The collapse of a subway construction site in Sao Paulo, Brazil, shows the necessity of understanding the geological reality of where one is building something. All too often, escavations ignore the possibility of a collapse or signs of impending collapse are papered over or ignored. Many workers die because of this.
&hearts As usual, workers at an escavation are killed by careless or inept engineering or supervision.
Written by Francesco Neves
Friday, 12 January 2007At least seven people are disappeared and feared dead following a landslide this afternoon at the São Paulo subway's Yellow Line in Nações Unidas avenue, on the city's west side. A truck and a microbus were swallowed by the crater of about 30 meters (100 feet).
Over my own lifetime, I have done rather a bit of backhoe work, escavating various places. Since childhood, I have enjoyed digging up the earth, burrowing into things, and my earliest lesson in all this came when I was in elementary school in Arizona. We used to dig pits and tunnels in the desert. Near where I lived for a while, the Pantano Wash was one place we were forbidden to go alone. During much of the year, it was dry but in summer or if we had a wet winter, floods would roar through it and since it came out of the surrounding mountains, storms far away could cause sudden floods which is why it was very dangerous to play there. The banks of this dry river were fairly high and the sandy bottom was dangerous due to sink holes and other hidden hazards.
One day, three of my classmates decided to ignore the nearly universal warnings we were all given about the dangers of that enticing arroyo. They didn't think it was much fun digging in the hard caliche soil of the desert so they decided to make a big tunnel into the bank of the arroyo.
They used shovels and sticks to dig and over time, their tunnel grew in size. I warned them it was not a good idea to dig so deep but they thought I was stupid. One Saturday morning, they were busy digging deep inside when the tunnel collapsed. It killed two of the children, one was able to work his way out and run off to get help which came much too late.
I watched construction of the observatory complex at Kitt Peak, too. It also was an important learning experience. Unlike the sandy subsoils of the arroyos in the Tucson valley, Kitt Peak is a mostly granite mountain, the ancient stub of great mountain building millions of years ago. Unlike the volcanic mountains that also surround Tucson, this mountain was very hard and solid in nature which is why it was selected for the observatory projects by AURA.
Due to their height and being on the highest peaks of the mountain, the observatories had to be well-anchored and to do this, they blasted away with explosives to break up the rock and of course, deep holes were drilled and all this was quite thrilling to watch, as a child. We used to use firecrackers to imitate the drilling and blasting work the adults were doing.
Of course, letting children play with firecrackers is now illegal but back in the 1950's, it was rather normal, actually. I do know a number of people who blew off a finger or two, playing with firecrackers.
&hearts All pictures here courtesy of BBC
Click on picture to enlarge
The escavation work on this subway shows many signs of careless engineering as well as construction techniques. Indeed, looking at these pictures, I notice the quality of the soil: a very fine, even silt! Judging from photos while unable to touch it, the earth seems to be mostly loose volcanic dust or it is river deposits that took down from the Andes, such fine silt. Either way, it is not very integrated. When I excavate on my own mountainside, it being mostly glacial compressed earth, there are many large, loose rocks brought down from Canada, mostly quartz and if I dig down more than 50', I hit bedrock which is the roots of what was once a mountain range taller than the Himalayas around 500+ million years ago. Since I am anchoring things on this mountain, I have to build a base from these materials such as is around my present house. Compacting the loose soil while running drainage pipes along the mountainside, filling them with loose gravel which relieves water pressure on the subsoils, protects my foundation, for example.
Many builders, to save a few bucks, don't bother with much drainage or reconstituting the soil/rock mix. This is why people have wet basements or cracks in foundations, for example. This is true of large projects like the one in Brazil: builders take chances because they save money by ignoring construction dangers. And one of the top things that are skipped is escavation safety. I often say, 'Because no one sees what has been dug out, this is often the place where builders cheat.'
Hidden things are the things most often skipped over, in other words. In the case of the Brazillian tragedy, the fault lies with many responsible people, first and foremost, the architect. Looking at the walls in the photos, one can see how they go straight up. If one is building into solid rock like the subways in Manhattan, for example, large holes like the one in Brazil can be made and supported, going straight down, because it is solid rock. But in Manhattan, we also have very high walls, some over 100', lining the Henry Hudson Parkway.
Just recently, part of this collapsed onto the roadway. Hydrolic pressure behind the walls pushed them outwards. One can see this all over the place. In Troy, NY, where my family lives, there have been a number of catastrophic mountain collapses in the past. Right now, near the RPI campus, there is a vertical wall built 100 years ago that is now collapsing outwards, threatening traffic. Engineers had to berm the base with a dozen loads of large rock.
Medieval builders figured out, the higher the walls, the more they needed support and besides the built-in piers, they designed flying butresses. Medieval castle builders understood the base of a curtain wall had to be very wide in order to support the towers and stone structures above. This seemingly simple idea is ignored these days as people imagine cement and rebars are suffient within themselves.
&hearts I have had huge OSHA fights with previous bosses over the years.
Excavation and trenching are among the most hazardous construction operations, and the weight of soil sloughing off the side of an unprotected trench can cause a crushing blow to an employee at the bottom, resulting in a serious injury or fatality. During 2000 alone, 38 construction workers died in excavation and trench cave-ins, according to the Bureau of Labor Statistics’ “Census of Fatal Occupational Injuries.” Last fall, for example, a 31-year-old Illinois man died after a piece of a trench wall broke loose and fell on him while he was working in a five-foot-deep trench. In Arizona, a 33-year-old man stepped out of a stacked trench box system when the trench collapsed, suffocating him. In Missouri, a worker was laying pipe in a 12-foot-deep trench with no shoring when the trench wall collapsed, burying him. Four hours later, rescuers finally were able to recover his body.According to Jim Boom, an occupational safety and health specialist in OSHA’s Directorate of Construction, most of these fatalities would have never have occurred if the people responsible for providing a safe workplace had complied with well-known safety requirements for trenches and excavations. OSHA’s excavation standard requires employers to provide sloping, benching, shoring, or shielding to protect employees in excavations five feet or deeper. The only exception is for a trench dug in stable rock where there is no loose soil or likelihood of a cave-in.
I have often said in the past (to bosses), "OSHA rules were written in blood." This truism is self-evident. The government writes rules only after enough people die that it overcomes resistance from employers who want to save money while risking lives. Even with many deaths, though, if an industry is powerful enough, politically like the oil or coal corporations, the deaths continue and the protections remain slim as we saw this week with yet another coal mining collapse that killed some miners.
If a trench has sloping sides, the danger is much less, the more vertical the sides, the more dangerous. This is due to gravity and pressure from the sides of the trench. Namely, Nature hates a vacuum and loves to fill them, swiftly if possible. Looking at the photos of this latest disaster, I noticed a big black ground-cloth lying on the road above. It has a lot of big tires lying on top of it. How peculiar!
Down below, next to the big rig lying on its side, are even more tires! I believe the foreman or someone above him saw the fissure growing at that spot and decided to cover it up so water wouldn't get in as if such a solution would stop water from getting in! Anyway, the people ordering this were not very bright. They probably left an opening for trucks to drive through and probably that big rig in the pit was driving over this spot when it finally turned into an avalanche.
When working with fine soil like this, what responsible people do is excavate a very large pit with sides sloping backwards into the hillside or making the hole like a cone, then pouring the cement and then backfilling it with fairly uniform rock fill. I did this with my house, for example, as well as other projects in the past. The expense is well worth it since the buildings or walls last much longer. I see collapsing retaining walls all over the place because people hate spending money on things one can't see. And nothing is more hidden than good drainage, good backfill.
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