What is a landslide?

A landslide is colloquially defined as a downward movement of rock, soil and debris. Although there are many types and sizes of landslides, the most commonly observed landslides on a day-to-day basis are small, shallow slides. Common landslides in Oregon include shallow translational slides, deep-seated rotational slides, slumps, block slides, earthflows, debris flows, rock avalanches, rock slides and rockfall.

Why do landslides occur?

Landslides can occur from many different influences, the most common being erosion, water and earthquakes. Water, or rapid snowmelt, that permeates into soil or rock may weaken a hillslope by increasing pressure in the voids, cracks and pores that already exist underground. Combined with gravity, this buildup of water pressure reduces the internal strength that already exists in soil and rock, while also "pushing" a mass downslope, much in the same way water puts pressure on dams or basements. Erosion can often drive landslides, especially when it causes slopes to become oversteepened and unstable, or it "undercuts" the base of an otherwise stable slope. The latter is common near rivers or in marine environments as the movement of water can erode the stabilizing, buttressing behavior of the toe of a slope. Earthquakes can cause significant landsliding as it causes significant vertical and horizontal seismic accelerations. This means that the shaking that occurs during an earthquake may put large destabilizing forces on soil and rock, sometimes driving already unstable terrain to fail. This last driver is of particular concern as the Pacific Northwest is due for a long-duration, strong Subduction Zone Earthquake from the Cascadia Subduction Zone fault off the Pacific Coast.

What are signs that a landslide is happening?

Unfortunately, the signs of an impending landslide are not always apparent until it is too late. However, most of the time, there are some telling signs of instability.

Tension Cracks: Look for tension cracks, which are a sign of impending failure, often characterized by deep cracks in the ground occurring at the top or along a slope. These cracks represent locations where chunks of ground are shifting away from one another. Tension cracks can sometimes expedite the onset of slope movements as they provide a direct path to introduce more water into the unstable slope.

Subsidence and Heave: Often, unstable slopes exhibit movement well before collapse, if collapse occurs. Typically, one might expect subsidence (settlement) at the top or upper part of a landslide. This occurs because the ground is moving downslope with gravity, while stable ground surrounding it remains relatively unaffected. Heave (upward or outward movement) typically occurs at the lower reaches of the slope, often where a landslide is buttressing against shallower ground. Sometimes, particularly at river and coastal environments, this heave will not be detectable as the lower portion of the landslide is being eroded by water faster than it can move outwards. In structures, cracks and spalling may occur from ground movements, but don't be alarmed - these can often be due to common ground settlements.

Bowed Trees: Nature can often give us many clues towards a given site's condition. One such example are trees, fir trees in particular, which are very adept at growing vertically, seeking their source of vitality: light from the sun. Often, ground movements may result in these trees tilting at an angle, unusual behavior for trees that are healthy. Recent ground movements will cause trees to lose their vertical structure, causing tilt that has not yet been corrected. Sometimes, slopes will move and then become dormant again. In these cases, some trees will recover, growing straight yet again, but keeping a bend in its trunk from the past slope instability. Other telltale signs of an unstable slope are natural changes in vegetation; since landslides are often driven by water, one might expect vegetation that thrives on wet environments where the water table is near-surface (Horsetail, Bigleaf Maple).

How do we stop landslides?

Fixing a landslide is often a function of environment, size, risk, and of course, your wallet. Often, many big landslides are prohibitively expensive to fix as the quantity of earth that must be arrested may be in the many millions of cubic meters (yes, you read that right). However, many landslides are often stabilized (or prevented) using several guiding principles. Namely, (1) get water out of the slopes, (2) keep water out of the slope, (3) armor and protect vulnerable locations, (4) reduce driving loads, and (5) replace or reinforce poor soils when feasible. Let's get into a bit more detail.

(1) Get water out of the slope. Remember, water is a main driver of slope instability. Therefore, getting it out of the slope is often the most efficient (and economical) measure of stabilizing a slope. For landslides, both large and small, this often includes installation of drains and outflow pipes, often augered or drilled, within a slope. Larger slides, however, may require kilometers of drains, while smaller slopes may require a few meters to be effective.

(2) Keep water out of the slope. Notice a trend here? Just as draining a slope is an effective means of improving slope stability, keeping water out is necessary to sustain stable conditions. This is often achieved by installing ditches, interceptor channels, membranes, geosynthetics or other measures to channel water to a location where it cannot cause harm to the hillslope. For small slides and road construction, appropriate grading, ditches and occasional culverts may be satisfactory.

(3) Armor and protect vulnerable locations.Often, erosion can destablize slopes by removing stabilizing soil in important locations, resulting in oversteepened regions. This can be due to wave impact, flow of rivers or streams, rainfall and even wind. Therefore, it is important to ensure that the soil in a given slope is protected from the erosive forces of nature. Commonly, riprap (boulders or concrete blocks) may be placed along areas where the movement of water can cause erosion. Also effective is the use of vegetation - the roots of trees, shrubs and other plants can confine and stabilize surficial soils, preventing erosion and rilling, especially from rain.

(4) Reduce driving loads. One technqiue to stabilize a slope is removing or replacing material from the upper portion of a landslide, reducing the mass that is driving failure. This could include removal of heavy soil and replacing with well-draining, lighter fill or even using lightweight materials, such as geofoam.

(5) Replace or reinforce poor soils. Whenever possible, poorly-draining, weak soils, such as soft clays or weak, fractured rock should be removed and replaced with stronger, freely-draining materials, such as sand or gravel. Sometimes, the use of reinforcements, such as geosynthetics (geotextiles, geogrids, geonets) or steel (soil nails, stabilizing piles, sheet piles) may be effective in stabilizing small slides as well. However, these options can become prohibitively expensive for very large landslides.

Where can I learn more about landslides?

Fortunately, there are many great resources to learn about landslides, especially in the state of Oregon.

See the location of active and dormant landslides around the State using SLIDO, the Oregon Department of Geology and Mineral Industries' (DOGAMI) webGIS database of landslides.

Check out the susceptibility of various geohazards, including landslides, at the Oregon Hazards Explorer for Lifelines Program (OHELP), which highlights the risk of potential landsliding during the next Cascadia Subduction Zone Event.

The US Geological Survey provides great background information about landslide hazards around the United States.

Dave Petley's The Landslide Blog provides scientific perspective on major landslide events occurring worldwide.