{"id":11070,"date":"2023-10-03T15:26:37","date_gmt":"2023-10-03T15:26:37","guid":{"rendered":"https:\/\/blog.geostru.eu\/liquefazione-e-sisma\/"},"modified":"2023-10-09T14:02:14","modified_gmt":"2023-10-09T14:02:14","slug":"liquefaction-and-earthquake","status":"publish","type":"post","link":"https:\/\/blog.geostru.eu\/en\/liquefaction-and-earthquake\/","title":{"rendered":"Liquefaction and earthquake"},"content":{"rendered":"

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Liquefaction and earthquake<\/h2>\n

Liquefaction is an extraordinarly complex and potentially destructive phenomenon<\/a> that can occur during a seismic event of significant intensity. The accelerations generated by a seismic event result in an increase in the neutral pressure of the interstitial water contained in saturated sandy soil.<\/p>\n

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Figure 1<\/strong> \u2013 Comparison between stable soil on the left and liquefied soil on the right (from Encyclopaedia Britannica Inc, 2012)<\/em><\/p>\n

The increase in neutral pressures leads to the cancellation of effective stresses and shear resistance of the soil. The temporary loss of cohesion in sandy soil causes it to behave like a fluid. This soil behavior can substantially weaken the bearing capacity of the soil, resulting in structures built on it sinking or tilting (Figure 1, 2, 3<\/strong>).<\/p>\n

\"liquefaction\"<\/strong><\/em><\/p>\n

Figure 2-<\/strong>Real Cases: A) Liquefaction in the city of Urayasu, Japan; B) cars stuck in boiling muddy water (from Yasuda et al., 2013); C) effects of sand liquefaction (Niigata, 1964); Kobe event, 1995.<\/em><\/p>\n

Undrained soil conditions<\/h2>\n

In static conditions, sandy soils are typically considered to be in a drained state, where the neutral pressure inside the soil has time to equilibrate with the surrounding conditions. However, during the phenomenon of liquefaction induced by a seismic event, the accelerations imposed on the soil create a rapid increase in neutral pressure that prevents it from dissipating within the available time frame.
\nEssentially, the sudden increase in neutral pressure during liquefaction, caused by the seismic forces at play, creates undrained conditions where the sandy soil loses its ability to rapidly dissipate the accumulated neutral overpressures (Fig.3).<\/p>\n

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Figure 3 –<\/strong> A) Stable pre-liquefaction condition, where sand grains are in contact with each other; B) Following the earthquake, cohesion between the grains is lost, and the soil exhibits liquid behavior (from Youd, 1992).
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This scenario can be compared to the case where the application of an external load occurs at a much higher rate than the rate at which neutral overpressures can dissipate (e.g., penetrometer test). During the liquefaction process, despite the inherently high permeability of the sand, it will be in a state defined as ‘undrained,’ where neutral overpressures do not have the opportunity to be dissipated.<\/p>\n

Legislation<\/h2>\n

According to the current Technical Regulations, the verification against liquefaction can be omitted if at least one of the four conditions listed below is met in the excerpt from the EC7\/EC8.<\/p>\n

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Fig.4<\/em><\/strong><\/p>\n

Methods for conducting checks<\/h2>\n

In the context of safety assessments related to liquefaction phenomena, different methodological approaches can be adopted, ranging from simplified methods to more sophisticated ones based on advanced analysis.<\/p>\n