Talk:Broken stormwater drain led to Guatemala sinkhole
Excerpts from e-mails
Dated 06-Mar-2007 01:56
I have been around and may be helping out the director of public works in the solution for this problem. The first thing that was needed was to stop the flow of water so we can assess the situation. I can give you good firsthand information being that I was there and have talked with all the engineers involved and am kind of involved myself.
The hole is the product of a broken stormdrain pipe. It was at the location where two main collector pipes joined together in a junction box to the main pipe (3.5m diameter) leading the water to the nearby canyon. There are differing opinions as to exactly how the problem began but all agree that sometime in the past 20 years probably due to seismic activity, either there was a small collapse of one of the pipes near the junction box or one of the collector pipes separated from the box. This began the washing of soil down into the pipe and creating a small cavern. This cavern grew with each rainstorm increasing the failure.
This area of the city was built on a lot of fill material that was not compacted well so the soil is quite soft. This aided in the loss of material as well as the final failure itself. I observed there to be about 15-20' of material that was still in tact over the cavern before the failure. When the sinkhole opened up what happened was that the cavern was so large and the majority of the cavern was on one side creating pressure on the box from the soil that still remained on the other side.The register (manhole) for the box buckled and the top 15' of dirt I mentioned before simply failed in shear and came straight down. When it opened up the top of the hole was about 20m wide at the top and tapered to a wider base. The hole was and still is about 60m deep not 330 feet. The reason we know this is because with the video feed we could see that the bottom of the hole was right at the flow line elevation of the pipe leaving the hole which was built 60m down.
The hole has grown. I have not been there in approx 1 week but a week ago the top of the hole had opened to about 25m and the base was about 40m in an oval shape. What was causing this was that the two collector pipes entering the original box entered the box about 15m higher than the pipe leaving so although the main pipe is 60m down at the bottom of the hole, the two collector pipes are only 45m down. The problem being that the pipes were still flowing at about 25% capacity and cascading the water 15m down into a small pool that it had created and the water was circulating against the side of the cavern further eroding the side and growing the hole. As the hole grew the opening at the top grew even faster to make up for the difference between the width at the top and the bottom. Another main factor to this is the proximity of a 4 story building that as of a week ago was hanging about 1m over the hole which means that the building itself is significantly over the base of the hole. The mayor and director of public works are looking to contingencies if the whole building falls into the hole. You can see how the hole has grown if you compare the aerial photos that show the hole going just to the other side of the road and the later photos with me that show that it has eaten the wall and tree that were in the previous photo. The 4 story building is the one seen in the background on the left of the picture that I am in. The construction photos are of the city laying a bypass pipe to take all the local sewer flows around the hole and try to dry it out once the flow in the collectors is stopped. I know this is probably more info than you were looking for but I did want to set a few things straight. The main things being the depth of the hole as well as the blame on recent rains. It is the end of the rainy season here and has not rained for several months! I guess information was scarce. I even found differing stories at the site amongst the differing workers until I spoke with the engineers who actually knew.
Dated 06-Mar-2007 21:37
WN: Is there any concern that similar ruptures and undermining may have happened elsewhere in the City and is there any effort to locate possible ruptures?
EH: I do know that this concern has been raised and that they are keeping it in the back of their heads on how to find and remedy this kind of hole in the future prior to becoming a disaster but for now I am only aware of the focus of energies on the current problem. Some of the engineers believe there to be similar problems around the city because the water coming out of these main collector pipes is "chocolate" meaning that it is murky with mud which hints to another location where earth is being washed out. It is not a definite indicator because much of the poorer sections of Guatemala City still have slopes of uncovered dirt (meaning no concrete or grass) which promotes heavy erosion. Often times after a heavy rain you will see "rivers" of brown water flowing down the street from the individual dirt residences. What makes the brown water so suspect right now is that there have not been any heavy rains recently and yet the water is still brown. This may be caused from collections of soil in the storm system that was deposited during rains and just now is being washed out or it may be from other failures in the system itself.
WN: Can you characterize the civil planning and regulation in the area, from what you've seen and learned from officials/engineers at the site - were the pipelines designed and built with good safety guidelines? ditto the land-fill. was the land-fill okayed for building upon?
EH: If you look at the city on Google Earth, especially with the terrain set to 3x exagerration, you will notice that the city itself is quite flat but there are significant canyons running as fingers throughout the city. The reason for this is that the original terrain of the city was not very flat and so there has been, over the years, LOTS of material moved in cutting hills and filling valleys. One must remember that the sinkhole happened in one of the older sections of the city. I believe their "municipal" regulations to be pretty good as far as third world regulations go. (This does not speak to the construction of individuals but rather the design process of the government itself. Individual construction has virtually no regulations and I would not classify most of their construction "safe") I have been told that this section of pipe was constructed somewhere between 20-50 years ago, depending on who I asked. Those involved would admit that the pipeline was not built with good safety guidelines nor could they be expected to have been built as such because of the state of the nation when the pipeline was built. I don't know all the surrounding circumstances regarding the permitting or safety process when it was constructed but if you notice the location of the failure you will see that it is very close to a canyon. This tells us that by nature it was placed in an area of deep fill because if you cut down a hill you must then put the dirt to the side of the hill and leven the cut hill the the area of fill and in essence you get twice the area. (I don't know if I explained that well, sorry) as far as planning goes, you can also see from Google earth that the roads are very grid oriented and then as they move to the canyon the grids get distorted. I believe the growth pattern (how it was to grow in a grid instead of random roads) was established but zoning was not planned resulting in a haphazard expansion. This also can be explained by the lack of zoning laws or construction regulations at the time. The one thing that con be said is that there was enough foresight to put in such a large pipe. I cannot speak to the correct use or sizing of the pipe because I have neither done nor read any hydrology study of the rain load of the area and the correct sizing of the pipe. I would generalize that as Guatemala became more of a central economic hub for central america, its municipal engineering increased as well just as you would expect for any developing nation. The main problem is that this pipe was probably designed and installed prior to this push for engineering excellence and most likely during a tumultuous time during the 36yr long civil war.
WN: Tell us a little about yourself ... and how you were involved in the recovery effort at the sinkhole.
EH: I am a registered professional civil engineer [...] My specialty there was in Land Development, meaning, Earthwork (cut/fill), watersheds, roads, and wet utilities (water, storm, sewer). I currently am a missionary [...] I actually only got involved in the sinkhole because I had heard of it and visited the site to see what happened and hopefully talk to one of the engineers in charge to learn what happened and make a contact to see if I could help or at least be able to read their reports. (things like this don't happen often and there are many interesting engineering lessons to be learned with them). I don't know what my future role with the reconstruction efforts will be. The City has many capable engineers. More than likely I will be an "expert friend" that will visit time and again to be a soundboard for ideas. Either way this process will be a long one.
Where in Guatemala city did this take place? Even just zone number would be nice.
22.214.171.124 17:11, 26 April 2007 (UTC) Ahnteis
Barrio San Antonio, Zona 6. 126.96.36.199 06:26, 20 May 2007 (UTC)