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The Coastal and Marine Geology geotechnical group investigates the causes of ground deformation and ground failure as a result of earthquakes, storms, and wave action. The coastal urban regions of the Pacific margin of the United States are growing rapidly, putting increasing demands on coastal infrastructure and lifelines, such as highways, utilities and community services. These structures and services, as well as the general population that are located in the coastal zone, are at risk from ground deformation and landslides which are triggered by a variety of phenomena including episodic disastrous earthquakes and storms, long-term and seasonal changes in water table as a result of changing land use, and coastal erosion caused by natural response to changes in sea level and changes in sediment transport patterns caused by construction of dams, breakwaters and other coastal structures. Coastal ground failures can be significant contributors to longshore sediment load. Hence, increase in the frequency of landslides can in some cases also contribute to environmental degradation through local amplification of sedimentation in nearshore habitats.
Our coastal ground failures study group focuses on characterization of (1) the geologic environment, and form of deformations; (2) measurement of geophysical and geotechnical properties of the ground; and (3) development of analytical, numerical, empirical and probabilistic models that describe and predict ground failures and their amplitude. These models are intended for scientists, engineers and urban planners.
Ground failure is a general term which encompasses all types of downward movement of material. Ground failures occur in every state of our country and are annually responsible for an estimated 25 to 50 deaths and $1 to $2 billion in property damage (FEMA 1995). Researchers suggest that approximately 40 percent of the United States' population is either directly or indirectly affected by landslide events. Ground deformation and failure of liquefied ground can be severely damaging to buried utilities, fire-suppression water mains, and foundations and has a major contributor to urban conflagration following earthquakes.
The west coast of the United States is particularly susceptible to ground failure due to the significant amount of precipitation and the earthquake potential. These two natural agents act as triggering events for landslides.
The principal natural factors, which play a role in ground failure potential, are topography, geology, and precipitation. Areas with steep slopes are more susceptible to landslides than flat areas, whereas, liquefaction of soil during earthquakes has typically occurred in relatively flat coastal plain areas. In general the more precipitation an area experiences, and the higher the ground water table, the greater the potential for ground failures. Anthropogenic (human) factors also influence landslide potential. The major human induced factors are development of hilly terrain, irrigation, construction of highways, buildings, and railroads, mining activity, and forestry practices.
Landslides also can be grouped by the way in which they move and divided into four general types:
Slides: Slides are characterized by the downward displacement movement of material along one or more failure surfaces.
Flows: Flows are similar to slides but differ in that they are characterized by high water content and the disintegration of the failure into fluid-like movement.
Lateral spreads: Lateral spreads are earthquake-induced movements associated with loose, sandy soils with high liquefaction potential. Lateral spreads occur when coherent surface soil is rafted on a weak liquefied layer and displaces down-slope. Lateral spreads can occur on very gentle slopes.
Falls and topples: Falls and topples are movements in which masses of rock or other material fall from cliffs or other steep slopes. Earthquakes commonly trigger this final type of movement.
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When large faults rupture and produce earthquakes, they generally deform the ground surface. Primary surface faulting, such as the 340 kilometer-long surface rupture associated with the 2002 Denali Fault earthquake, is the direct effect of movement on a seismogenic, or earthquake-producing fault. Rupture on nearby faults induced by the primary event (sympathetic rupture) may also produce surface faulting. Away from a surface fault rupture, ground deformations in soil and rock can produce secondary damage features that extend for tens and even hundreds of kilometers from the sesmogenic fault. Ground failure and ground deformation associated with dynamic movements can form by a variety of mechanisms: (1) shaking-induced compaction of deposits in fills, sedimentary basins and river valleys; (2) liquefaction of loose gravely-sands, sands and silts and (3) displacements of ground on non-liquefiable soils.
Liquefaction is a phenomenon associated with earthquakes in which sandy to silty, water-saturated soils behave like fluids. As seismic waves pass through saturated soil layers, the structure of the soil distorts and void spaces between soil particles (pores) collapse, causing deformation and ground failure. In general, young, loose sediment and areas with high water tables are the most susceptible to liquefaction. Sand boils and fissures are a common sign of liquefaction. Sand boils and fissures form when saturated sediment below the surface is ejected to the surface by elevated pore water pressure. Lateral spreads involves the movement of a relatively coherent surface crust of soil due to the liquefaction of underlying sediment. Horizontal movements of up to several meters is commonly observed after earthquakes in fills, coastal alluvium and river embankments. These deformations are a major contributor to economic loss and loss of services during earthquakes. Flow failure is another type of liquefaction-related ground failure and the most destructive. Flow failures occur when liquefied soils completely disintegrate during deformation and flow long distances down-slope, sometimes at high speed. Liquefied ground looses its capacity to support structure. The loss of bearing strength may result in the settlement, tilting or collapse of building.
Click on the Play button to view a video clip of the
formation of sand boils in a blast-induced liquefaction
experiment at Treasure Island, San Francisco, California.
Liquefaction most commonly occurs in earthquake-prone, low-lying areas with saturated soils. Generally, young saturated soils beneath rivers, lakes, bays, and seafloor are highly susceptible to liquefaction. Port and harbor facilities built on these soils are especially vulnerable to liquefaction effects. Many port and harbor facilities have been severely damaged from earthquake-induced liquefaction.
Permanent seismic deformation can occur in any soil or rock when driving stresses exceed the mobilized capacity of the ground to resist shearing. During earthquake motions, permanent displacements accumulate in the ground for each cycle that induces slip along the failure surface. It is common for the shear strength of the ground to dramatically fall during this type of loading, causing increased displacements with subsequent cycles. In the most damaging case, the shear strength can fall below the down-slope stress, so that even when earthquake shaking stops the landslides continues to displace and even disintegrate into a high speed flow. All earthquake-induced landslides are caused by a combination of dynamic displacements during shaking and deformations associated with the interaction between the local sloping ground and changes in the internal properties of the slide material.