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Liquefaction of Tokyo bay

Liquefaction describes a phenomenon where a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.

The Liquefaction is most often observed in saturated, loose (low density or uncompacted), sandy soils. This is because a loose sand has a tendency to compress when a load is applied; dense sands by contrast tend to expand in volume or 'dilate'. If the soil is saturated by water, a condition that often exists when the soil is below the ground water table or sea level, then water fills the gaps between soil grains ('pore spaces'). In response to the soil compressing, this water increases in pressure and attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if the loadings rapidly applied and large enough, or is repeated many times (e.g. earthquake shaking, storm wave loading) such that it does not flow out in time before the next cycle of load is applied, the water pressures may build to an extent where they exceed the contact stresses between the grains of soil that keep them in contact with each other. These contacts between grains are the means by which the weight from buildings and overlying soil layers are transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose all of its strength (the ability to transfer shear stress) and it may be observed to flow like a liquid (hence 'liquefaction').

The pressures generated during large earthquakes with many cycles of shaking can cause the liquefied sand and excess water to force its way to the ground surface from several metres below the ground. This is often observed as "sand boils" also called "sand blows" or "sand volcanoes" (as they appear to form small volcanic craters) at the ground surface. The phenomenon may incorporate both flow of already liquefied sand from a layer below ground, and a quicksand effect whereby upward flow of water initiates liquefaction in overlying non-liquefied sandy deposits due to buoyancy. Tohoku earthquake and tsunami has altered this phenomena to happen in Japan, Tokyo Bay areas. We will be focusing on the liquefaction-induced damage in Tokyo Bay area.

The effects of soil liquefaction on the built environment can be extremely damaging. Buildings whose foundations bear directly on sand which liquefies will experience a sudden loss of support, which will result in drastic and irregular settlement of the building causing structural damage, including cracking of foundations and damage to the building structure itself, or may leave the structure unserviceable afterwards, even without structural damage. Where a thin crust of non-liquefied soil exists between building foundation and liquefied soil, a 'punching shear' type foundation failure may occur. The irregular settlement of ground may also break underground utility lines. The upward pressure applied by the movement of liquefied soil through the crust layer can crack weak foundation slabs and enter buildings through service ducts, and may allow water to damage the building contents and electrical services.

The 2011 Tohoku-Pacific Ocean earthquake

with a magnitude of Mw=9.0 occurred in the Pacific Ocean about 130 km off the northeast coast of Japan’s main island on March 11, 2011. Liquefaction occurred in a wide area of reclaimed land along Tokyo Bay, though the epicentral distance was very large, about 380 to 400 km. Much land has been reclaimed in the Tokyo Bay area since the seventeenth century. Liquefaction has been induced during past earthquakes, such as 1923 Kanto Earthquake and 1987 Chibaken-toho-oki Earthquake. However, the Tohoku-Pacific Ocean earthquake is the first on record to cause liquefaction in such a wide area and to severely damage houses, lifelines and roads. The effect of the very long duration of the main shock and of the aftershock, which hit 29 min later, on the occurrence of liquefaction and the associated damage to houses, is discussed. Remarkable phenomena, such as the buckling of sidewalks and damage to sewage facilities, due to a kind of sloshing around of the liquefied ground, are cited, and the effectiveness of soil improvement in the prevention of liquefaction is discussed.

Damage to infrastructure due to liquefaction in Tokyo Bay:

The FL-value (safety factor against liquefaction) came to 1 or more at most depths in the neighborhood of Urayasu Station, where no liquefaction damage was observed, and in the Akemi-Hinode (southeast) area, where only minor damage was seen. In other places, however, the FL-value was estimated to be lower than 1. Particularly in Mihama-Irifune, Takasu and Akemi-Hinode (northwest), there are sequences of layers with an FL-value lower than 1 until the depth of nearly 20 m. These results agree with the observed damage.

Liquefaction prevention : Methods to mitigate the effects of soil liquefaction have been devised by earthquake engineers and include various soil compaction techniques such as vibro compaction(compaction of the soil by depth vibrators), dynamic compaction, and vibro stone columns. These methods result in the densification of soil and enable buildings to withstand soil liquefaction. Existing buildings can be mitigated by injecting grout into the soil to stabilize the layer of soil that is subject to liquefaction.

Dynamic compaction is a method that is used to increase the density of the soil when certain subsurface constraints make other methods inappropriate. It is a method that is used to increase the density of soil deposits. The process involves of dropping a heavy weight repeatedly on the ground at regularly spaced intervals. The weight and the height determine the amount of compaction that would occur. The weight that is used, depends on the degree of compaction desired and is between 8 tonne to 36 tonne. The height varies from 1m to 30m.

Vibro stone columns, are an array of crushed stone pillars placed with a vibrating tool into the soil below a proposed structure. This method of ground improvement is also called vibro replacement.