Liquefaction
Liquefaction means the transformation of any substance into a liquid state.
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Liquefaction
7.1 7.1.1
Introduction to Liquefaction Causes of Liquefactions
Liquefaction means the transformation of any substance into a liquid state. When soils around structures’ foundations are subject to cyclic loading due to earthquakes or adverse ocean storms, they may lose their shear strength, causing a catastrophe such as local or global failures of foundations and even the collapse of entire structures. Nowadays, liquefaction is an important soil failure mode relevant for the geotechnical design and integrity of both offshore and land-based structures. Under static monotonic loading, drainage helps soils to prevent excess water pressure building up. However, under cyclic loading, especially for loose saturated sand, it tends to densify and settle, break down the soil structure and cause a tendency toward volumetric compression. Because the duration of the shaking is too brief, the water does not have time to dissipate. A higher pressure load may then be taken by the water pressure, leading to a reduction in effective stress and a decrease in shear resistance and stiffness. This is mainly because, under repeated loading, the loose particles in sand tend to compact more tightly, resulting in a decrease in volume and an increase in pore-water pressure that cannot be dissipated under undrained or partly drained conditions. Figure 7.1 shows the development of pore-water pressure and shear stress–strain behavior as functions of time under undrained cyclic loading with a constant cyclic shear stress. The cyclic loading generates a pore pressure characterized by a permanent pore pressure component, up, and a cyclic pore pressure component, ucy. Each load cycle gives an additional incremental pore pressure, which reduces the effective stresses in the soil, resulting in increased average, ca, permanent, cp, and cyclic, ccy, shear strains with time [390]. And after a number of load cycles, the stress path reaches the failure envelope as shown in Fig. 7.2. Figure 7.3 shows the excess pore-water pressures under different consolidation ratios (kc = r1/r3, where © Springer International Publishing AG 2018 J. Jia, Soil Dynamics and Foundation Modeling, https://doi.org/10.1007/978-3-319-40358-8_7
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7 Liquefaction
r1 and r3 are the maximum principal stress and confining pressure), measured by the stress-controlled torsional shear test method. By observing this figure, it is also found that, under anisotropic consolidation condition (kc = 1.5 and 2.0), the excess pore-water pressure cannot reach the confining pressure r3, and the liquefaction cannot occur. Furthermore, the peak value of the pore-water pressure decreases with an increase in initial consolidation ratio. It should be mentioned that, for the cases without liquefaction occurrence, the number of load cycles to induce liquefaction Nf is defined as the load cycles corresponding to a deformation (i.e., strain) of 5%. In addition, it is important to consider the simultaneous occurrence of both dynamic shear stress and applied axial dynamic bidirectional lo
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