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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

Arched roof.(a) A symmetrical semicircular two-hinged arch with a rectangular cross section (width b, thickness h). The vertical and horizontal forces at points A and B are Ay, By, Ax, and Bx respectively. Weight of the beam in water is W, radius of the semicircular arch is Ra, span of the arch is L (= 2Ra) and lower case w represents the weight of a unit length. (b) A section of the semi-circular arch given in Fig 4a. N and V are the normal thrust and shear forces, respectively, at any point on the arch, and θ is the angle V makes with the horizontal. β is the angle of the tangent at any point on the arch to the horizontal axis.
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pone.0122349.g005: Arched roof.(a) A symmetrical semicircular two-hinged arch with a rectangular cross section (width b, thickness h). The vertical and horizontal forces at points A and B are Ay, By, Ax, and Bx respectively. Weight of the beam in water is W, radius of the semicircular arch is Ra, span of the arch is L (= 2Ra) and lower case w represents the weight of a unit length. (b) A section of the semi-circular arch given in Fig 4a. N and V are the normal thrust and shear forces, respectively, at any point on the arch, and θ is the angle V makes with the horizontal. β is the angle of the tangent at any point on the arch to the horizontal axis.

Mentions: To examine this case, consider a symmetrical two-hinged arch (Figs 3b and 5a) with a rectangular cross section (width b, thickness h). Based on the symmetry of the structure, one easily finds that the vertical reactions at the points A and B, are Ay = By = W / 2, where W is the arch weight in water. Next, we denote the unknown horizontal reaction F at points A and B by /Ax/ = /Bx/ = F.


Bubble-induced cave collapse.

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

Arched roof.(a) A symmetrical semicircular two-hinged arch with a rectangular cross section (width b, thickness h). The vertical and horizontal forces at points A and B are Ay, By, Ax, and Bx respectively. Weight of the beam in water is W, radius of the semicircular arch is Ra, span of the arch is L (= 2Ra) and lower case w represents the weight of a unit length. (b) A section of the semi-circular arch given in Fig 4a. N and V are the normal thrust and shear forces, respectively, at any point on the arch, and θ is the angle V makes with the horizontal. β is the angle of the tangent at any point on the arch to the horizontal axis.
© Copyright Policy
Related In: Results  -  Collection

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

pone.0122349.g005: Arched roof.(a) A symmetrical semicircular two-hinged arch with a rectangular cross section (width b, thickness h). The vertical and horizontal forces at points A and B are Ay, By, Ax, and Bx respectively. Weight of the beam in water is W, radius of the semicircular arch is Ra, span of the arch is L (= 2Ra) and lower case w represents the weight of a unit length. (b) A section of the semi-circular arch given in Fig 4a. N and V are the normal thrust and shear forces, respectively, at any point on the arch, and θ is the angle V makes with the horizontal. β is the angle of the tangent at any point on the arch to the horizontal axis.
Mentions: To examine this case, consider a symmetrical two-hinged arch (Figs 3b and 5a) with a rectangular cross section (width b, thickness h). Based on the symmetry of the structure, one easily finds that the vertical reactions at the points A and B, are Ay = By = W / 2, where W is the arch weight in water. Next, we denote the unknown horizontal reaction F at points A and B by /Ax/ = /Bx/ = F.

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