On January 10, 2005, residents of La Conchita worst nightmares became true. Heavy rain triggered a massive landslide that swallowed 13 houses and damaged 23 other homes. By the time rescuers pulled out the last bodies, ten were confirmed dead. California’s natural landscapes are filled with rock cliffs and verdant rolling hills. However, a deep, dark secret is lurking underneath the veneer if beauty. The sad truth is that many California slopes have the potential to catastrophically move.
A landslide is defined as the movement of rock, soil, or debris down a slope. There are several different types of landslides and many reasons why slopes fail.
Landslides involve widespread movement of the slope. Landslides can be characterized as shallow or deep.
Shallow landslides are located in the soil profile and move downslope in a block. Shallow landslides usually occur when a permeable surface layer becomes saturated above a less permeable soil or bedrock layer. The extra weight of the water combined with reduced internal friction cause the top layer to slide over the bottom layer.
Deep landslides occur below the rooting depth of vegetation and generally include failure or the bedrock. Some landslides are very slow moving while others can move rapidly with catastrophic consequences. When you inventory a site, you may be able to observe evidence of past landslides. The upper part of the slope will be a steep scarp while the toe will be steep due to the deposition of material during the movement.
Why Slopes Fail
There are many reasons why landslides occur and slopes fail. Many are natural conditions that occur only during the right combination of weather events. Other causes may be man-made.
Man-made Causes of Slope Failure
Deforestation and construction may create conditions favorable for landslides by altering the natural equilibrium of the slope and changing conditions that permitted slope stability.
Earthwork that alters the shape of the slope could trigger a landslide.
Saturation of the soil through man-made means. For example broken pipes or leaking swimming pools could trigger slope movement the way that excessive rain can. Extra soil water causes the slope to lose it’s natural friction and slide.
Core Drilling Core drilling, also known as coring, involves removing a sample of the site’s soil and geology to identify the site’s underlying statiography. A trained engineer can identify potentially hazardous conditions and suggest mitigation solutions.