Landslide

Landslides are a collective term for a range of mass-wasting processes in which rock, soil, or debris moves downslope under the influence of gravity. They occur in environments with both steep and gentle gradients, from high mountain ranges and river valleys to coastal cliffs and even the seabed, where they are known as submarine landslides. Although gravity is the ultimate driving force, landslides are usually the consequence of a complex interaction between geological, hydrological, climatic and human factors that together push a slope from a stable to an unstable condition.

Definition and General Characteristics

In common usage, “landslide” has often been used to describe almost any form of downslope movement. In technical terms, however, it refers to a set of specific processes within the broader field of mass movement. These processes include rockfalls, rockslides, debris slides, mudflows, debris flows, earthflows, and slow, deep-seated slope deformations.
Landslides may involve:

• Rock masses, often fractured or jointed.
• Soil and regolith, including colluvium and weathered material.
• Mixed debris comprising rock fragments, soil, vegetation and, in some cases, artificial structures.

Landslides can occur suddenly and catastrophically, moving at speeds of many kilometres per hour, or they can creep so slowly that motion is measurable only over months or years. Their scale ranges from small slips affecting a garden slope to regional events deforming entire valley sides or coastal sections.

Causes and Triggers

Although the force of gravity is always present, a landslide typically occurs when the balance between resisting forces (shear strength) and driving forces (shear stress) is disturbed. This can happen through a reduction in strength of the materials forming the slope, an increase in load, or both.
Natural factors that destabilise slopes include:

• Increased water content: Intense or prolonged rainfall, rapid snowmelt, or glacier melt can saturate soils, raise groundwater levels and increase pore-water pressure. This reduces effective stress and shear strength, making failure more likely.
• Hydrostatic and pore-pressure changes: Aquifer recharge in wet seasons, or infiltration of water into fractures and joints, can elevate pressures and weaken rock masses.
• Erosion and undercutting: Rivers, waves, or glacier action can erode the base of a slope, removing support and steepening the profile.
• Weathering: Physical weathering (such as freeze–thaw cycles) and chemical weathering (such as mineral dissolution or saline intrusion into groundwater) break down rock and soil structure over time.
• Vegetation loss: Wildfires, disease or natural disturbances may remove deep-rooted vegetation that previously reinforced soil and anchored colluvium to bedrock.
• Earthquakes and ground shaking: Seismic activity can directly trigger landslides by inducing liquefaction in saturated granular soils or by widening fractures and weakening slope materials.
• Climate variability: Seasonal or long-term climatic changes alter patterns of rainfall, temperature and snowpack, influencing the frequency and timing of slope failure.

Usually, landslides are associated with a specific triggering event, such as an exceptional storm or an earthquake, but sometimes the trigger is subtle or not clearly identifiable.

Human Influence and Land Degradation

Human activities often increase landslide susceptibility and intensify the consequences. Key influences include:

• Earthworks and construction: Road cuts, terracing, quarrying and excavation modify the geometry of slopes, steepening them or removing support. Buildings and infrastructure also impose additional loads.
• Mining and resource extraction: Surface and underground mining can change stress regimes, alter drainage patterns and create spoil heaps that are prone to failure.
• Removal of vegetation: Logging, overgrazing, and land-clearance for agriculture or urbanisation reduce root reinforcement and change the infiltration and runoff characteristics of the land.
• Changes in land use and abandonment: Shifts from traditional agriculture to abandonment, or rapid urban expansion without adequate planning, can promote instability. Degraded land surfaces allow faster infiltration and erosion, increasing the likelihood of slope failure.
• Climate change and extreme events: Anthropogenic climate change is associated with more frequent and intense rainfall events in many regions, raising the probability of rainfall-triggered landslides. Thawing of permafrost and glacial retreat also destabilise high-mountain slopes.

The combination of land degradation and extreme weather often leads to an increase not only in the frequency of landslides but also in secondary hazards such as debris floods and river blockages.

Classification of Landslides

To provide a clearer framework, geologists have refined the classification of landslides into distinct types based on the nature of movement and the material involved. A widely adopted scheme recognises six main types of movement, each of which can occur in rock or soil:

• Falls: Sudden detachments of blocks or fragments from steep slopes or cliffs, moving mostly through free fall, bouncing or rolling. Rockfalls are common in mountainous and coastal cliff environments.
• Topples: Forward or rotational movement of rock or soil blocks pivoting about a point or axis, often triggered by undercutting or loss of support at the base of a vertical or near-vertical face.
• Slides: Downslope translation of relatively intact masses along one or more discrete shear surfaces.
  • Planar slides occur along roughly planar surfaces such as bedding planes or joints.
  • Rotational slides occur along curved, spoon-shaped surfaces.Movement may be rapid and catastrophic, or progressive and episodic.
• Spreads: Extension and lateral displacement of a cohesive strata over a softer, more deformable layer. Quick-clay slides in formerly glaciated regions are a characteristic example, where liquefaction of sensitive clays leads to rapid lateral spreading.
• Flows: Movement of material that behaves in a fluid or plastic manner. The flow may consist of water-saturated debris, mud, or, in some cases, dry granular material that has become fluidised.
• Slope deformations (creep): Slow, distributed movements affecting thick sections of slope or entire hillsides. Although movement rates may be only millimetres per year, they can progressively damage buildings, roads and other infrastructure.

Many real events are complex landslides, in which different movement types coexist or succeed one another. A failure may begin as a rockfall or topple, disintegrate into a debris slide, and evolve into a debris flow as it entrains water and additional material downslope.

Flows, Debris Flows and Earthflows

Flow-type movements are particularly destructive due to the combination of mobility, volume and impact forces.
Debris flows and mudflows develop when slope material becomes saturated or otherwise fluidised. These flows can:

• Travel at high velocity down confined channels and valleys.
• Entrail boulders, trees, houses and vehicles.
• Block rivers and streams, forming temporary dams and lakes.

Failure of such natural dams may cause catastrophic downstream flooding and a “domino effect”, in which successive blockages and outburst floods greatly increase the scale of damage. Mountainous areas with steep, narrow valleys are particularly vulnerable.
Earthflows primarily involve fine-grained materials such as clays, silts, fine sands or volcanic ash. They may:

• Move at rates from a few millimetres per year to many kilometres per hour.
• Form elongated bodies with lobate toes that thicken as they decelerate.
• Accelerate during periods of high rainfall when pore-water pressures rise.

As earthflows advance, the development of fissures enhances infiltration, allowing rapid responses to subsequent storms and sustaining movement even during relatively dry seasons.
Large, rapid rock avalanches (sturzstroms) represent another extreme form of flow-like movement, in which huge rock masses travel long distances at high speeds, often following valley floors.

Impacts, Risk and Mitigation

Landslides are a major natural hazard, causing loss of life, destruction of property, disruption to infrastructure, and degradation of ecosystems. Residential areas built on unstable slopes or at the foot of steep hillsides are especially at risk. Historical events have buried entire neighbourhoods, destroyed transport corridors, and reshaped river systems through damming and avulsion.
Landslide mitigation refers to a range of policies, engineering measures and planning practices aimed at reducing risk. These may include:

• Hazard assessment and mapping: Identifying unstable slopes, susceptible geological units, and areas with a history of landsliding.
• Land-use planning and regulation: Restricting or prohibiting construction in high-risk zones, and guiding development towards safer areas.
• Engineering solutions: Slope regrading, retaining structures, drainage works, rock bolting, netting and protective galleries for roads and railways.
• Vegetation management: Promoting or restoring deep-rooted vegetation to improve soil reinforcement, while avoiding practices that increase instability.
• Monitoring and early warning: Instrumentation to measure ground movement, pore-water pressures and rainfall thresholds, combined with warning systems and evacuation plans.
• Public awareness and preparedness: Education campaigns informing communities about warning signs, safe behaviour and response procedures.