Sunday, June 20, 2010

The Big Sinkhole in Guatemala City

It made world news when a giant sinkhole opened up in Guatemala City during Tropical Storm Agatha. It swallowed a multi-storey apartment building and several homes. This spectacular photo of the hole circulated for days on the Internet:

Click the photo to see larger and much larger versions.

There has been a lot of discussion since over what may have caused it. Geologists and engineers have been studying the collapse and several theories have been put forward. The common thread is that an underground flow of water eroded the soil until a huge cavern was hollowed out and finally the surface collapsed into the hole. The questions then were was this natural or man-caused? Can it happen elsewhere? Are other caverns like this likely to exist in other parts of Guatemala City?

When all the facts are taken together, the root cause is clear, and it's something that the engineers have been warning the city about for 20 years. From 1975 to 1985 the Guatemala City rebuilt and modernized its storm drain and sewer system. From 1986 on, the system was largely ignored and not maintained. Even worse, due to lack of controls and building codes, people built right over the top of some of the facilities, sealing up manhole covers and building over them, sealing up gas vents, filling and building over overflow basins.

For years, the engineers who designed and built the system have been complaining and trying to raise the alarm but it's fallen on deaf ears. Residents have noted cases where during heavy rains, water will erupt from manholes, blowing the covers completely off. This is partly due to the overflow basins not functioning as designed. Hissing noises and foul smelling gases are noted coming out of manhole and storm drain gratings. This is caused by the gas vents being sealed off and the gases underground building up considerable pressure. Residents have noted odd vibrations and occasional loud rumblings in the earth. The loud rumblings are the sound of soil collapse as caverns like the one in the photo open up underground but the surface has not yet collapsed. The warning signs have been there for a long time.

These caverns can occur wherever there is an uncontained flow of water underground. The lack of maintenance and vandalism to the system mentioned above causes the storm drain system to operate at higher pressures than it was designed for. Manhole covers being blown off is a clue. This greatly increases the chance of an underground leak or completely broken pipes or galleries. If there is a leak, the flow will gradually erode the soil. If a pipe is broken, the amount of soil removed can be very large. Where does the soil go? Often it goes back into the pipe. Storm drains are not under continuous pressure, the pressure varies, often to zero, so water can flow out of a leak and then back into the same pipe or gallery, carrying away the soil. Over a period of years the amount of soil carried away can produce the kind of sinkhole shown in the photo above.

So there's really no mystery to this. This failure mechanism is nothing new. It's been known to engineers since Roman times. The bad news is that it's a virtual certainty there are more caverns waiting to collapse, and more forming each time it rains. Fixing it pretty much means rebuilding the system--again, and then maintaining it.


catherine todd said...

Excellent article and explanation. Why is it no one listens "until the roof literally falls in on their heads?" And even then, what do they do? Stand outside in the rain and complain it's raining, when they created the weather! (i.e. they created the problems).

ShutterSparks / KW2P said...

It's just human nature. The only reason, and I mean the ONLY reason, things like this happen fairly rarely in the USA and Europe is that public officials, engineers, and other responsible parties are held responsible and held accountable by the legal system. Something like this would result in brutally expensive lawsuits and probably criminal charges and jail time. Here in Guatemala, the legal system does not function at all so politicians don't care. Nobody is held accountable.

greenman-23 said...

couple of issues...that trouble me...

1st is the claim above that a sewer pipe was sunk 30m deep? 30m..? who on earth sinks a sewer 30m (100ft!) deep. Highly questionable claim.

2nd sinkholes formed in this manner are usually kettle shaped... the cavern dug by the water being much larger than the hole created by the eventual collapse above.. there are exceptions but such exceptions occur where a sinkhole cavern has formed beneath fractures in rock.. I don't know the geology of Guatamala but from the photo the substrate looks clay like..

so not so clear cut at all...

ShutterSparks / KW2P said...

Well, there's no question about how it formed. That was clear before the news came out confirming that there was a collapsed storm drain at the bottom of the pit. The depth of the storm drain struck me as extreme too but that's what the diagrams I've seen show, 30 meters. The soil is mostly clay throughout Guatemala (karst / decomposed limestone) but I don't know what the composition of the soil is at the site of the sinkhole. There are volcanoes all around there so surely there's plenty of volcanic ash in the soil, not straight clay, but I don't have details.

The material I've read implies that this sinkhole formed rather quickly, but it's hard to know if that's being said for political reasons or if it's the truth. If it did form quickly, it might explain the the cylindrical shape. Also bear in mind that during the storm, the storm drains were overflowing, blowing off manhole covers, etc. so this cavity would have been completely filled to the top with water and even pressurized slightly before the collapse occurred.

ShutterSparks / KW2P said...

To shed more light on this I inquired about the soil composition where this collapse occurred. I was informed that the surficial composition here consists of one to two meters of topsoil which is a clay and sand mixture. Below that is a 200 meter deep layer of loosely compacted pumice sand (volcanic ash) which erodes very quickly in running water.

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