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Bubble-induced cave collapse.

Girihagama L, Nof D, Hancock C - PLoS ONE (2015)

Bottom Line: Using familiar theories for the strength of flat and arched (un-cracked) beams, we first show that the flat ceiling of a submerged limestone cave can have a horizontal expanse of 63 meters.Using familiar bubble dynamics, fluid dynamics of bubble-induced flows, and accustomed diving practices, we show that a group of 1-3 divers submerged below a loosely connected ceiling rock will quickly trigger it to fall causing a "collapse".In these experiments, a metal ball represented the rock (attached to the cave ceiling with a magnet), and the bubbles were produced using a syringe located at the cave floor.

View Article: PubMed Central - PubMed

Affiliation: Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, Florida, United States of America.

ABSTRACT
Conventional wisdom among cave divers is that submerged caves in aquifers, such as in Florida or the Yucatan, are unstable due to their ever-growing size from limestone dissolution in water. Cave divers occasionally noted partial cave collapses occurring while they were in the cave, attributing this to their unintentional (and frowned upon) physical contact with the cave walls or the aforementioned "natural" instability of the cave. Here, we suggest that these cave collapses do not necessarily result from cave instability or contacts with walls, but rather from divers bubbles rising to the ceiling and reducing the buoyancy acting on isolated ceiling rocks. Using familiar theories for the strength of flat and arched (un-cracked) beams, we first show that the flat ceiling of a submerged limestone cave can have a horizontal expanse of 63 meters. This is much broader than that of most submerged Florida caves (~ 10 m). Similarly, we show that an arched cave roof can have a still larger expanse of 240 meters, again implying that Florida caves are structurally stable. Using familiar bubble dynamics, fluid dynamics of bubble-induced flows, and accustomed diving practices, we show that a group of 1-3 divers submerged below a loosely connected ceiling rock will quickly trigger it to fall causing a "collapse". We then present a set of qualitative laboratory experiments illustrating such a collapse in a circular laboratory cave (i.e., a cave with a circular cross section), with concave and convex ceilings. In these experiments, a metal ball represented the rock (attached to the cave ceiling with a magnet), and the bubbles were produced using a syringe located at the cave floor.

No MeSH data available.


Related in: MedlinePlus

Laboratory experiment for bubble-induced cave collapse (with a convex ceiling).Now, the apparatus is a cylindrical chamber (radius and width are 6.4 cm and 3.3 cm, respectively) with a convex ceiling. It is again filled with water (colored). The chamber is equivalent to a cave with a circular cross section. Air bubbles are again released at the bottom of the chamber using a syringe. The tube on the left side of the chamber removes water displaced by the bubbles. In this demonstration, the bubbles cause a flow past the metal ball and accumulate away from the metal ball, due to the convex shape of the cave ceiling. (a) Experimental set up before bubble release. (b) Bubbles released at the bottom using a syringe. (c) Point at which the metal ball loses its buoyancy due to the bubbles. (d) The ball falling to the bottom of the chamber, due to gravity.
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pone.0122349.g011: Laboratory experiment for bubble-induced cave collapse (with a convex ceiling).Now, the apparatus is a cylindrical chamber (radius and width are 6.4 cm and 3.3 cm, respectively) with a convex ceiling. It is again filled with water (colored). The chamber is equivalent to a cave with a circular cross section. Air bubbles are again released at the bottom of the chamber using a syringe. The tube on the left side of the chamber removes water displaced by the bubbles. In this demonstration, the bubbles cause a flow past the metal ball and accumulate away from the metal ball, due to the convex shape of the cave ceiling. (a) Experimental set up before bubble release. (b) Bubbles released at the bottom using a syringe. (c) Point at which the metal ball loses its buoyancy due to the bubbles. (d) The ball falling to the bottom of the chamber, due to gravity.

Mentions: We constructed two small Plexiglas caves, one with a concave roof (Fig 10) and one with a convex roof (Fig 11). The radius and width of the cylindrical chamber are 6.4 cm and 3.3 cm, respectively; whereas the metal ball’s radius is approximately 0.75 cm. Bubbles are released at roughly 2-3ml/s. The concave roof, though more realistic, allows for bubbles to accumulate near the rock, which enhances the buoyancy reduction process by an amount we have not taken into account. The convex cave (Fig 11), on the other hand, allows an investigation of the process without the abovementioned bubble accumulation, despite its unrealistic configuration.


Bubble-induced cave collapse.

Girihagama L, Nof D, Hancock C - PLoS ONE (2015)

Laboratory experiment for bubble-induced cave collapse (with a convex ceiling).Now, the apparatus is a cylindrical chamber (radius and width are 6.4 cm and 3.3 cm, respectively) with a convex ceiling. It is again filled with water (colored). The chamber is equivalent to a cave with a circular cross section. Air bubbles are again released at the bottom of the chamber using a syringe. The tube on the left side of the chamber removes water displaced by the bubbles. In this demonstration, the bubbles cause a flow past the metal ball and accumulate away from the metal ball, due to the convex shape of the cave ceiling. (a) Experimental set up before bubble release. (b) Bubbles released at the bottom using a syringe. (c) Point at which the metal ball loses its buoyancy due to the bubbles. (d) The ball falling to the bottom of the chamber, due to gravity.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4388830&req=5

pone.0122349.g011: Laboratory experiment for bubble-induced cave collapse (with a convex ceiling).Now, the apparatus is a cylindrical chamber (radius and width are 6.4 cm and 3.3 cm, respectively) with a convex ceiling. It is again filled with water (colored). The chamber is equivalent to a cave with a circular cross section. Air bubbles are again released at the bottom of the chamber using a syringe. The tube on the left side of the chamber removes water displaced by the bubbles. In this demonstration, the bubbles cause a flow past the metal ball and accumulate away from the metal ball, due to the convex shape of the cave ceiling. (a) Experimental set up before bubble release. (b) Bubbles released at the bottom using a syringe. (c) Point at which the metal ball loses its buoyancy due to the bubbles. (d) The ball falling to the bottom of the chamber, due to gravity.
Mentions: We constructed two small Plexiglas caves, one with a concave roof (Fig 10) and one with a convex roof (Fig 11). The radius and width of the cylindrical chamber are 6.4 cm and 3.3 cm, respectively; whereas the metal ball’s radius is approximately 0.75 cm. Bubbles are released at roughly 2-3ml/s. The concave roof, though more realistic, allows for bubbles to accumulate near the rock, which enhances the buoyancy reduction process by an amount we have not taken into account. The convex cave (Fig 11), on the other hand, allows an investigation of the process without the abovementioned bubble accumulation, despite its unrealistic configuration.

Bottom Line: Using familiar theories for the strength of flat and arched (un-cracked) beams, we first show that the flat ceiling of a submerged limestone cave can have a horizontal expanse of 63 meters.Using familiar bubble dynamics, fluid dynamics of bubble-induced flows, and accustomed diving practices, we show that a group of 1-3 divers submerged below a loosely connected ceiling rock will quickly trigger it to fall causing a "collapse".In these experiments, a metal ball represented the rock (attached to the cave ceiling with a magnet), and the bubbles were produced using a syringe located at the cave floor.

View Article: PubMed Central - PubMed

Affiliation: Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, Florida, United States of America.

ABSTRACT
Conventional wisdom among cave divers is that submerged caves in aquifers, such as in Florida or the Yucatan, are unstable due to their ever-growing size from limestone dissolution in water. Cave divers occasionally noted partial cave collapses occurring while they were in the cave, attributing this to their unintentional (and frowned upon) physical contact with the cave walls or the aforementioned "natural" instability of the cave. Here, we suggest that these cave collapses do not necessarily result from cave instability or contacts with walls, but rather from divers bubbles rising to the ceiling and reducing the buoyancy acting on isolated ceiling rocks. Using familiar theories for the strength of flat and arched (un-cracked) beams, we first show that the flat ceiling of a submerged limestone cave can have a horizontal expanse of 63 meters. This is much broader than that of most submerged Florida caves (~ 10 m). Similarly, we show that an arched cave roof can have a still larger expanse of 240 meters, again implying that Florida caves are structurally stable. Using familiar bubble dynamics, fluid dynamics of bubble-induced flows, and accustomed diving practices, we show that a group of 1-3 divers submerged below a loosely connected ceiling rock will quickly trigger it to fall causing a "collapse". We then present a set of qualitative laboratory experiments illustrating such a collapse in a circular laboratory cave (i.e., a cave with a circular cross section), with concave and convex ceilings. In these experiments, a metal ball represented the rock (attached to the cave ceiling with a magnet), and the bubbles were produced using a syringe located at the cave floor.

No MeSH data available.


Related in: MedlinePlus