Numerical prediction of blast induced wave propagation in the soil

Surface or subsurface explosions cause vibrations that can have detrimental effects on structures at large distances. To control these effects, accurate prediction of soil vibrations is needed. The shock non-linearity and high velocities and pressure levels associated with blast loading considerably complicate the prediction strategies. Confronted with the limitations of empirical and analytical modeling, an explicit numerical hydrocode simulation seems the only option to predict the highly non-linear events in the immediate surroundings of an explosion, while taking the geometry of the site in to account. This paper deals with the optimal configuration and customization of an existing hydrocode tool for blast-induced soil wave propagation simulations. Autodyn, a hydrocode analysis tool of the ANSYS Workbench, offers a wide array of solver techniques using explicit time integration and adapted material models, design and calculation tools. It enables the simulation of the non-linear propagation of a shockwave and the very large deformations associated with the energy freed by an explosion. A 2D axisymmetric Autodyn model of a surface explosion experiment at the Poelkapelle Explosive Ordnance Disposal site, using a combination of Euler and Lagrange solvers, delivers promising results, but also demonstrates a number of shortcomings. These are related to solver and software limitations and model configuration restrictions. Considerable errors are caused by the treatment of model boundaries and solver transitions and can be minimized by adopting a partially spherical geometry and meshing strategy. This calls for an adaptation of currently available Autodyn model configuration tools. The Arbitrary Lagrangian Eulerian solver scheme can replace Eulerian techniques in parts of the model. It improves the poor material boundary and surface treatment, while still allowing large soil deformations, by adding a mesh rezoning step to a traditional Lagrange scheme. The high calculation cost, due to the use of explicit hydrocode solvers, can be reduced by limiting their use to the shock wave domain and the hydrodynamic and elasto-plastic soil deformation zone. The vibrations at the borders of this reduced hydrocode model can be used as the input at the boundaries of a model of the linear elastic wave propagation zone, thus replacing the non-linear part of the global model by an equivalent elastic source. The linear zone can be simulated using boundary elements. The prediction of blast-induced soil vibrations can be improved by the balanced combination of Euler, Lagrange and ALE solvers in Autodyn, leading to a robust and economic model of the non-linear calculation domain. Several limitations call for adaptations of the Autodyn tools, in order to enable an improved simulation of the non-linear events near the explosion. Future work includes the development of an adapted linear elastic boundary element model, the determination of the appropriate zoning of both non-linear and linear elastic models and the development of an appropriate soil model. Furthermore, a supplementary series of underground blast tests, including short range surface vibration measurements, are currently planned to enable the validation of the adapted hydrocode model.

Jaar van publicatie:2009