Building on Quicksand

Reliable systems have always been built out of unreliable components. Early on, the reliable components were small such as mirrored disks or ECC (Error Correcting Codes) in core memory. These systems were designed such that failures of these small components were transparent to the application. Later, the size of the unreliable components grew larger and semantic challenges crept into the application when failures occurred. As the granularity of the unreliable component grows, the latency to communicate with a backup becomes unpalatable. This leads to a more relaxed model for fault tolerance. The primary system will acknowledge the work request and its actions without waiting to ensure that the backup is notified of the work. This improves the responsiveness of the system. There are two implications of asynchronous state capture: 1) Everything promised by the primary is probabilistic. There is always a chance that an untimely failure shortly after the promise results in a backup proceeding without knowledge of the commitment. Hence, nothing is guaranteed! 2) Applications must ensure eventual consistency. Since work may be stuck in the primary after a failure and reappear later, the processing order for work cannot be guaranteed. Platform designers are struggling to make this easier for their applications. Emerging patterns of eventual consistency and probabilistic execution may soon yield a way for applications to express requirements for a "looser" form of consistency while providing availability in the face of ever larger failures. This paper recounts portions of the evolution of these trends, attempts to show the patterns that span these changes, and talks about future directions as we continue to "build on quicksand".Comment: CIDR 200

A micromechanical model of collapsing quicksand

The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e. at the particle scale) of granular/soil materials. We present a contact dynamics method, accounting for the cohesive nature of fine powders and soils. A modification of the model adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of loose systems is able to simulate "quicksand" behavior of a collapsing soil material, in particular of a specific type, which we call "living quicksand". We investigate the penetration behavior of an object for varying density of the material. We also investigate the dynamics of the penetration process, by measuring the relation between the driving force and the resulting velocity of the intruder, leading to a "power law" behavior with exponent 1/2, i.e. a quadratic velocity dependence of the drag force on the intruder.Comment: 5 pages, 4 figures, accepted for granular matte

Shifting landscapes: from coalface to quick sand? Teaching geography, earth and environmental sciences in UK higher education

In this paper we examine contemporary academic working lives, with particular reference to teaching-only and teaching-focused academics. We argue that intensification in the neoliberal university has significantly shifted the structure of academic careers, while cultural stories about those careers have not changed. We call for academics to re-examine our collective stories about standard academic career paths. Challenging the stories and making visible the ways that they create and multiply disadvantage is a crucial step in expanding the possibilities for academic identities and careers. The paper begins by describing teaching-focused academics within the context of the wider workforce. We then draw on narratives of those in these roles to illustrate the processes that (re)inscribe their marginalisation. We uncover the gendering of the teaching-focused academic labour market. We end the paper by suggesting interventions that all academics can take and support to address the issues we highlight

Spartan Daily, May 12, 1954

Volume 42, Issue 138https://scholarworks.sjsu.edu/spartandaily/12032/thumbnail.jp

Sixth annual conference on alaskan placer mining

An abridged format of papers, presentations and addresses given during the 1984 conference held on March 28-29, 1984, compiled and edited by Daniel E. Walsh and M. Susan Wray

Installing Farm Drainage Systems

Exact date of bulletin unknown.PDF pages: 2

Living quicksand

The image of quicksand merciless swallowing a victim has inspired the fantasy of kids and helped writers and moviemakers to get rid of evil figures. Is this really possible? This is still disputed since till today it is not even clear what quicksand exactly is. In soil mechanics, the "quick-condition” is usually described as a liquefaction due to high water pressure essentially possible with any soil. However, previous studies have detected anomalous rheological properties from natural quicksand. Pushed by these contradicting points of view we set off to Lençois Maranhenses in North-East Brazil, where quicksands are common, to investigate rheology and strength in situ. We found that along very quiet drying lakes cyanobacteria cement an impermeable crust above a suspension of grains. Beyond a critical pressure, the crust fails releasing water from the collapsing colloidal structure and radically changing the depth dependence of the shear strength from a constant to a linear function. The sedimenting solid fraction and the rapid increase of shear strength can indeed trap an intruder endangering his life if the basin is sufficiently deep. As opposed to some previous studies, we find that this quicksand condition cannot be restored once it has collapsed. Finally, we also show some preliminary results from a contact dynamics model specially designed to mimic the living quicksand behavio

A micromechanical model of collapsing quicksand

The discrete element method constitutes a general class of modeling techniques to simulate the microscopic behavior (i.e. at the particle scale) of granular/soil materials. We present a contact dynamics method, accounting for the cohesive nature of fine powders and soils. A modification of the model adjusted to capture the essential physical processes underlying the dynamics of generation and collapse of loose systems is able to simulate "quicksand” behavior of a collapsing soil material, in particular of a specific type, which we call "living quicksand”. We investigate the penetration behavior of an object for varying density of the material. We also investigate the dynamics of the penetration process, by measuring the relation between the driving force and the resulting velocity of the intruder, leading to a "power law” behavior with exponent 1/2, i.e. a quadratic velocity dependence of the drag force on the intrude