factors controlling slope stability
A slope is stable when the resisting forces (friction and cohesion) exceed the driving force (the downslope component of gravity). Stability is reduced by:
- Slope angle: steeper slopes have a larger driving force; undercutting (by rivers, the sea or excavation) steepens and removes support.
- Water: the master variable. Water raises the pore pressure between grains, reducing the effective stress and so the friction, and it adds weight; saturation can liquefy soil into a flow.
- Rock and soil strength: weak, weathered or clay-rich materials resist less; strong, intact rock resists more.
- Bedding and joint orientation: planes (bedding, joints, faults) that dip out of the slope provide a slip surface, so they are dangerous; planes dipping into the slope are far more stable.
- Vegetation: roots bind soil and intercept water, increasing stability; clearing it reduces stability.
Triggers of failure
A slope close to failure is triggered over the edge by an event that tips the balance:
- Heavy rainfall or snowmelt (raises pore pressure), the commonest trigger.
- Earthquakes (shaking adds stress and can liquefy).
- Undercutting by erosion or excavation (steepens and removes toe support).
- Loading the top of the slope (construction, spoil), or volcanic activity.
Reducing the hazard
Mass movement is managed by engineering the slope and by planning:
- Reduce the slope angle by regrading or benching (terracing) to cut the driving force.
- Improve drainage (surface and horizontal drains) to lower pore pressure, the most effective single measure since water is the key control.
- Add retaining structures: retaining walls, gabions, rock bolts and anchors, and soil nailing to hold the slope.
- Protect the toe against undercutting (sea walls, revetments, rock armour).
- Plant vegetation to bind soil and intercept water.
- Hazard mapping and land-use planning keep development off unstable ground, reducing exposure.
Examples in context
Example 1. A coastal landslide. Where soft, water-bearing strata dip seawards and the sea undercuts the cliff toe, slumps and slides are common; drainage and toe protection (sea walls, rock armour) are the standard remedies.
Example 2. Soil creep on a hillside. Tilted fence posts and bent tree trunks record the slow, continuous downslope creep of soil, a reminder that mass movement is not only sudden catastrophic failure.
Try this
Q1. State three types of mass movement. [3 marks]
- Cue. Any three of: rockfall; (translational) slide; slump (rotational slide); flow (mudflow or debris flow); creep.
Q2. Explain why water reduces slope stability. [2 marks]
- Cue. Water raises the pore pressure between grains, which reduces the effective stress and so the friction holding the slope together, and it adds weight; saturation can turn soil into a flow.
Q3. Name the most effective single engineering measure to stabilise a wet slope, and why. [2 marks]
- Cue. Improving drainage, because water (pore pressure) is usually the controlling factor, so removing it restores friction and resistance.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksExplain how water, slope angle and the orientation of bedding each affect the stability of a slope, and how they can combine to cause a landslide.Show worked answer →
A levels-of-response answer; treat each factor, then their combination.
- Water
- Water raises the pore pressure between grains, reducing the friction (effective stress) that holds the slope together, and it adds weight; saturation can turn soil into a flow. Heavy rain or snowmelt is therefore a common trigger.
- Slope angle
- The steeper the slope, the greater the downslope component of gravity (the driving force) relative to the friction resisting movement, so steep slopes are less stable. Undercutting (by a river, the sea or excavation) steepens the slope and removes support.
- Bedding orientation
- If bedding planes (or joints or a fault) dip out of the slope (daylight on the slope face), rock can slide along them; beds dipping into the slope are far more stable. Weak layers (clay) parallel to the slope provide a slip surface.
- Combination
- A steep slope with bedding dipping out of the face becomes unstable when heavy rain raises pore pressure along a clay bedding plane, reducing friction until the driving force exceeds resistance and the slope fails along that plane.
Top-band answers explain water as reducing effective stress and adding weight, slope angle as increasing the driving force, and out-of-slope bedding as providing a slip surface, then combine them into a failure.
Eduqas 20215 marksDescribe two types of mass movement and outline three engineering methods used to stabilise an unstable slope.Show worked answer →
Give two movement types, then three stabilisation methods.
- Types of mass movement (any two)
- A rockfall is the free fall or bouncing of detached blocks from a steep cliff. A slump (rotational slide) is the movement of a mass along a curved (concave) failure surface, leaving a back-tilted block. A flow (mudflow or debris flow) is the movement of saturated material as a fluid. Creep is the very slow downslope movement of soil.
- Engineering methods (any three)
- Reduce the slope angle by regrading or benching (terracing) to lower the driving force. Improve drainage (surface drains, horizontal drains) to lower pore pressure. Add retaining structures (retaining walls, gabions, rock bolts and anchors, soil nailing) to hold the slope. Protect against undercutting (sea walls, revetments). Plant vegetation to bind soil and intercept water.
- Use
- Lowering the angle and improving drainage tackle the driving force and the water; retaining structures resist the movement directly.
Markers reward two distinct movement types and three valid engineering measures (regrading, drainage, retaining structures, vegetation) correctly described.
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Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)