What causes slopes to fail, and how can mass movement hazards be reduced?
Mass movement: the types of slope failure (rockfall, translational and rotational slides, slumps and debris flows); the balance of driving and resisting forces on a slope; the factors that trigger failure (slope angle, rock and soil type, water content, discontinuities, weathering, earthquakes and human activity); the recognition of warning signs; the engineering methods used to stabilise slopes and reduce risk.
A focused answer to the OCR H414 dot point on mass movement. Covers the types of slope failure (rockfall, translational and rotational slides, slumps, debris flows), the balance of driving and resisting forces, the triggers of failure (slope angle, rock type, water, discontinuities, earthquakes, human activity), warning signs, and the engineering methods used to stabilise slopes.
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What this dot point is asking
OCR wants you to describe the types of slope failure, to explain slope stability as a balance of driving and resisting forces, to state the factors that trigger failure, to recognise warning signs, and to describe the engineering methods used to stabilise slopes.
The answer
Types of slope failure
Mass movement (the downslope movement of rock and soil under gravity) takes several forms:
- Rockfall. Blocks detach from a steep cliff and fall or bounce down.
- Translational slide. Material slides down a planar surface (often a bedding plane or joint) parallel to the slope.
- Rotational slide (slump). Material rotates down a curved (concave) failure surface, tilting backward.
- Debris flow. A fast-moving, water-saturated mixture of rock, soil and water flows down the slope.
The balance of forces
Slope stability is a contest between two sets of forces:
Factors that trigger failure
Anything that increases the driving force or decreases the resisting force can trigger failure:
- Slope angle. Steeper slopes have a larger downslope gravity component.
- Rock and soil type and discontinuities. Weak materials, and planes of weakness (bedding, joints, faults) dipping out of the slope, ease sliding.
- Water content. Water adds weight (driving force) and, crucially, raises pore water pressure, which reduces friction between grains (resisting force); it can also lubricate weaknesses. This is why heavy rain is a common trigger.
- Weathering. Weakens the material over time.
- Earthquakes. Shaking adds a sudden destabilising force.
- Human activity. Cutting into the base of a slope (removing support), loading the top, or altering drainage.
Warning signs and stabilisation
- Warning signs include tension cracks at the top of the slope, bulging at the toe, tilting trees or poles, and new springs or seepage.
- Engineering methods improve the force balance:
- Drainage removes water, lowering pore pressure (the most effective measure where water is the trigger).
- Regrading reduces the slope angle, lowering the driving force.
- Retaining walls, rock bolts, soil nails and gabions add support (resisting force).
- Vegetation binds soil with roots and removes water.
Examples in context
Example 1. Rainfall-triggered debris flows. Prolonged heavy rain saturates steep slopes, raising pore pressure and adding weight until they fail as fast debris flows, a common and deadly hazard in mountainous, wet regions.
Example 2. Drainage stabilising a slope. Because water is so often the trigger, installing drainage to lower the pore water pressure is frequently the single most effective way to stabilise an unstable slope, restoring the friction that resists sliding.
Try this
Q1. State when a slope fails, in terms of forces. [1 mark]
- Cue. When the driving forces exceed the resisting forces.
Q2. Explain two ways water reduces slope stability. [2 marks]
- Cue. It adds weight (increasing the driving force) and raises pore water pressure, which reduces the friction holding the grains together (reducing the resisting force).
Q3. Describe one engineering method to stabilise a slope and how it works. [2 marks]
- Cue. For example drainage: removing water lowers pore pressure and weight, increasing friction (resisting force) and reducing the driving force.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/02 20206 marksExplain how the balance of driving and resisting forces controls slope stability, and explain why heavy rainfall is a common trigger for landslides.Show worked answer →
A level-of-response answer; set up the force balance, then explain the role of water.
- The force balance
- A slope is stable when the resisting forces (the friction and cohesion holding the material in place, plus any support) exceed the driving forces (the downslope component of gravity acting on the mass). Failure occurs when the driving forces exceed the resisting forces.
- How water reduces stability
- Heavy rainfall adds water to the slope, which has several destabilising effects. The water adds weight to the slope material, increasing the driving force. More importantly, water raises the pore water pressure between grains, which pushes the grains apart and reduces the friction (effective stress) holding the material together, lowering the resisting force. Water can also lubricate planes of weakness.
- Result
- By increasing the driving force and decreasing the resisting force at the same time, heavy rainfall can tip the balance so that driving forces exceed resisting forces and the slope fails. This is why landslides so often follow prolonged or intense rain.
Top-band answers set up stability as resisting versus driving forces and explain that water both adds weight (driving) and raises pore pressure to cut friction (resisting).
OCR H414/01 20194 marksDescribe two engineering methods used to stabilise a slope and explain how each improves stability.Show worked answer →
Choose two methods and link each to the force balance.
Drainage. Installing drains to remove water from the slope lowers the pore water pressure and reduces the weight of the slope material. This increases the friction holding the material together (raising the resisting force) and reduces the driving force, improving stability. Water is usually the key factor, so drainage is often the most effective measure.
Regrading (reducing the slope angle), retaining walls or rock bolts. Reducing the slope angle lowers the downslope component of gravity (the driving force). A retaining wall or rock bolts add physical support (resisting force) to hold the material in place.
Markers reward two valid methods (drainage, regrading, retaining structures, rock bolts, soil nails, vegetation) each correctly linked to increasing the resisting force or reducing the driving force.
Related dot points
- Engineering geology: the engineering properties of rocks and soils (strength, jointing and discontinuities, weathering and the behaviour of clays, sands and gravels); the purpose and methods of site investigation (desk study, boreholes, trial pits and core logging); the ground conditions that cause problems for foundations (weak or compressible soils, swelling clays, solution cavities in limestone, made ground and high groundwater); the role of foundations and the ground model.
A focused answer to the OCR H414 dot point on engineering geology. Covers the engineering properties of rocks and soils, the purpose and methods of site investigation (desk study, boreholes, trial pits, core logging), the ground conditions that cause foundation problems (weak or swelling soils, solution cavities, made ground, groundwater), and the role of foundations and the ground model.
- Earthquake hazards: the primary and secondary hazards (ground shaking, surface rupture, liquefaction, landslides and tsunamis); the distinction between hazard, vulnerability, exposure and risk; the factors that determine the impact of an earthquake (magnitude, depth, ground conditions, population density, building design and preparedness); monitoring and mitigation (building codes, land-use planning, early-warning systems and education); the limits of earthquake prediction.
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- Volcanic hazards: the hazards of an eruption (lava flows, pyroclastic flows, ash falls, lahars, volcanic gases and sector collapse) and how they relate to magma type and the Volcanic Explosivity Index; the methods of monitoring a volcano (seismicity, ground deformation, gas emissions and thermal anomalies); the use of hazard maps, exclusion zones and evacuation to mitigate risk; the comparison with earthquakes in terms of predictability.
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- Surface processes: mechanical weathering (freeze-thaw, exfoliation and abrasion) and chemical weathering (solution, hydrolysis and oxidation); the difference between weathering and erosion; transport by water, wind and ice and its effect on the rounding and sorting of sediment; how the maturity and texture of a sediment record its transport history.
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- Groundwater: porosity and permeability and how they differ between rock types; aquifers, aquitards and the water table; confined and unconfined aquifers; the calculation of porosity from pore and total volumes; the use of a simple form of Darcy's law to relate groundwater discharge to hydraulic conductivity, hydraulic gradient and area; the issues of over-abstraction and contamination.
A focused answer to the OCR H414 dot point on groundwater. Covers porosity and permeability and how they vary between rock types, aquifers, aquitards and the water table, confined and unconfined aquifers, calculating porosity, using a simple form of Darcy's law for groundwater flow, and the issues of over-abstraction and contamination.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)