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Show Liquefaction and Ground Failure in Sensitive Clays Shallow Ground Water Lowe ( 1990a) states that " ground shaking tends to increase the pressure in the pore water between silt grains, which decreases the stresses between the grains. The loss of intergranular stress can cause the strength of some soils to decrease nearly to zero. When this happens, the soil behaves like a liquid, and therefore is said to have liquefied." Four types of ground failure can occur during liquefaction: loss of bearing strength, ground oscillation, lateral- spread landslides and flow landslides. The type and severity of the failure depends greatly on the surface slope. Under some conditions, clays can become unstable by leaching salts. These are referred to as sensitive clays. During earthquakes they can lose their strength, resulting in ground failures similar to those occurring during liquefaction. Anderson and others ( 1982, 1986 and 1990) and Lowe ( 1990a and 1990b) suggest that large areas within Salt Lake, Davis and Weber Counties east of the lake have a moderate to high potential for liquefaction during earthquakes. These areas adjacent to the lake have sensitive clay soils susceptible to liquefaction. Regarding flooding related to local and distant earthquakes, liquefaction, and wind tides, Atwood and Mabey ( 1990) point out the following: " Engineered structures ( such as dikes and causeway embankments) founded on the lakebed, particularly those designed to provide protection from the lake water, pose special engineering- geology problems." These problems include settling, flooding, soil compaction and erosion. Ground water is, by definition, water beneath the surface of the ground which fills fractures and pore spaces in rocks and the voids between grains in unconsolidated sediments. Ground water is considered shallow when it occurs at depths less than 30 feet. Lowe ( 1990a and 1990b) suggests that ground water adjacent to the lake, at depths less than 10 feet, may cause flooding of basements and other related problems. In the vicinity of GSL, the water table, or the top of the saturated ground, fluctuates in response to the level of the lake. During times of high- lake levels, the water table is higher than during times of low- lake levels, and larger areas around the lake will be affected. Wind Tides and Seiches Sustained winds blowing across the surface of GSL push the water to the shore or dike and causeway where it " piles up," forming what is known as a wind tide or wind setup. The height or magnitude of the setup depends on the speed, direction, fetch, depth of lake at that point and duration of the wind. Wind setup exceeding two feet is not uncommon, and can cause localized increased flooding and damage. The combined effects of wind setup and high waves ( wave runup) can produce adverse impacts to elevations five to seven feet above the static lake elevation and locally even higher. As these winds cease or diminish, the water begins to oscillate back and forth in the lake, similar to water sloshing from end to end in a bathtub. This movement is referred to as a seiche. The period of the oscillation, or the time it takes to move from high to low and back to high, is about six hours 116 |