What is a landslide? Types, causes, effects & prevention

This guide explains landslides, outlining what they are and the various types, the factors that can cause landslides to occur and the main landslide prevention techniques used in engineering and construction. Use the links below to jump to the sections you’re most interested in:

• What is a landslide?
• Types of landslide movement
• What causes a landslide?
• The varying effects of landslides
• Preventing a landslide
• Repair of failed slopes

What is a landslide?

A landslide is defined as the movement of a mass of earth, rock or debris down a slope. They are a type of “mass wasting” which is the name for any down-slope movement of soil and/or rock under the direct influence of gravity. This movement is caused by the self-weight of the material, under the force of gravity, and can take the form of falls, topples, sliding, spreading or flows (mudflows or waste flows).

Types of landslide movement

Although landslides often have the same negative effects on their surroundings, there are many diverse types with varying sizes, shapes, movement speeds and characteristics. The four main types of landslides are:

1. Falls
2. Topples
3. Slides (including rotational landslides and translational landslides)
4. Flows 

The landslide types each have their own unique cause, read on to get a better understanding of each type and why they occur.

   1. Falls

With vertical or steep slopes such as cliffs, a landslide will often take the form of a fall – this is where material collapses from the slope, falls downwards, and collects towards the base. As a result, we can recognise the occurrence of a fall and differentiate it from the other landslide types by the accumulation of earth, rock and debris at the bottom of a steep slope. 

     2. Topples

As the name suggests, a topple is a type of landslide where the rock, earth or debris rotates forwards away from slope and literally topples downwards. Under normal circumstances, there will be an axis at or close to the bottom of the slope about which the rotation occurs.

     3. Slides

In cases where a slide landslide occurs, the failure in the slope takes place at depth below the surface. The material above this point then slides downwards along a slip surface (or ‘rupture’). This type of slide landslide can either be rotational or translational.

   4. Rotational

We can spot rotational slides in cases where the slip surface is curved (often like the surface of a spoon) – this is referred to as a listric rupture or slip surface. As a result of this appearance, rotational landslides are sometimes called ‘slumps’.

   5. Translational

Where a landslide occurs along the surface of a fault, joint or bedding plane, it is said to be translational or ‘planar’. Often causing huge damage, translational landslides can result in the rapid movement of material down a slope. The speed of the slide ultimately depends on how deep the underlying fault is.

   6. Flows

If water – or another form of fluid – is involved, then a flow can occur. This is a movement of fluid along with any other material that is being carried in it. A flow could consist of mud, debris or even rock (known as a rock avalanche).

What causes a landslide? 

Landslides are caused by agitation to the layers of earth beneath the surface. There are four common factors that can trigger a landslide: the weight, angle, geometry and strength of the external force. All of these factors can place pressure upon the internal strength of the soil, leading to the stability to fail and a landslide to happen.

Let’s take a deeper look into the different cause of landslides:

Water

Water plays an integral part in the many causes of landslides, and is actually the most common cause of a landslide trigger mechanism. Soil that is already facing instability can be at risk of sliding when water increases the hydrostatic pressure while at the same time weakening the soil further. An increase in water can originate from a number of natural causes and other human related activities including: rainfall, snowmelt, stream erosion, burst water mains, floods.

Soil strength and slope angle

Different soil types can support different slope angles. The maximum stable slope face angle depends upon the internal strength of the soil and the forces acting on it. Some slopes that have been stable for many years can suddenly fail due to a trigger mechanism such as a change in soil strength or surcharge loading.

External activity

A landslide can be caused by a number of external factors other than water and soil stability.  Primarily earth vibrations such as earthquakes, volcanic eruptions and human activity (mining or traffic) are the obvious causes. A slope’s strength can also be impacted by the removal of vegetation, weathering – where a material’s strength is reduced due to freeze thaw cycles and mineralogical changes – and the arrangement of the rock layers..

The varying effects of landslides

Landslides are not uncommon. They occur in all regions across the globe, with varying degrees of severity. Many are minor slope failures with minimal consequences, where even local people may be unaware of the landslide. At the other end of the scale are humanitarian disasters involving catastrophic loss of life and destruction. In between these extremes are a variety of landslide types, including both minor and major failures, resulting in differing degrees of damage to buildings, properties, businesses, people and animals.

Landslide prevention techniques

The easiest way to prevent a landslide is to avoid building in areas that are – or might become – prone to landslides. However this is not always an option; unique assessments are often used to show the conditions for a landslide exist. In such cases, there are measures that can be taken to reduce the potential for a landslide to occur, such as steep slope reinforcement, installing structures using pile foundations and checking surface drainage.

Surface drainage

Surface drainage can be installed to reroute surface water away from a slope, thus reducing the likelihood of a landslide. In high rainfall areas, erosion protection via surface drainage – both at the toe and on the surface – can be very effective, positively impacting the maintenance of the angle of the slope. An alternative option is almost the opposite of this, where removing material from the top of the slope or adding a toe berm can reduce the effective slope angle and loading, thus reducing the risk of a landslide and increasing safety.

Construction of safe steep slopes

Sometimes it is desirable to construct earth slopes or embankments with steep faces. Steep slopes  reduce land take and are lower in cost than constructing vertical retaining walls. The steep slope can be made stable and prevent landslides by the inclusion of soil reinforcement in the form of high strength geogrids. Tensar have developed systems for the design and construction of reinforced soil slopes up to 70° face angle: TensarTech Natural Green and TensarTech GreenSlope.

Ultimately, the only way to increase the strength and stability of slopes prone to landslides is by major engineering works. Several options are available including soil nailing and earth retaining structures.

Repair of failed slopes

Landslides affecting infrastructure are not an uncommon occurrence, as road and rail embankments can slide, and steep faced cuttings in soil and rock can fail. Where landslides occur in embankments or soil cuttings, the failed material is often removed from site and replaced with a material that has greater strength – usually quarried rock or aggregate. This is expensive and disruptive, and clearly not ideal.

An alternative approach to landslide repair, first introduced in the 1980’s, involves re-use of the failed soil material but with the inclusion of soil reinforcement in the form of Tensar geogrids. This solution has been widely used across the globe for reinstatement of failed slopes and landslides in highways, rail and other infrastructure works, both in urban and rural areas. The TensarTech Slope Repair System includes design advice and support from Tensar’s experienced design team.

Next steps

This guide has explained what a landslide is, the different types of landslides, the varying effects, how to prevent them and how failed slopes are repaired. If you’ve found this post useful, you may also want to read some of our other articles:

• What is the shear strength of soil?
• Types of pavement and road construction methods
• Geogrids: A different way of thinking

To find out more about how Tensar products support slope design and construction processes, visit our Tensar geogrids and geosynthetic construction solutions pages.

Got a burning question about geotechnical engineering? 

Why not drop us a line at tensarinfo-intl@cmc.com and the answer to your question may feature in a future episode of Ground Coffee!