Earthquake Risks III: Liquefaction Potential in the Willamette-Puget Lowland Amanda Tondreau ES473 Environmental Geology – Spring 2009 INTRODUCTION SOIL CHARACTERISTICS ABSTRACT Liquefaction occurs when the strength and stiffness of soils are reduced by earthquakes or other loading. Liquefaction can cause major damages to buildings, roads, and other structures in the Willamette-Puget Lowland. Liquefaction Susceptibility (1) no susceptibility; bedrock; low (2) <6ft. of liquid material; moderate >25 ft. of liquid material; high Liquefaction is an earthquake-related hazard that causes unstable land and poses risks to building infrastructure in urban areas. Saturated sediments, ranging from gravel to silt, are more prone to liquefaction during an earthquake compared to those that are unsaturated and well drained. Unconsolidated sediments deposited within the last 10,000 years, during the Holocene era, are particularly vulnerable to liquefaction in the Willamette-Puget Lowland. Liquefaction processes commonly result in buried pipelines and other objects floating to the surface, and commonly cause foundational failure in roads and buildings. Analysis of paleo-liquefaction features preserved in the geologic record provides a technique that allows use of past events to predict future risks of damage. This poster focuses on geotechnical aspects of seismically-induced liquefaction hazards in western Washington and Oregon. Session • Conditions leading to liquefaction: • Each particle is in contact with surrounding particles. • Weight of the covering particles creates contact forces between soil particles. • During an earthquake water is trapped and prevents soil particles from contact with one another. • An increase in water pressure also reduces contact weakening the soil. • The contact forces become smaller and can also loose contact altogether. Figures 1 and 4. Deposits Relative Ranking Fills1 Very High Holocene2 Alluvium3 Mod. to High Holocene Beach Deposits Moderate Pleistocene4 Alluvium Low Pleistocene Glacial Deposits Very Low Clean sands and silty sand are most susceptible to liquefaction. The best ways to reduce the hazards are to avoid liquefaction susceptible soils, build liquefaction resistant structures, and to improve the soil by using soil development techniques. Puget Lowland Willamette Valley Figure 1. VOCABULARY Figure 2. Ground Failure CAUSES Fill: accumulation of sand in a stream channel, more is deposited than is washed away. Holocene: beginning at the end of the last Ice Age about 11,000 years ago and characterized by the development of human civilizations. Alluvium: Sediment deposited by flowing water, as in a riverbed, flood plain, or delta. Pleistocene: 1.8 million to 10,000 years before present • Saturated cohesion less soils are temporarily turned into a liquid state. • Liquefaction is from the result of earthquake-induced ground shaking. • Buildup of excess water pressure during the shaking. • Depth of the water table is important to how saturated the soil will become. • Soil below the water table is the most susceptible to liquefaction. Figure 3. Particle in contact with another REFERENCES Grant, Paul W., Perkins, William J., Youd, Leslie T. “Evaluation of Liquefaction potential in Seattle, Washington.” US Geological Survey Professional Paper 1560: 441-473. Vessely, Andrew D, Riemer, Michael, Arango, Ignacio. “Liquefaction susceptibility of soft alluvial silts in the Willamette Valley.” Oregon Geology Volume 58.Issue 6 November 1996: 142-145. THREE IMPORTANT FACTORS Sand boils – an effect of liquefaction and unstable ground related to seismic shaking The strength of the underlying soil deposit The location of the water table The severity of the earthquake ground shaking CONTACT Name: Amanda Tondreau Email: firstname.lastname@example.org Figure 4.The blue block represents the water level.