Contact Maturing and Aging of Silica Sand

Abstract

For more than three decades, sand has been observed to alter its engineering properties over time, but no consensus has been reached on the driving mechanisms behind this phenomenon. Silica sand freshly deposited or after recent disturbance tends to undergo delayed changes in small strain stiffness, penetration resistance, or liquefaction resistance. In the same category of phenomena is a delayed increase of the shaft resistance of displacement piles after installation (pile setup). Micromechanical behavior at grain scale and the contact scale is coming to be understood as the most plausible mechanism among the proposals suggested in the available literature, but only very limited research has been conducted at the contact scale in studies of sand aging. A static fatigue hypothesis is advocated in this thesis; it suggests that delayed fracturing of micro-morphological features on grain surfaces at contacts, such as asperities and mineral debris, is a key contributor to aging of silica sand. The static fatigue process at inter-grain contacts induces changes in micromechanical properties of the contacts, a process termed maturing in this research, and it triggers rearrangements of sand grains over time. Maturing of contacts and rearrangement of grains are hypothesized to be the cause of the observed changes in macroscopic engineering properties of sand over time. To support this hypothesis, this research focuses on exploring micromechanical behavior of inter-grain contacts through micro-scale experiments complemented with numerical simulations. The following major tasks were accomplished: (1) Micro-scale laboratory experiments were conducted to study time-dependent response of inter-grain contacts under sustained loads; they produced the first set of data of its kind. (2) Laboratory experiments on sand grain assemblies were performed to provide evidence that the contact behavior induces aging effects in sand; factors affecting rates of aging, such as loads, pore fluid acidity and grain sizes were explored. (3) Simulations of a single inter-grain contact were performed with the distinct element method, and possible consequences of contact fatigue/maturing were demonstrated. (4) Finally, a preliminary finite element framework was developed to explore the evolution of grain surface textures to shed light on the effects of pore fluid chemistry on aging rates.