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Ministry of Energy Mines and Responsible for Core Review

What causes Landslides?

Some slopes are susceptible to landslides whereas others are more stable. Many factors contribute to the instability of slopes, but the main controlling factors are the nature of the underlying bedrock and soil, the configuration of the slope, the geometry of the slope, and ground-water conditions.

 

Three distinct physical events occur during a landslide: the initial slope failure, the subsequent transport, and the final deposition of the slide materials. Landslides can be triggered by gradual processes such as weathering, or by external mechanisms including:

 

  • Undercutting of a slope by stream erosion, wave action, glaciers, or human activity such as road building,
  • Intense or prolonged rainfall, rapid snowmelt, or sharp fluctuations in ground-water levels,
  • Shocks or vibrations caused by earthquakes or construction activity,
  • Loading on upper slopes, or
  • A combination of these and other factors.

 

Once a landslide is triggered, material is transported by various mechanisms including sliding, flowing and falling. Landslides often occur along planes of weakness that may parallel the hill slope. In bedrock, planes of weakness are usually beds, joints or fractures. Soils such as silt and clay are weaker than rock and commonly have complex or multiple planes of weakness.

 

Weakness planes and landslides
Types of weakness planes and associated landslides: (1) slope failure in glacial sediment resulting in slumps; (2) parallel bedding in rock causing slides; and (3) fracturing of rock promoting falls.

 

Effects of human activity

 

Landslides may result directly or indirectly from the activities of people. Slope failures can be triggered by construction activity that undercuts or overloads dangerous slopes, or that redirects the flow of surface or ground-water.

 

Thuya Creek landslide
The Thuya Creek landslide near Little Fort occurred in 1972 and removed much of the roadway (photo courtesy of the Ministry of Transportation and Highways).
 

 

Poor road construction in this logged area near Boston Bar Creek probably contributed to slope failures in the early 1980s (photo courtesy of D. VanDine).
 

Poorly planned forest clearing may increase rates of surface water run-off or ground-water infiltration. Inefficient irrigation or sewage effluent disposal practices may result in increased ground-water pressures, which in turn can reduce the stability of rock and sediment.

 

People increase the risk of landslides by modifying the landscape, for example, by building on unstable slopes or in the path of potential landslides. Unfortunately, many people are unaware of their exposure to landslide risks.

 

Next Page - What are the different types of landslides?

Landslides in British Columbia was originally produced as an Information Circular 1993-7 by the BC Geological Survey Branch of the B.C. Ministry of Energy, Mines and Petroleum Resources in cooperation with the B.C. Ministry of Health, the B.C. Ministry of Transportation and Highways, the B.C. Ministry of Environment, Lands and Parks, the B.C. Ministry of Forests, the B.C. Provincial Emergency Program, and with the assistance of the Geological Survey of Canada.

Copies are available from: 
Publications Office,

BC Geological Survey