The sinkhole washout in Guatamala City - a note from Naples



Posted by Fulvio Feb. 26, 2007

A couple of days ago we received news of a problem in Guatamala City in Central America where there had been a huge, deep sinkhole in the middle of a town seen in the photo above. Even I was stupefied to see the size of the gigantic crater. Our original article posted yesterday noted that the sinkhole was absolutely enormous, 30 meters in diameter and 100 meters deep and that it pulled in two, possibly three people, killing them and a number of houses and buildings were lost. The local authorities attributed the cave-in to a collapse in their sewer mains, noting the powerful smell of sewer gas that issued from the enormous cavity. So, if this, is indeed the fact, we have to ask several questions. First, given the dimensions of this cave-in and the volume of earth that was eroded and carried away, easily more than 70,000 cubic meters of soil and rock based just upon the cylindrical size of the hole, and noting that the hole was actually larger as the washout deepened, we have to ask if the sewer line was large enough to also carry off that volume of material.
The second question deals with the fact that the hole was at least 100 meters deep. So if this is the case, then we ask how deep beneath the surface the sewer line that is said to have caused the huge washout was located. If it was located closer to the surface than 100 meters, and in my opinion it probably surely was,  then the sewer line could not have been the sole cause of the enormous size of the cave-in because the washout would have been only a few meters in size given the rate of liquid flowing in a sewer main.
So, it seems that the sewer line rupture may have been the cause of the initial erosion, but that alone wouldn't account for the size of the gigantic hole that resulted. The big question that remains is, "Where did all that earth and rock go? "

Here in Naples, in the past, we have verified many such events like this also caused by broken sewer lines and water mains but the resulting washouts and cave-ins have been in proportion to the size and flow in the mains themselves. But if the washout reached huge proportions in both diameter and depth, we have always found that there was a pre-existing cavern beneath the surface that allowed the formation of the extreme sinkhole or cave-in allowing the eroded earth to continue to wash out and flow into the cavern below.

The photos from the Guatamala City cave-in are startlingly like those we have taken of similar gigantic cave-in's and sinkholes here in Naples. Like many of you already know, Naples sits above a honeycomb of quarried cavities, ancient aqueducts, passageways and other voids that have been down there for centuries. A conservative estimate is more than 1,000,000 cubic meters of artificial cavities runs beneath the city. These cavities have ceilings of yellow tuff sandstone and the surface level has a varying strata of volcanic pozzolane. Once erosion begins to wash out this material following a sewer or water main rupture the washout doesn't progress much beyond 10 meters or so in ordinary circumstances. However if the ceiling of one of the artificial cavities washed through then we see photos exactly like those in the Guatemala disaster. Compare the picture of the Guatemala sinkhole with those photos taken by our friend Enzo di Luzio here in Naples. These are huge washout sinkholes that have resulted from erosion into the roof of gigantic cavities beneath the city . . . then draw your own conclusions!     

Translated by Larry Ray



by Napoli Underground


Well-known member
Here's Tony's more considered view.

Tony Waltham said:
Sinkhole in Guatemala, 2007

The site is in the San Antonio barrio north of Guatemala City. It collapsed just before 2.00am in the early hours of February 23, 2007, killing three people and destroying one street and five buildings. The collapse lies within a basin of weak Quaternary volcanic deposits, mainly pumice and ash, which are very weak, crumbly and easily eroded, yet, like loess, they have a degree of cohesion that allows them to stand in very steep to vertical cliffs, and also allows subterranean chambers to be opened up in them.

The sinkhole is a little over 20m in diameter at the surface. It may be about 50m deep (not the 100m of some press reports). Its walls are vertical through about 10m depth, to a major overhang on one side, with vertical walls visible for about another 5m on the other sides. Retreat and degradation of the sinkhole walls have not been reported.

The sinkhole is within 100 m of the head of a steep-walled, fluvial canyon (known as a barranca) entrenched about 30m deep into the volcanic sediment plain. Another canyon, twice as deep, lies 250m away on the other side.

Collapse of the sinkhole was a classic dropout, and was triggered by a major sewer failure after a period of heavy rains. Residents of the area report that they had hear noises underground for a month prior to the final breakthrough to the surface. Storm-water and/or sewage continue to flow through the base of the sinkhole. Over 200 people have been evacuated from houses within 100m of the collapse.

It appears that the cave and collapse constitute a classic example of a large piping failure. In such an event, the insoluble sediment is progressively washed out through its own pore spaces by seeping water; first the fines are removed; this increases the porosity along the optimum flow route, so that progressively coarser material is removed; eventually a large open pipe is formed and rapid underground erosion then progresses out of control. Large piping failures and collapses are well known in the loess lands of China (and also caused the 1976 failure of the Teton Falls Dam is Idaho). Piping failures expand from the bottom upwards, so a large cave could have progressively developed under San Antonio, until its unsupported roof failed instantly.

The question that remains concerns the destination of the pipe and the large amount of failed material that has gone from the now-open sinkhole. The pipe could resurge in a nearby barranca canyon, but nothing has been reported. It could have fed out to the base-level Rio Las Vacas, somewhat further away and about 80m down, via a pipe along the base of the weak ashes. It could feed down into karstic caves within Cretaceous limestone in a faulted block, between older lavas, beneath the soft ashes; limestones are known nearby, but the geology at depth below San Antonio remains uncertain. Finally, there could be old mines in the area, comparable to many others known in the vicinity.

This note has been compiled largely from the words of Ric Finch, a retired geology professor and a member of NSS familiar with Guatemala.

Tony Waltham, March 6 2007


Tony, thank you for the fascinating information and the professor's detailed explanation of the geological dynamics in this area. His analysis definitely confirms Fulvio's insistance that there had to be a cavity beneath the huge sinkhole. And his possible explanations about where the washed out material could have gone are most interesting. I translated his article for Fulvio and know he will appreciate your efforts in contributing to the explanation of this cave-in.

All the very best, Larry Ray, translator for