This Thesis examines the mechanical response of large objects -called intruders or anchors that are embedded into a granular packing and subjected to dynamic loadings. By using a numerical approach based on a discrete element method, the study focuses on a canonical test comprising a plate-shaped intruder, placed horizontally and being uplifted vertically. The research is articulated into three projects.
The first project considers steady and quasi-static loading conditions, whereby the intruder is uplifted at a constant velocity. Its purpose is to validate the numerical method against previously established models for the maximum drag force, also known as uplift capacity. Incidentally, this project establishes to what extent it is possible to downscale/upscale the size of the intruder relative to the grain size. This result is important as most laboratory and numerical tests, starting from those presented in this study, are performed with object-to-grain size ratios much smaller than real applications.
The second project considers the mobility response under cyclic loading, whereby the object is subjected to a cyclic uplift force. A series of numerical tests exploring a range of loading frequency and magnitude reveals the existence of three possible mobility responses. The object can either move up steadily, not move up at all or exhibit a creep trajectory. Furthermore, this study points out a phenomena of elasto-inertial resonance inducing a fluidisation of the packing even at low loading magnitudes.
The third project considers loading patterns including some acceleration of the object. This reveals a new contribution to the drag force, which we named “inertial drag”. We show that this contribution results from gradual mobilisation and acceleration of grains in the packing above the object. We further find that achieving a complete grain mobilisation takes a finite period of time, controlled by the elasto-inertial stress propagation from the object to the free surface.
These three projects highlight fundamental differences between the drag force in quasi-static loading and dynamic loading conditions. A number of analytical models, built from identified micro-mechanical processes, are proposed to rationalise these effects