Academic journal article Journal of Geoscience Education

Laboratory Apparatus for the Demonstration of Quicksand

Academic journal article Journal of Geoscience Education

Laboratory Apparatus for the Demonstration of Quicksand

Article excerpt

ABSTRACT

The use of quicksand as a convenient plot device in television and movies often leads to misconceptions, even among students taking introductory earth science courses. Because quicksand is a familiar natural phenomenon, exploring the underlying mechanisms provides an exceptional opportunity for student learning. Studying quicksand is facilitated by an easily constructed experimental apparatus and illustrative exercises, both of which are detailed here.

INTRODUCTION

Quicksand is a simple natural phenomenon that occurs when the upwards flow of water through a cohesionless sediment (e.g., silt, sand, gravel) eliminates the shear strength or that sediment, causing it to act as a viscous fluid (e.g., Bowles, 1984; Charbeneau, 2000). Regrettably, students often arrive at introductory geology classes with the perception that quicksand is an almost malevolent phenomenon that entraps and engulfs the unwary. This misconception rises from the use of quicksand as a convenient plot device in popular fiction (i.e., books, movies, television). In the fictional context, encounters with quicksand occur in terrains ranging from deserts to swamps, and lead to either the picturesque death of a villain or a dramatic rescue. In reality, the natural occurrence of quicksand is limited to locations where water rises to the land surface, such as swamps, streams, or along beaches. As a threat to life, the density of quicksand is greater than that of water, hence humans are actually more buoyant in quicksand than in water. However, the high viscosity of quicksand greatly inhibits movement, making it difficult to extricate oneself or to move into a prone floating position. Because quicksand is such a well known, misunderstood, and relatively simple phenomenon, hands-on exploration of the underlying physical processes provides an excellent learning opportunity.

Here, we provide a strategy for classroom investigation of quicksand that is appropriate for use in introductory geology, soils, and environmental science courses. This approach is also applicable to more advanced courses in hydrogeology, engineering geology, and geotechnical engineering, where loss of strength in a sediment is a matter of importance. First, we describe the fundamental processes that lead to the formation of quicksand and the closely related phenomenon or liquefaction. We also place these processes into context for both natural and anthropogenic scenarios. Then, we present a simple apparatus that has been designed to facilitate classroom investigation. Lastly, we describe laboratory exercises designed to study quicksand, liquefaction and the more general case of quick conditions" in varying levels of detail.

BACKGROUND

Quicksand is a naturally occurring subset of "quick conditions", a term that refers to the loss of shear strength in response to excess pore water pressure (e.g., Bowles, 1984; Charbeneau, 2000). The occurrence of quick conditions is limited to saturated sediments that lack cohesion (e.g., silt, sand, gravel). The strength of such sediments results entirely from friction between grains, which is dependent on the pressure forcing the grains together, known as the inter-granular normal pressure. At any point within a given sediment, the normal pressure is applied by the overburden pressure and counteracted by water pressure within the pores (figure 1). The normal pressure goes to zero if pore water pressure is equal to, or exceeds the overburden pressure. The resulting lack of friction between grains causes the sediment to act as a thick fluid with zero shear strength (e.g., the sediment becomes quick). By definition, pore water pressure is negative in unsaturated sediments, thus preventing the occurrence of quick conditions. Cohesive sediments (those that contain clay) do not become quick when overpressured, but may lose strength dramatically through soil heave or blow-out, particularly in excavations below the water table (e. …

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