Tuesday, March 6, 2007

A rupture in the underground stormwater drain system opened a huge sinkhole on February 23, killing three people and bringing down twelve houses in Guatemala City.

Teenagers Irma and David Soyos and their father, 53-year old Domingo Soyos were killed when their house collapsed into the sinkhole. Nearly a thousand people were evacuated from the San Antonio neighborhood after the collapse.

Wikinews interviewed Eric Haddox, a civil engineer who has visited the site of the sinkhole and spoken to the engineers working on fixing the drain. Mr. Haddox, who specialises in the building of earthworks, roads, water supply and sewage systems, and is working as a missionary in Guatemala, visited the site following the collapse to help in the recovery effort.

Mr. Haddox told us that the size of the hole is much smaller than the 330 feet depth originally reported and that the erosion causing the collapse is believed to have happened over a long time, and not just during the recent rains as initially suspected.

There are also concerns that a four-story building less than a metre from the edge of the hole may collapse as the earth under the building continues to be eroded.

Before the collapse, a junction box linked two collector pipes to a 3.5m main pipe leading to a nearby canyon in a system believed to be 20 to 50 years old. The surrounding earth had been filled in artificially to level the ground, but the fill was not well compacted before being built upon. Such leveling of the ground is widespread in Guatemala city.

It is thought that, at some point in the last 20 years, either one of the collector pipes ruptured or was detached from the junction box, possibly because of seismic activity. Water gushing out of the break following rainstorms gradually eroded the loosely compacted soil, creating an expanding cavern around the junction box. On February 23, the roof of this cavern collapsed, creating the sinkhole, 20m wide at the top and tapering out towards the bottom, which is about 60m (204 feet) deep, not 330 feet as originally reported.

“Things like this don’t happen often and there are many interesting engineering lessons to be learned with them”, Mr. Haddox said.

The sinkhole has continued to expand even after the collapse, since the collector pipes continue to carry water, which cascades 15m down the sinkhole to the main pipe, further eroding the sides of the sinkhole. The hole was about 25m wide at the top and 40m wide at the bottom a week ago.

A bypass pipe is being laid to divert the water away from the junction to arrest further erosion. The sinkhole will then have to be drained before repair work can begin.

Authorities are also concerned that similar breakages and undermining may be happening at other locations, Mr. Haddox said. Muddy water has been seen coming out of the main collector pipes, but it is not certain whether this is due to ruptures elsewhere or simply mud from the surface that has been washed into the drainage system.