Saturating the Soil

A series of storms has saturated the soil in many areas, causing landslides that destroyed or damaged homes in San Clemente, Pacific Palisades and elsewhere. Here is a look at why seemingly stable ground can begin to slip. A. When rainfall is less than 4 to 6 inches, over a short period of time, there tend to be few problems. B. When rainfall reaches 6 to 10 inches, soil starts becoming saturated and can absorb less water. Small mudslides with a few feet of soil washing away can occur. C. With more than 10 inches, real problems begin. These include large mudslides during the storms and, later in the year, the chance of massive landslides as water undermines bedrock layers of compacted earth.

The Worst Case A major long-term risk of saturated soil is that it will be weakened and later give way. In some areas of the Southland-especially in places such as Pacific Palisades, Malibu, the Palos Verdes Peninsula and Laguna Beach-the underlying bedrock is not the usual igneous rock but compacted earth. Heavy rainfall from the recent storms has seeped into this compacted earth. The seepage loosens the soil, making the area vulnerable to a big slide. Source: Caltech