Influence of the void ratio and the confining on the static liquefaction in slopes in changi sand
Authors: Alfonso Mariano Ramos Cañón
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Failure of slopes under monotonic undrained conditions of loading is an instability process which occurs in loose sands. It is well known that the behavior of sands is greatly influenced by their initial conditions, such as the initial density and confining pressure (Hyodo, Tanimizu, Yasufuku, & Murata, 1994). A common approach used to tackle issues associated with liquefaction is based on phenomenology. Moreover, the most used methodologies for evaluating the potential of liquefaction are based on field test such as SPT and CPT (Youd et al., 2001). These methodologies disregard important aspects like the anisotropy of stresses in the evaluation of susceptibility of liquefaction. An alternative and more rational way to understand the static liquefaction is based on sets of laboratory tests, and based on these results it’s proposed a generalization of the behaviour. For exampleWanatowski and Chu (2007) depict a relation between the stress ratio hL = qL /pL and void ratio for undrained triaxial tests isotropically and anisotropically compressed (qL and pL are the main pressure and the deviatoric stress in the onset of static liquefaction, respectively). On other hand, Chu and Wanatowski (2008) propose a mathematical equation between hL and the Been-Jefferies critical state parameter (ψ = e − ec ). This equation is obtained by assuming that the increase of plastic volumetrics train is equal to the increase of total volumetric strain and both of them are equal to zero in the peak of deviatoric stress. It means that the dilatancy is zero in the peak of undrained effective stress path. However, Lade (1994) demonstrates that sands subjected to undrained loading show a behavior that must be modelled by a flow rule highly non associative. A similar approximation for detecting the onset of static liquefaction is to characterize the instability line for a specific type of sand. The instability line can be defined as the boundary in which large strains are rapidly generated due to the inability of a soil element to sustain a given stress or load (Chu, Leroueil, & Leong, 2003). The instability line was firstly proposed by Vaid and Chern (1985) defining it as the locus of points at which flow liquefaction is initiated for the same initial void ratio under monotonic undrained triaxial tests. Many researchers have analysed the instability line (Lade, 1994; Chu & Wanatowski, 2008; Chu, Leroueil, & Leong, 2003; Wanatowski & Chu, 2007; Hyodo, Tanimizu, Yasufuku, & Murata, 1994; Andrade, 2009; Andrade, Ramos, & Lizcano, 2013). Andrade (2009) and Ramos, Andradeand Lizcano (2011) using two different elastoplastic constitutive models concluded that the slope of the instability line is not a constant of the material but it is a state parameter
In this work, a criterion for detecting the onset of static liquefaction derived by Andrade, Ramosand Lizcano (2013) and Ramos, Andradeand Lizcano (2011) was extended to an isotropic elastoplastic constitutive model with kinematic hardening and bounding surface theory (Manzari-Dafalias model). The application of this criterion matches very well with experiments reported in the literature, allowing to generate numerical simulation for different conditions with a high level of confidence. By making use of numerical simulations previously validated with experiments, it is possible to supply the deficiency of the experimental approximation and to have a greater spectrum of the behavior of the sandy slopes subjected to static liquefaction. As result, the influence of void ratio, mean pressure and initial anisotropy on the onset of static liquefaction is investigated. Finally, despite of the multiple variables involved in the process of generation of static liquefaction, the behavior of the sand can be condensed in a linear relation between Δq/p0 and h0 . This kind of relation can be used in the practice of geotechnical engineering to quantify the degree of security of a slope given the initial conditions of stress and void ratio. The paper is organized as follow. The first section of this paper shows the instability criterion for detecting the onset of static liquefaction using the Manzari-Dafalias elastoplastic constitutive model. The ‘Validation’ section presents numerical simulations for prediction of static liquefaction and compares them with laboratory experiments. Finally, the influence of the void ratio, mean pressure and the initial stress anisotropy on the onset of static liquefaction is presented based on several simulations conducted with the constitutive model and the instability criterion. Findings from the application of the criterion are highlighted in the ‘Conclusions’ section.