In recent years, bio-inspired burrowing robots and other intruder problems in granular media have attracted significant attention. Many of these, especially related to traditional penetration problems in geotechnical engineering, cover vertical penetration. Modelling these types of problems numerically using the discrete element method (DEM) is typically done ignoring gravity by controlling the stresses in the selected representative volume. Additionally, most problems involve infinitely long intruders from a modelling point of view. However, in horizontal penetration there is enough evidence to show that intruders are affected by an uplift force that affects the penetration and needs to be considered. In this paper we use the DEM to demonstrate the impact of considering vertical uplift and gravity for a finite-length intruder penetrating in a dense granular media through particle level and macro-behaviour. Additionally, through the numerical study, important mechanisms emerge during horizontal penetration, including four different distinct stages on the surrounding soil, or the extent of disruption, that are fundamentally distorted when gravity is ignored.
Graphical abstract
In geotechnical applications the success of the discrete element method (DEM) in simulating fundamental aspects of soil behaviour has increased the interest in applications for direct simulation of engineering scale boundary value problems (BVP's). The main problem is that the method remains relatively expensive in terms of computational cost. A non-negligible part of that cost is related to specimen creation and initialization. As the response of soil is strongly dependant on its initial state (stress and porosity), attaining a specified initial state is a crucial part of a DEM model. Different procedures for controlled sample generation are available. However, applying the existing REV-oriented initialization procedures to such models is inefficient in terms of computational cost and challenging in terms of sample homogeneity. In this work a simple but efficient procedure to initialize large-scale DEM models is presented. Periodic cells are first generated with a sufficient number of particles matching a desired particle size distribution (PSD). The cells are then equilibrated at low-level isotropic stress at target porosity. Once the cell is in equilibrium, it is replicated in space in order to fill the model domain. After the domain is thus filled a few mechanical cycles are needed to re-equilibrate the large domain. The result is a large, homogeneous sample, equilibrated under prescribed stress at the desired porosity. The method is applicable to both isotropic and anisotropic initial stress states, with stress magnitude varying in space.
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