MPM modelling of soil structure interaction problems in liquefied granular materials: the role of visco-plasticity

  • Flessati, Luca (Technische Universeit Delft)
  • Marveggio, Pietro (Politecnico di Milano)

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In many practical cases, granular materials may experience a phase transition and start behaving like a viscous fluid potentially accumulating large displacements and moving at large rates. This phenomenon is likely to happen either when large strain rates are observed [1] or when large pore water pressure develops [2]. A typical case of the former one is the fast propagation of landslides, whereas of the latter one is the seismic induced liquefaction. In the literature are present many works describing the disastrous consequences of liquefaction on existing structures. To evaluate the hazard associated to this phenomenon is crucial being able (i) to numerically reproduce the dynamic interaction between soil and structures and (ii) to reproduce the mechanical behaviour of soil under fluidized/almost fluidized conditions. The first one can be reliably be reproduced by using the Material Point Method (MPM), while the second one still represents a challenge, since the standard approaches commonly employed lack of either the biphasic nature of soil (rheologies formulated in total stresses) or the time dependency of material behaviour (rate independent plasticity). In this paper, the authors considered a case study concerning the uplift of a buried pipeline in a uniform liquefied granular material layer. The problem is numerically studied by using the MPM code Anura3D and by implementing a simple visco-plastic constitutive law relating effective stresses to strain rates and by using an uncoupled approach. Despite of the straightforwardness of the formulation, the numerical method can very well reproduce small scale experimental test results [3].