I have been taking geodynamics as a part of master course in winter session 2013. Geodynamics is one of the courses that suit my interest because I am really curious about dynamic behavior of soil and its impact on earth structures during earthquake. Soil liquefaction is one of the most important phenomenons that usually becomes devastating to the structures and eventually claims several human lives. 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
Factors affecting soil liquefaction
Mechanism of Soil Liquefaction
Liquefaction is one of the most important, interesting, complex, and controversial phenomenon whereby a saturated soil significantly 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. It can cause spectacular damage including slope failure, bridge and building foundation failures, and floatation of buried structures. Nowadays, many researchers around the world have paid their attention extensively to such a devastating phenomenon to reconcile a smooth path.
Figure 1: Mechanism of soil liquefaction (Reference: Public Works Research Institute, Ibaraki Prefacture, Japan, After 2011 Great East Earthquake)
The figure 1 elucidates the liquefaction mechanism schematically. It shows that when the loosely packed sand is disturbed very slowly, the natural arrangement of particles will be disordered, reducing the volume. As shown in figure, if this sand layer is located below the groundwater level and pore water flows between sand grains, then water is discharged to the outside as the volume is reduced. On the other hand, when the earthquake load shakes many times in a short period, the sand grains try to quickly reduce their volume but the discharge of pore water cannot keep pace. 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):
Flow liquefaction produces the most dramatic effects of all...