Anchor Marco Werman speaks with geo-scientist Timothy Bechtel, about the massive sinkhole that swallowed an intersection and a three-story factory in Guatemala as the country was hit by Tropical Storm Agatha.
MARCO WERMAN: I'm Marco Werman, this is The World. The Central American nation of Guatemala is still struggling to recover from Tropical Storm Agatha. The storm caused flooding and landslides all over the country. At least 180 deaths have been blamed on it. The storm also seems to have played a role in creating a giant sinkhole in Guatemala City. The hole is more than 60 feet wide, and about 100 feet deep. It opened up during the storm, swallowing up a clothing factor and an intersection. It's not the first sinkhole to appear in Central America after a major storm, but it's bigger than most and almost perfectly round. Timothy Bechtel teaches geosciences at Franklin & Marshall College and the University of Pennsylvania. He says the hole could still get bigger.
TIMOTHY BECHTEL: It's still very dangerous. If you've seen the photographs of it, it's really quite spectacular because the walls are nearly vertical. It almost looks photoshopped, but it's not. That's for real. The problem now is those walls are much steeper than their angle of repose. The angle of repose is the angle at which a material will be stable in the long term. Probably anybody who has built a sand castle is familiar with angle of repose. If you build a nice vertical wall out of sand it doesn't stay that way for very long. So those vertical walls are likely to begin losing material and trying to attain their angle of repose. That angle is going to depend on the soil type, but it's going to be probably something on the order of 35 to 45 degrees. So that hole could get a lot bigger.
WERMAN: Do you think certain regions of the world are more prone to sinkholes than others?
BECHTEL: Oh absolutely. The areas that are prone to sinkholes are really very well known, they are the areas that are underlain by limestone or dolomite or gypsum bedrock, any of the types of rock that are soluble. The cavities develop underground and it creates an unstable situation. If you picture the bedrock being like Swiss cheese, full of cavities, and there's soil above it, the soil is going to want to drop down into the cavities and water is always the big factor in these things, that's why Tropical Storm Agatha was involved. There's some indication that a broken sewer line may be involved. There was a big sinkhole like this in 2007 in Guatemala City and one of the main causes there was a broken sewer line. What happens is you get excess in the soil in a particular spot and it will flush the soil down into the underlying cavities. That's what creates the sinkhole. The sinkhole actually starts down at the top of rock. Soil drops into the cavities, there will be a little opening above the cavities in the rock and that opening will eat its way upward, pieces will fall off the roof and get lost down into the cavities and that void will stope, or eat its way upward until it breaks through at the surface. So when we see that opening at the surface, essentially that vertical tunnel in Guatemala City has probably been eating its way up from the top of rock for days, if not months or years.
WERMAN: I'm wondering what the human role is in all of this. You mentioned this broken sewer line. Was that an exception or do humans have a lot to do with these sinkholes as well.
BECHTEL: Sadly it's kind of a rule. It's very much a rule. Sinkholes do occur naturally. They happen all over the world and have for millennia, even in the absence of humans. But human activities contribute greatly to the incurrence of sinkholes. When we pave, or put up roofs, it takes rainfall that would have been distributed and there would be infiltration across a much larger area, we concentrate the runoff and that water infiltrating in a concentrated fashion, if it infiltrates above a cavity in the rock it will take soil with it and the sinkhole will eat its way upward. And in addition, we have a lot of underground utilities that carry water or sewage, things that if they break will put a lot of water into the subsurface and that water will start flushing soil down into the cavities. So it's really man's concentration of water in various places that leads to most sinkholes these days.
WERMAN: Now Timothy, one report I read said the sinkhole understandably spooked the locals in Guatemala City, but excited geologists. So does it excite you and why?
BECHTEL: Excites and saddens. It's exciting because I've been fascinated with earth processes, particularly sinkhole processes since I was a kid. I grew up in a house with a sinkhole in the backyard.
WERMAN: Oh, so this is personal.
BECHTEL: A little bit, yes. So I find it scientifically fascinating and exciting, but from a human standpoint it's very much a tragedy. I think the people in Guatemala City have a right to be very nervous. I think the occurrence in 2007 and this recent one really the rule rather than the exception given the geology and man's activities in that location, there are going to be more of these sinkholes.
WERMAN: What can the officials in Guatemala City do to prevent further sinkholes?
BECHTEL: Without knowing a whole lot about Guatemala City, I can only speak about it in general terms. It's really too late to move the city. People are already living there. The biggest thing that could be done is avoid concentrated infiltration in any fashion. I think they ought to be very vigilant about keeping on top of their underground infrastructure, their water lines and sewer lines. When those leak, those commonly lead to sinkholes.
WERMAN: Timothy Bechtel, a consultant with Enviroscan, Inc. a geophysical consulting firm in Pennsylvania, thank you very much for your time; very interesting stuff.
BECHTEL: You're welcome. Thank you.
WERMAN: There's a link to a video of the giant sinkhole in Guatemala City at the world dot org.