| 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.
|
 |
| 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.
|
 |
| 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 Home
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 |
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 Branch
Ministry of Energy, Mines and Petroleum Resources
P.O. Box 9333. Stn Prov Gov't
Victoria, B.C.
V8W 9N3
This page was updated:
March 08, 2006. |
