Miguel Angel Cabrera

The growing size of offshore wind turbine monopiles and environmental concerns have led to reconsideration of their installation method by impact-driving. In comparison, vibratory driving offers the advantages of lower noise emissions, easier scaling up of the hammer, less pile fatigue, potentially quicker installation, and reduced risk of pile-run. However, vibratory driving has not yet been adopted in the majority of industry projects due to a lack of confidence in vibrodriveability analyses and the subsequent lateral behavior. This emerges from a lack of experience and, specifically, the small number of publicly available testing campaigns. This paper presents the results of a testing campaign conducted in a geotechnical centrifuge model of a newly developed miniature vibrodriver for hollow steel piles installed in dense, fine, dry sand. The experimental model allows assessment of the role of vibrodriving parameters, such as frequency, eccentric moment, and static weight, on pile driveability. This parametric study elucidates their interdependent effects and allows a critical examination of the semianalytical models in use in the industry and their assumptions.

Granular flows can occur under low inertia conditions, called the quasi-static regime, and extend to highly inertial systems, called the inertial regime. In the latter, granular flows, particularly those having a variety of grain sizes-property known as polydispersity-have not been extensively studied. Existing rheological laws for monodisperse flows effectively capture volume and friction variations across inertial ranges, assuming the grains diameter as the flow characteristic length. For polydisperse materials, this assumption is less intuitive, and rheological laws cannot be extended straightforwardly. In this work, we employed the Discrete Element Method to study granular flows across varying inertial levels, aiming to identify a physically based length scale that represents the grain scale for polydisperse flows. We show that the average branch length (i.e., distance between the centers of contacting grains) is a representative value of the material's grain size distribution, remaining nearly constant across the explored range of inertia. Moreover, we show that monodisperse and polydisperse flows follow common inertial volume and friction laws when the average branch length is considered as the characteristic length. The findings of this work propose a new perspective for understanding the characteristic length of granular flows, providing a comprehensive interpretation based on the grains contacts. They also permit to extend rheological laws, initially proposed for monodisperse flows, to polydisperse flows by considering the characteristic length scale as the average branch length. Finally, Our results are useful for choosing the characteristic length that controls large-scale flows where polydispersity plays an important role.

Dynamic pile installation is a challenging aspect of the realization of offshore infrastructure. Due to the large-scale and specialized hardware involved, researchers have resorted to centrifuge modelling to study this process at a manageable scale and in a controlled environment. However, in centrifuge models, a discrepancy between the dynamic and diffusive timescales is introduced. By using a viscous fluid, the discrepancy is mitigated, thereby synchronizing both timescales. Hydroxypropylmethylcellulose (HPMC) is a versatile and affordable thickening agent, widely used in centrifuge modelling of dynamic processes. However, the rheological behavior of HPMC solutions can vary based on the type of HPMC and the solution concentration. This study investigates how the rheological behavior of HPMC solutions is influenced by the concentration and the type of HPMC. We present a detailed preparation protocol for preparing aqueous HPMC solutions and propose a generic power-law function that unifies experimental results from our study and those of previous works. We investigate the implications of varying degrees of shear thinning, as observed across different HPMC products, by examining the pore pressure response during the dynamic installation of a pile in dense sand saturated with fluids of similar dynamic viscosity. Our findings support the use of viscous fluids for studying dynamic events in the centrifuge but also emphasize the importance of selecting a viscous fluid whose rheology does not vary considerably within the range of expected shear rates. In broader terms, this work streamlines the manufacturing of viscous fluids for use in the centrifuge and advances research efforts related to modelling dynamic phenomena in geotechnical engineering.

The dynamic properties of loose sands under low stresses are an unexplored topic in soil dynamics because these soil conditions are uncommon in most geotechnical structures on Earth. However, low densities and low-stress conditions prevail on other planets, like, for instance, the surface of Mars, for which particular attention is presently given through the InSight NASA mission. This work presents a new procedure for measuring the dynamic properties of loose sand under low stress by using the dynamical mechanical analysis (DMA) tester, a technique commonly used in asphalt engineering but not in geotechnical engineering. Compared to traditional geotechnical methods (resonant column and cyclic triaxial tests), DMA investigates a broader range of strains using a single apparatus. In this work, we assess the dynamical properties of loose fine sand D_r ≈ 0.2, considered a possible Mars regolith analog, by varying the input strain from γ = 10⁻⁶ to γ = 10⁻² while applying confining pressures from σ₃ = 3 kPa to σ₃ = 30 kPa. The results validate the proposed procedure, showing an increment of the shear modulus as the confining pressure increases. Furthermore, they highlight DMA’s advantages for studying the dynamic properties of granular soils under low stress and strain.

The collapse dynamics and runout of columns of elongated grains in two dimensions are numerically investigated in dry and immersed conditions, by means of an unresolved finite elements/discrete elements model. The elongated grains are modeled as rigid aggregates of disks. The column aspect ratio is systematically varied from 0.125 to 16 in order to span short and tall columns. To analyze the effect of the initial grain orientation, columns with an initial grain orientation that is either random or aligned with a given direction are both considered. Collapse dynamics, both in dry and immersed cases, are found analogous to that previously observed for circular grain columns, particularly with respect to the power law dependency for the runout as a function of the column aspect ratio. The effect of the fluid mainly results in a decrease of the runout distance. Interestingly, the collapse dynamics and runout are not significantly affected by the initial orientation of the grains, except maybe in the extreme case where the grains are all horizontally oriented, which geometrically prevents the collapse. Finally, a scaling based on the front propagation energy is proposed allowing one to unify the runout of short to tall and dry to immersed columns in a single description, regardless of the initial grain orientation.

Granular flows are a complex process, involving a wide range of grain sizes, materials, varied viscous fluids, among others. For this reason, the simulation of granular flows requires a certain level of simplification, allowing the isolated study of its governing variables and extending the global observations to field events. Here, we present the planar setup as an alternative for studying simplified processes associated to granular flows. The planar setup consists of two windows separated by a thin gap and enclosing a granular assembly. We present two examples where the planar setup is adapted for the study of the competing action of segregation and disaggregation in a fractured grain under shear flow, and for the study of the stability scenarios of a flow impacting a permeable obstacle. The close visualization of the kinematics at the particle scale provides an ideal opportunity for describing the mechanisms behind the grain disaggregation or controlling the obstacle stability. Both examples highlight the advantages of the planar setup for the study of granular flows.

The granular column collapse is a simplified version of granular flows such as landslides, avalanches, and other industrial processes mobilized in air or within a fluid. In this configuration, the particles collapse in an accelerating phase, reaching a state of constant spreading velocity until they decelerate and stop. Granular flows commonly involve particles of different sizes, a property termed polydispersity. Understanding the role of polydispersity remains a challenging task that is often analyzed with nearly monodisperse systems and demanding a series of simplifications when coupled with a fluid in a numerical model. Here, we study the effect of particle-size polydispersity in dry and immersed granular columns, using a finite element method-discrete element method model for fluid-particle interactions. We show that the velocity of the column collapse and runout distance decrease with an increase in the level of polydispersity in immersed conditions, and remain nearly independent of the level of polydispersity in dry conditions. Moreover, we find that the runout scales with the spreading front kinetic energy, weighted by the ratio between the particles' density and the density difference between particles and fluid. This scaling helps in identifying the governing processes in polydisperse granular columns, unifying the runout description of both dry and immersed collapses, and indicating that the column initial packing fraction is the governing parameter.