So I found this video online and had to share it. I think it's a great video to stick in a presentation for anyone teaching a trench program. I also posted it to get everyones take on what they think happened. My thoughts are 1. The strut located at the corner of the "T" (angled strut) didn't have the collar locked and was over extended in my opinion. As for the other struts perhaps they weren't shot at the correct PSI and not plumb.
Without a doubt a great video to include in any trench training be it awareness, ops or tech.
I do agree the angled struts seem to be over extended and I wonder if poor length calculations led to using a strut to short or if they decided that a short distance over maximum recommeded length was acceptable. Either way, if this wasnt intentional, its a very expensive lesson learned.
Watching it again you can hear one guy say 200...205. Thats about 1000 lbs of force being applied when the strut fails.
Good catch Up&Over... I watched that video a dozen times and didn't catch that. I'd hate to think that was intentional and I think I hate more that such vital things were missed to make that happen. I've done about 20 training sessions in "live" trenches and 2 actual trench operations and never had anything even close to that happen. I so curious to hear more feedback on this post.
The inside corner of an L trench certainly can be a challenge. A key point to remember in a trench is the sheeting and shoring system is designed to hold back a failire; not push back the trench wall.
The failure of this trench occures because the force of the corner struts exceeds the force /friction of the sidewall struts. I don't think it has anything to do with overextension of the corner struts. It has everything with too much pressure. Typically, you should only pressurize the corner struts at about 1/2 - 3/4 the pressure of the sidewall struts.
Great video and a great lesson.. Imagine if guys were working in the trench at the time failure. This could have been a fatal training accident.
Let me also say, not knowing the context of the video, this could have been an intentional failure. I have my doubts because the cameraman is exited he caught it on video, but who knows.
Great catch Jeff...Watching that video yet again I now caught the corner strut essentially pushing the panel and throwing the other struts. Also great point regarding the the reduced pressure you'll be shooting the corner struts at. Being that is the weakest point of the trench over pressure will certainly cause big problems. I'm amazed nobody was in the trench at the time of this event. It doesn't look like they were simulating a rescue otherwise there would have been guys in the trench and it would not have been a good scene.
They experienced a "dynamic" event. Hopefully they took a lesson from it. Trying to figure out the system failure; obviously over pressuring the diaganol struts. Had a hard time here on my iphone seeing, I could not tell. Also, had questioned if all the sidewall struts were pressured up to 200 and locked off?
Again, looking at a smartphone, but at 4 seconds in does anyone else see the top far interior thrust block and sidewall strut move? Right as the guy says "see it moving"? I think the operation needed to be reassessed at that point.
If you are installing “Thrust Block” shoring systems the way the one in this video is being used. STOP!
If you are instructing “Thrust Block” shoring systems the way this one has been installed. STOP!
This system will not come close to holding back the forces of any active soil. I have conducted several tests on this system and have failed it at very low forces each time. The problem is found neither with the strut pressure setting of the struts that are perpendicular to the wall nor at the collars of the diagonal struts. The problem is that this system does not provide adequate lateral resistance to the forces created by loading of the diagonal struts. In the video a complete system failure is reached by simply increasing the air pressure on the diagonal struts. The fact that the system could not resist the mere force of the strut air pressure should be seen as a CLUE. Any shoring system that can not stand up to the forces created by normal air pressure could not possibly resist the forces of active soil. The air pressure (commonly set at 250 psi) on a Paratech strut creates less than 2,000 pounds of force. The force created by active soil on a section of wall (the size of the one seen in the video) could be many times that force. I have seen, several, ways to resolve the (vector) forces created in this system. Some are adequate while others are not. In many cases, this shoring system is being taught and is being installed in a manner that is dangerous to anyone in the trench.
The video depicts a snap shot view of what is a much bigger problem. The video simply shows the failure of an improperly installed shoring system. The bigger picture is that improperly installed and designed trench rescue shoring systems are being used by firefighters all across this country. As is the case in this video…many trench rescue shoring systems are substantially weaker than their individual component parts. Unlike our rope rescue and building collapse shoring equipment, trench rescue shoring has no national standard and no testing mechanism in place. This fact should be a BIG concern to the fire service but it remains our hidden secret.
In Michigan we have been conducting destructive tests on trench rescue equipment and system designs for a number of years. We do this through the MUSAR Training Foundation in cooperation with the International Union of Operating Engineers (Local #324) and Michigan State University. We have started a blog which will publish some of the test results and system designs. We hope to dispel some myths and generate some discussion. To see another shoring failure the go to www.musartf.org, click on blog and click on trench then scroll down to Excavation Shoring. In that blog you will find a Youtube link. Future blogs will bring videos of wale and trench panel failures.
I do not consider myself to be a trench shoring expert. Instead, I consider myself to be a student of trench rescue. Over the past twenty plus years I have shored more than 500 live trenches and have responded to more than 20 trench emergencies. I feel that these experiences provide a basis for learning. I continue to work with structural and soil engineers in a pursuit of trench rescue safety and efficiency.
If a shoring system fails under normal air (strut) pressure it should be considered a "psuedo" shore. A psuedo shore will not hold back the potential force of the soil (lateral force per square foot x depth x tributary area). What you are doing by only pressurizing the diagonal struts to 1/2-3/4 pressure of the sidewall struts is simply allowing the psuedo system to stay in place. As the soil on the outside wall becomes active (surcharge/vibrations from rescuers on the lip, drying of the soil and loss of cohesion and adhesion during the rescue operation etc.) it will create forces that this system can not possible resist.
Ron, I'll be brave and take the bait. How then do you shore an "L" shaped trench? Not questioning your research, but would like to know the better solution. The shown method looks like the system straight out of Buddy Martinette's text, I first learned that system years ago at Mason Dixon weekend, and just last year did it in a TEEX course for refresher training. What is your alternative?
The method that failed in the video relies on the friction created from the strut pressure (perpendicular to the side wall) on the inside corner panels. As the video shows that resistant force often cannot withstand the vector force from the diagonal strut air pressure. Soil pressure (which will be much gretaer than the strut pressure) from the outside wall which is transferred to the diagonal strut(s) causes the same type of catastrophic system failure. The lateral resistance needed must be more than the potential force of the soil pushing on the diagonal struts. I have not asked Buddy Martinette how he resolves that force but I did discuss this with Jon Rigolo (Virginia Beach Fire Dept.) who is Buddy's former partner in Spec Rescue. Jon agrees that the corner panels must have a mechanism added for proper resistance. He uses a technique similar to #2 described below. There are a couple of alternatives that I have tested. I'll try to explain both but to learn how to install them properly takes a minimum of a day of instruction.
1. Corner Brackets- Our structural engineer designed corner brackets which we have fabricated from aluminum. Go to www.musartf.org click on Course Descriptions then on Trench rescue Technician for a look at the Corner Brackets in a 10’ deep L-trench. We keep a set of panel together on the trench rescue rig which have pre-drilled holes (at one foot increments) for the corner brackets. Once you determine the depth of the trench and the vertical spacing for the struts you bolt the corner brackets at appropriate distances to the Finnform panels. It takes a couple of minutes to bolt these on but this method typically speeds up the overall shoring process by fifteen to twenty minutes. As you can see from the picture on the MUSAR website, the two bolted panels are slid on rails and stood up on the inside corner as one unit. This is a simple and easy process that saves huge amounts of time throughout the development of the shoring system. The panels stay together so you don’t have to line up one thrust block with the other and then readjust it to the level of the wales on the opposing walls. Our structural engineer feels that one diagonal strut does a better job of resolving the forces in both directions. If you are using air bags, backfill or back-shores to fill the void created by the inside corner cave-in (see photo with airbag) the bracketed corner panels create a one piece unit which does not separate with pressure from behind. All struts can be pressured to 250 psi and locked from outside the trench. We typically have an 8’ deep L-trench completely shored in less than one hour. I have tested this system to more than 25,000 t pounds of force on the diagonal strut and have yet to fail the system.
2. Trucker Ratchet Straps- Another method used to resist the vectors on the diagonal struts is done with 5 ton trucker tie down straps. After the system is in place and the diagonal struts have been shot at 100-150 psi the top of the corner panels can be secured by looping the ratchet strap around the strong-backs and ratcheting them tight. To secure the bottom of the corner panels two rescuers have to enter the trench with shovels and a ratchet strap. The rescuers have to work the strap around the strong-backs at the bottom and tighten that strap. The diagonals can then be brought up to full pressures. I have tested this system up to 20,000 pounds of force and have not failed it, however, as the force on the load cell on the diagonal strut reaches about 5,000 pounds the straps settle and stretch cause some lateral movement (a couple of inches) of the (perpendicular) struts and the sidewalls. Once the straps are settled and stretched the movement stops. This movement and the fact that rescuers have to enter the trench before the shoring is complete are the down sides to this method.
Trying to explain these techniques (in writing) is a lot harder than showing you how to do them. I hope that this is enough to give you an idea of how to resolve the vectors which can fail the L-trench shoring system as shown in the Rescue 2 video. As I mentioned in my first post, untested trench rescue shoring systems are being used and taught on a daily basis. Without a national Trench Rescue Shoring Standard that is based on quantitative (destructive) testing... firefighters will continue to “make stuff up”.
For a more details contact me by e-mail at email@example.com
Great info. I checked out the corner brackets and like that idea. So i'm on the same page the panels hinge out and are simply dropped in place. The force from the diagonal strut is directed first at the corner (of course) then it terminates at either upright. I like that the brackets also keep direct force from hitting the corner.
Paratech recommended we shot are struts at 210 PSI rather than 250 PSI. They stated 210 PSI is sufficient. Any thoughts on that?
PS. Those panels must be a bitch to carry weighing in at over 400 lbs. together. "Rookie Job"
There has only been one study done on the effect of strut pressure on trench rescue shoring systems. It was conducted by Dr. LaBaw (2009) and the Montgomery County (Maryland) Fire Department. The study concludes that there can be too much pressure or too little pressure on struts (go figure). There does not seem to be much difference with pneumatic struts like the Paratech (Gray) when they are shot between 200 and 250 psi regardless of soil type. We have found that struts shot at the higher pressures actually have a positive effect on the strength of the strong-backs and Finnform panels. Our structural engineer can explain the physics behind that but I can't. If Paratech says to shoot them at 210 panels that seems reasonable to me.
We bring the panels to the trench lip separately and bolt them together there. That way you only have to tip them in and not carry them. I have not weighed a 4'x8' Finnform with a strong-back attached but I have lifted and carried a bunch of them. I'd be surprised if they weigh much over a hundred pounds each. I don't have any problem handling the two bolted panels and I an old guy.
Much like Trenchdog said there not very heavy (panels) and once you get the corner braket together it becomes a two maybe 3 man job with one extra excepting the corner braket.
Mike perhaps a thread on "Spot shoring" speaking of strut pressures???
What if you nail the thrust blocks in place to the whaler?
I'm a little confused by your question. It might just be a matter of terminology. To me...thrust blocks go on the strong-backs and corner blocks go on the whalers.
Give me a shout on my email firstname.lastname@example.org to discuss this some more. There was an email address typo on the previous post.
Now I feel like a wimp...Our are 1" inform with standard 2x12 uprights (strong backs ) and feel far more than 100 lbs. Any idea where I can get a PDF copy of Dr. LaBaw's study? I see you can purchase the book however I'd like to get a copy on PDF.
I don't mind airing my dirty laundry here, maybe everyone can learn something form the discussion... We have used the same cut for both corner and thrust block. If you take a thrust block (defined to you as on the block on the inside corners of an "L"-shaped trench, straddling the strongback) and flip it over, it becomes our corner blocks (that capture the corner of the whalers on the outsdie of the "L"-shaped trench). The 45 degree cut is the same, it just has a notch where a strongback would fit if it was used on the inside of the "L". Diagnoal struts hit it the same way if it has the notch or not.
Originally Posted by TrenchDog
So basically, we use a thrust block on the inside of the "L" and a thrust block turned backwards on the oustide of the "L" in place of a corner block. My bad for letting our methods dictate the terminology of the discussion.
What I suppose makes more sence to you would be; "What if you nail the corner blocks in place to the whaler?"
I didn't want to get too picky with terms but just wanted to get an understanding of your question. I think that nailing the corner block to the whalers can a bit of stability to the system if everything else is done right. What are you trying to accomplish with it?
Also Ron, your discription of the two methods you would use are very good and detailed. You worried about being able to convey your message, but between the picture on the MUSAR website and the discription, I think I got it. Any idea when this method will start to trickle down into training, I really like the single diagnoal strut method hitting the corner... problem is in a real world trench rescue envolving a patient inside the corner of the "L" the inside corner of the "L" is the section of dirt that is probably covering him. The method looks real good when shoring a "L" corner that is still intact, but I would find it hard to produce in a collapsed situation. Your thoughts?
I also emailed you, for I am a self admitted Trench Nerd.