What causes slope failure and ground subsidence, and how are they managed?
The types of mass movement (rockfall, slide, slump, flow), the factors that control slope stability (slope angle, rock type and structure, water, vegetation, undercutting), the ground hazards of subsidence and collapse (mining, dissolution), and the prediction and prevention of these hazards.
A focused WJEC and Eduqas A-Level Geology T1 answer on the types of mass movement, the factors controlling slope stability (slope angle, rock type and structure, water, vegetation and undercutting), the ground hazards of subsidence and collapse from mining and dissolution, and how these hazards are predicted and prevented.
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What this dot point is asking
This dot point covers the ground hazards that follow from weathering, water and human activity. WJEC wants the types of mass movement, the factors that control slope stability, the causes of subsidence and collapse, and how these hazards are predicted and prevented. It applies F2 weathering and erosion and links to engineering geology in G4.
The answer
Types of mass movement
What controls slope stability
A slope fails when the downslope driving force exceeds the resisting strength. The controls are:
Subsidence and collapse
Subsidence is the sinking of the ground surface, caused by the collapse of underground voids (old mine workings), the dissolution of soluble rocks (limestone, gypsum) creating cavities and sinkholes, and the compaction of sediments when groundwater is over-abstracted.
Prediction and prevention
Hazards are reduced by hazard mapping (former mining, soluble ground, unstable slopes), slope stabilisation (drainage to remove water, regrading to a gentler angle, retaining walls, rock bolts and netting, replanting), grouting or filling of voids, controlled groundwater abstraction, and monitoring for early movement.
Examples in context
The Aberfan disaster (1966) in south Wales, where a saturated colliery spoil tip flowed downhill onto a village, is a tragic example of a water-triggered flow and of the danger of unstable spoil. Coastal landslides along the Dorset and Yorkshire coasts show undercutting by the sea triggering slumps in clay-rich cliffs. Sinkholes in limestone and gypsum areas, and subsidence over old coal workings, illustrate dissolution and void-collapse subsidence.
Try this
Q1. Name the four main types of mass movement. [2 marks]
- Cue. Rockfall, slide, slump and flow.
Q2. Explain why heavy rainfall makes a slope more likely to fail. [3 marks]
- Cue. Water adds weight, raises pore-water pressure (reducing friction on slip surfaces) and lubricates clays, all reducing slope strength.
Q3. State two causes of ground subsidence. [2 marks]
- Cue. Collapse of old mine workings, and dissolution of soluble rock (limestone or gypsum) forming cavities and sinkholes.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20196 marksExplain the factors that control the stability of a slope and how they can lead to a landslide.Show worked answer →
Set out each factor and say how it raises the chance of failure, because the marks track named controls.
Slope angle: steeper slopes have a larger downslope component of gravity, so they are less stable and fail more readily.
Rock type and structure: weak rocks and the orientation of bedding, joints and faults matter. Beds or joints dipping out of the slope (parallel to the slope face) provide planes along which rock can slide; impermeable layers beneath permeable ones create slip surfaces.
Water: heavy rain saturates the slope, adds weight, raises pore-water pressure (reducing friction on potential slip surfaces) and lubricates clays, all reducing stability. Water is the most common trigger.
Vegetation: roots bind the soil and remove water, so removing vegetation reduces stability.
Undercutting: erosion or excavation at the base (by a river, the sea or construction) removes support and steepens the slope.
So a steep slope of weak or unfavourably dipping rock, saturated by rain, stripped of vegetation and undercut at the base, is primed to fail as a landslide.
Markers reward slope angle, rock type and structure (adverse dip), water and pore pressure, vegetation and undercutting, each linked to reduced stability.
WJEC Eduqas 20215 marksDescribe the causes of ground subsidence and how the hazard can be reduced.Show worked answer →
Give the causes, then the mitigation, because the question asks for both.
Subsidence is the sinking of the ground surface. It is caused by the collapse of underground voids, for example old mine workings collapsing beneath the surface, or by the dissolution of soluble rocks such as limestone and gypsum, which creates cavities and sinkholes. Over-abstraction of groundwater can also lower the water table and compact sediments, causing subsidence.
The hazard is reduced by mapping former mining and soluble ground and avoiding building there, by filling or grouting voids to support the ground, by managing groundwater abstraction to prevent compaction, and by monitoring the ground for early movement so action can be taken.
Markers reward collapse of mine voids, dissolution of soluble rock forming sinkholes and groundwater-related compaction as causes, and mapping, grouting, controlled abstraction and monitoring as mitigation.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)