April 2005
	In this issue:  Coastal Landslides in North America   New Service Pack

Coastal Landslides in North America
We all know that rainfall can cause landslides. This was again vividly evident in the news during February this year when we witnessed catastrophic landslides during intense rainfall in both California in the U.S., and British Columbia in Canada. Property destruction and tragic loss of life were the results of the various landslides. Over approximately a seven-month period, the Malibu California area received over 585 mm (23 inches) of cumulative precipitation. Then in February, 2005 the area received an additional 228 mm (9 inches) over a period of about four days, at which time the landslides occurred.

Sometimes the sliding mass in these situations can be well above the water table, which is rather perplexing. The quandary is then, "why does a slope become unstable even though the rainfall has not led to full saturation in the sliding zone?" The answer lies in the soil suction or negative pore-water pressure. Suction manifests itself in the strength of the soil – the higher the suction the higher the strength and vise versa.

Recently the engineers at GEO-SLOPE did some numerical analyses in the context of the California rainfall events. The diagram on the left shows an imaginary situation during the seven months of prolonged rainfall as modeled with SEEP/W and SLOPE/W. The water table remains fairly deep and the factor of safety is 1.35, indicating that the slope is stable even after the prolonged, although modest, rainfall. The diagram on the right shows the situation after the four days of intense rainfall in February. The factor of safety drops to unity (1.0) indicating the hillside has become unstable. Interestingly the water table has risen, but not into the zone of instability.

The decrease in factor of safety results from the strength loss related to the loss of suction as illustrated in the graph below. Note the strength loss along the entire slip surface, from before until after the intense rainfall. The strength loss is the highest at the slide crest (Slice 1) where the suction was the highest before the intense rainfall started. 

Is this perhaps what happened in Malibu, California? Most likely it was at least a strong contributing factor. The key to doing this type of analysis is to be able to correctly model transient unsaturated seepage flow and to use the results in a stability analysis, and secondly to relate the soil strength to the soil suction. This can be conveniently done with SEEP/W and SLOPE/W in the GeoStudio integrated environment.

To view additional details about this analysis, see "Why do slopes become unstable after rainfall events?" on our web site.

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