Title: Shake and sink: liquefaction without pressurization
ArXiv ID: 1802.04391
Date: 2018-02-14
Authors: 원문에 저자 정보가 제공되지 않았습니다.
📝 Abstract
Soil liquefaction is a significant natural hazard associated with earthquakes. Some of its devastating effects include tilting and sinking of buildings and bridges, and destruction of pipelines. Conventional geotechnical engineering practice assumes liquefaction occurs via shear-driven compaction and consequent elevation of pore pressure. This assumption guides construction for seismically hazardous locations, yet evidence suggests that liquefaction strikes also under currently unpredicted conditions. Here we show, using theory, simulations and experiments, another mechanism for liquefaction in saturated soils, without necessitating high pore fluid pressure or special soils, whereby seismically triggered liquefaction is controlled by buoyancy forces. This new mechanism supplements the conventional pore pressure mechanism, enlarges the window of conditions under which liquefaction is predicted to occur, and may explain previously not understood cases such as liquefaction in well-compacted soils, under drained conditions, repeated liquefaction cases, and the basics of sinking in quicksand. These results may greatly impact hazard assessment and mitigation in seismically active areas.
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Liquefaction occurs in saturated soils when an initially rigid soil, which supports structures, changes rheology under earthquake-induced shaking [Diaz-Rodriguez et al, 1992; Wang et al, 2010; Youd et al, 1978] to a liquid-like slurry, in which structures such as buildings sink and tilt [O'Rourke et al, 1989] and structures such as pipelines float [O'Rourke et al, 1989;Ambraseys et al, 1969;Huang et al, 2013]. Liquefaction is of crucial importance in geoengineering [Youd et al, 1978;Youd et al, 2001;Hausler et al, 2001;Seed et al, 2003;Sawicki et al, 2009], and constitutes a basic physics question for the two-phase system of fluids and grains [Khaldoun et al, 2005; Brzinski et al, 2013; Huerta et al, 2005; Lohse et al, 2004].
The conventional mechanism for explaining liquefaction requires un-drained and undercompacted saturated soils. It assumes that during earthquakes the induced cyclic shear causes pore structure in under-compacted porous soils to collapse. The un-drained condition leads to trapping of fluid in compacting pores, so pore pressure increases until its value may approach the total stress [Youd et al, 2001]. High pore pressure values lead to loss of strength and liquefaction of soils. Indeed, this mechanism was confirmed in many liquefaction instances [e.g. Obermeier et al, 1996;Holzer et al, 1989], yet it fails to predict many other observed situations:
Liquefaction in pre-compacted soils. An example of such a scenario is the liquefaction event in Kobe, Japan, following the 1995 Great Hanshin Earthquake (M=6.9). Soga [1998] reviewed the damage in the port facilities that were built on a reclaimed island. It was found that soils that were vibro-compacted, and therefore are not expected to be compactive, were still liquefied. This occurrence is baffling if liquefaction is explained solely by a mechanism that involves compaction of initially loose soils.
Recurrent liquefaction events [e.g. Wakamatsu et al, 2012;Ambraseys et al, 1969;Youd et al, 1978]. As explained by Obermeier (1996): “liquefaction has a strong tendency to recur at the same site” but “An apparent contradiction to recurrent liquefaction at the same site is the observation that liquefaction commonly densifies sediments. Densification should reduce the liquefaction susceptibility. Worldwide engineering measurements before and after occurrences of liquefaction indicate that thick zones … densified substantially whenever liquefaction was severe [Koizumi, 1966;Ohsaki, 1970].” Thus recurrent liquefaction is not predicted by the conventional mechanism.
Far-field liquefaction that occurred despite small seismic energy input. Experiments find that liquefaction via the conventional mechanism requires a minimum energy density input: at least 30 J/m 3 . However, about half of the sites that underwent liquefaction during earthquakes received less energy than that, probably by orders of magnitude less [Wang, 2007;Manga et al, 2012]. 4. Liquefaction produced under drained conditions [Goren et al, 2010; Goren et al, 2011; Lakeland et al, 2014]. Although the conventional mechanism explains liquefaction by pore pressure increase in rapidly compacting soils, most liquefaction demonstrations actually show fully drained liquefaction and involve no pore pressure rise. See e.g. the demo of Illinois Geological Survey https://www.youtube.com/watch?v=cONq231dn6w
The above observations are important in that they are both widespread and fundamentally inconsistent with the conventional mechanism of liquefaction. This paper presents an alternative liquefaction mechanism that requires neither compactive soils, nor high pore pressure. The new mechanism occurs in saturated soils. It is triggered by seismic accelerations and controlled by buoyancy effects. Although in nature liquefaction may occur solely by the mechanism we propose, we show below that the new mechanism may also combine with, and be enhanced by, the conventional mechanism, i.e. by elevated pore pressure.
In what follows we present experiments, theory and simulations. All of those investigate a system similar to the common table-top systems used to demonstrate the process of liquefaction to students, (see point 4 above). Our model system comprises an intruder, placed on top of a partially or fully water-saturated granular layer. The system is shaken at specified accelerations and amplitudes to simulate a building or a structure experiencing seismic shaking.
Under a certain range of shaking conditions, the medium liquefies and the intruder sinks.
We define liquefaction based on measurements of the intruder sinking. This definition is similar to that used in the field, where the occurrence of liquefaction is identified based on the phenomenology typical of liquefaction, such as sinking and floating of structures. We do not define here liquefaction by its narrower mechanistic definition (sometimes used by engineers) which identifies liquefaction with high pore pressure, as the m
