I have been taking geodynamics as a part of master course in winter session (2013). Geodynamics is the one that suits my interest because I am really curious about dynamic behavior of soil and its impact on earth structures during earthquake. Consequently, soil liquefaction is one of the most important and interesting phenomena usually occurs during earthquake which becomes devastating, and eventually claims several human lives as shown in figure 1. This report has been prepared as a part of home assignment given by Yokohama sensei based on his lecture delivered. This assignment includes the following issues:
Mechanism of soil liquefaction
Testing method of liquefaction strength
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The tendency for densification causes excess pore water pressure to increase and effective stress to decrease. Consequently, the sand particles become suspended in pore water, the strength of the sand is suddenly reduced, and the sand behaves like a liquid. After the earthquake has finished, the pore water is continuously discharged, and the ground sinks by an amount equal to the lost water, and then stabilizes almost in the previous or dense state. Such an interesting phenomenon is called liquefaction, and is known to occur easily in a loose sand layer below the groundwater level. The liquefaction phenomenon that results from this process can be divided into two groups (Kramer 1996):
Figure 2: Mechanism of soil liquefaction (Reference: Public Works Research Institute, Ibaraki Prefacture, Japan)
Flow liquefaction produces the most dramatic effects of all the liquefaction related phenomena tremendous instabilities known as flow failures. Flow liquefaction can occur when the shear stress required for static equilibrium of a soil mass is greater than the shear strength of soil in its liquefied state. Once triggered the large deformation produced by flow liquefaction are actually driven by static shear stresses. The cyclic stresses may simply bring the soil to an unstable state at which its strength drops sufficiently allow the static stress to produce the flow failure. Flow liquefaction is characterized by its sudden nature, the speed with which it develops and the large distance over which the liquefied material moves.
Cyclic mobility is another phenomenon that can also produce unacceptably large permanent deformation during earthquake loading. In contrast to flow liquefaction, cyclic mobility occurs when the static shear stress is less than shear strength of the liquefied soil. The deformations produced by cyclic mobility failure develop incrementally during earthquake shaking. In contrary to flow liquefaction, the deformations produced by cyclic mobility, called lateral spreading, are driven by both cyclic and static shear stresses. Moreover, level-ground liquefaction is a special case of cyclic mobility, in which failure is caused by the upward flow of water, which occurs when the seismically induced excess pore pressure dissipates. Depending on the length of time required to reach hydraulic equilibrium, level-ground liquefaction failure may occur even after ground shaking has ceased. The excessive vertical settlement with consequent flooding of low-lying and the presence of sand boils are characteristic of the level-ground liquefaction failure.
TESTING METHOD OF LIQUEFACTION STRENGTH
The liquefaction strength of an element of soil depends on how close the initial state of the soil is to the state corresponding to failure and on the nature of the loading required to move it from the initial state to the failure state (Kramer, 1996). Many researchers have pointed out...