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Geotechnical Aspects of Landslides Triggered by Rainstorms

Southern California is a geologically complex region with mountainous terrains. Many houses and roads are developed in hilly areas where may be subject to potential landslides.  Stability of slopes becomes a concern to many people who live in hilly areas during rainy seasons, especially during the anticipated El Nino winter storms this year.

From a geotechnical point of view, landslides can be categorized into (1) surficial landslides; (2) deep-seated landslides; and (3) earthquake-induced landslides. Most of surficial landslides and deep-seated landslides are triggered by intense rainstorms and related to saturation of soils.

Surficial landslides (popularly called mudslides) are shallow landslides of water-saturated soils and rock fragments that rush downslope as muddy slurries.  The driving force of such landslides is the seepage force generated from the storm water that saturates, softens and drags down the topsoil typically ranging from 1 to 5 feet in depth.

Rainfall soaking into ground can eventually cause deep-seated landslides long after the rain has stopped. This kind of movement is usually triggered by cumulative rainfall during long period of time, i.e. weeks to years. Deep-seated landslides can be as deep as hundreds of feet below ground surface.

Features that might indicate potential landslides on or near slopes include: saturated ground, abnormal seeps, new cracks, unusual bulges on ground surface, soil creeping away from foundations, tilting or cracking of concrete floors, broken underground utilities, leaning trees or poles, sunken road pavement, and other signs of slope movement.

For hillside development projects, geotechnical engineering and geological investigations are critical to protect the properties from potential landslide hazards.  Geotechnical engineers analyze slope stability for all types of landslides (surficial, deep-seated and earthquake-induced).  The results of slope stability analyses serve as the basis of geotechnical engineering recommendations to mitigate the project site conditions.  Slope stabilization methods typically include: improving surface drainage, installing sufficient subdrain system, reducing the steepness and height of slopes, construction of buttress fills, installing retaining systems, installing soldier piles, slope anchorage, and ground improvement.

Figure 1
Figure 1 – Surficial landslide triggered by rainfall
























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Sean Lin, PhD, PE, GE has over 17 years of experience as in the field of geotechnical engineering. Sean received his PhD in Earthquake Engineering, from USC and specializes in seismic site response analysis as it applies to hospitals, schools and high-rise buildings. His professional experience includes soil mechanics and foundations, soil reinforcement, geosynthetics, pile design and driving analysis, slope stability, and liquefaction analysis. Sean has experience in engineering projects for commercial, residential, water/wastewater, military, educational and industrial facilities. An avid photographer, Sean enjoys taking site photos, and is an active member of his photography club!

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