Dynamic behavior of vibrated dry sand : sphere penetration experiments and discrete element modeling of vibrofluidization
(2010)
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- Authors
- Supervisors
- Holeyman, Alain
- Abstract
- (en) The research focuses on dynamic properties of vibrated dry sand within the framework of vibrodriving and vibrocompaction processes applied to granular soils. Original experiments able to characterize the behavior of dry sand subjected to vertical vibration are first presented. In particular, the volume change and the motion pattern displayed by vertically vibrated sand particles are discussed. When cohesionless soil placed in a cylindrical container is vertically vibrated under the gravitational field (g), the UCL experiments performed on dry Fontainebleau sand allow one to distinguish three different types of dynamic behavior, depending on the acceleration amplitude (a): the densification (a/g < 1), the instability surface (a/g ≈ 1), and the vibro-fluid behaviors (a/g > 1). In the densification range, the sand simply settles. When the acceleration is increased beyond 1g, granular convection is observed and there is an instability in the sand mass leading to the emergence of an inclined free surface. If the acceleration is further increased, the free surface progressively flattens. There is an impressive dilatation of the whole sample and grains saltation is noticed. The sand becomes fully vibro-fluidized. Results of sphere penetration experiments (SPE’s) are also presented. The sinking logs of the SPE’s conducted at UCL do not follow Stokes law, as initially suggested by the 1962 Barkan experiments. An exponentially decreasing penetration profile is observed on one hand in the densification range although no refusal depth was reached. In the vibro-fluid regime, on the other hand, the sphere quickly sinks into the vibrated medium and stabilizes at a certain equilibrium penetration. Hence, the concept of vibro-viscosity coefficient, proposed by Barkan (1962), is to be reappraised in our opinion. In order to emphasize the evolution of the internal state of sand during vibration, one resorts to Discrete Element Modeling (DEM). Numerical simulations of vibrated granular sample are conducted under lateral periodic boundary conditions, considering only the interactions between the grains. In this way, one tries to track down the fundamental origin of the vibro-fluidization. A stratified analysis is used to describe the assembly behavior from the microscopic to the macroscopic scale. When the sample is vibrated at low acceleration amplitude (a/g = 0.5), it swiftly reaches a steady vibratory regime. There are successive compacting and dilating phases corresponding respectively to upward and downward acceleration phases of each cycle. The contact network is still preserved during the whole cycle and the granular material seems to behave like an elastic solid with regards to the macro- scale of the assembly. Once the acceleration amplitude is set to 1.02g, the contact network is cyclically disintegrated and the force chains get unbound resulting in shear strength degradation. Cyclic vibro-fluidization is therefore noticed. At the same time, the emergence of “fluid” stresses is observed, underscoring the dual nature of vibrated dry cohesionless soil. Hence, vibrations result in a particular state of granular matter, called the “vibro-fluid” state. It is characterized by a phase transition corresponding to the transformation of a solidlike material into a complex fluid identified by an abrupt change in its properties resulting in shear strength degradation.
- Affiliations
Citations
Denies, N. (2010). Dynamic behavior of vibrated dry sand : sphere penetration experiments and discrete element modeling of vibrofluidization. https://hdl.handle.net/2078.5/148161