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.

Little Rain

The forecast for yesterday (Saturday) called for lots of rain but it didn't happen, at least not in Rio Dulce. We got a few light showers.

There are several tropical waves moving across the Caribbean and northern South America so there's always the possibility of thunderstorms but for the time being conditions are not right for the formation of tropical storms.