Prevention & Mitigation Of Landslides

Reasons

Worldwide, landslides cause death and millions of $ in damage. Unfortunately, very little effort to mitigating this natural hazard in the U.S. One region in the U.S. - Greater Los Angeles, California - has state and local regulations regarding landslides and public use. There has been a 92-97% reduction in landslide losses for new construction. Nearby counties and several states now have more stringent landslide regulations.

Preventive Medicine

Prediction is the first necessity. Often a problem isn't known unless it is looked for. This is done with surface and subsurface data by geologists. Emphasis has changed from correction to prevention. It is now required by law to analyze the construction site and mitigate problem areas like landslides before the site is developed. This is cheaper for the developer and a necessity due to the population explosion in hillside areas

Property Purchase Stage

Developer should secure a geologic feasibility report before purchasing hillside property. This is generally done with and is required by law in California.

Report may be requested by home buyer. The Natural Hazards Disclosure Act, (June 1998) requires that a "Natural Hazard Disclosure Statement" be provided to a potential buyer of California real estate if the property is in a state-mapped hazard areas. These hazard areas include special Seismic Hazard Zones (earthquake induces liquifaction or landslides), flooding, wildfires, and dam inundation. Visit the State of California's Seismic Hazard Zonation Program website (http://www.conservation.ca.gov/cgs/shzp/Pages/shmprealdis.aspx) for more information.

Planning and Design Stages

Geologic factors should be considered in zoning, road access, storm drainage, sewage disposal, etc. It is best to modify the natural landscape as little as possible

Treatment of Existing Natural Landslide Areas

Massive grading is the most economic correction method. There are many factors that go into determining if a slope is stable or not, but two of the most important are:

• The driving force must be reduced
• The resisting force must be increased

Buttresses or buttress fills may be required to restrain unstable areas. In all cases,proper drainage is required. This helps to maintain the natural flow of water away from the slope and prevents saturation of the fill.
Massive hillside grading at the Mont Calabasas development in Calabasas, California. In the forground is a very large artificial fill slope (roughly 200' in height). Note the drainage terraces (light grey) evenly spaced up the slope. These terraces will draw water away from the slope and prevent slope saturation. Photo by S. Leyva © 2015


Terrace drains with interceptors on a slope in a housing development in Santa Clarita, California. Photo by S. Leyva © 2006.

Treatment of Cut Slope Stability Problems

Dip slopes are slopes where the bedding is dipping in the same direction as the slope. Dip slope problems are one of the most common as it is easy for the beds to slide down-slope. Temporary solutions to failures in these types of slopes include guniting the slope, deep rooted vegetation, sandbagging toe, and plastic sheeting.

Bedding in a dip slope dip in the same direction and angle as the slope, making it easier for the slope to fail.


Removing part of the slope and replacing it with an engineered artificial slope and buttress stablizes the slope by blocking the bedding.

Cut slopes are formed when part of a slope is removed and left as is. Sometimes this is stable, and sometimes not - it depends on the orientation of the beds in relation to the slope, the type of material, and how the cut was made. Slopes in which the beds are dipping in the opposite direction of the slope are typically stable. Beds dipping in the same direction are typically not (see above).

Remedies for cut slope instability include:

Flatten the slope to the dip angle

Construct a buttress with a compacted backfill

Construct a buttress fill with subdrainage

Treatment of Erosion Problems

Slopes must be properly prepared and drained. Terraces and interceptor drains must protect raw cut and fill slopes. They also must be well maintained and kept free of debris and trash. It is also important to have established root systems and continuous vegetation on the slopes - this will prevent slope erosion. Leaving vegetation in its natural state adjacent to home will reduce erosion. Poor drainage may produce rills and gullies as well as landslides

The basic responsibility is the homeowners - don't assume someone else will do it for you.

Steps the Homeowner may take

1. Check that thorough geologic soil studies were made prior to development
   
2. Check that all recommendations were heeded
3. Check that the community has strong building and grading codes
4. Check that someone (developer or community) maintains drainage control devices
5. Don't make major land modifications without professional advice

<< back		END! >>