Cdash
Structural
- Dec 27, 2002
- 2
I am assisting some local fire department personnel trying to wade through what is needed for trench rescue shoring. This would be shoring that the firefighters/rescue personnel assemble to go into a trench that most likely collapsed to try to rescue a victim. This would be short term shoring meant for a matter of hours to maybe a day (after more than a few hours, things will shift to recovery and change the urgency and the willingness to take risks). There isn't really much of a standard for this stuff and some of what is taught to rescuers in classes seems questionable at best.
My background is structural, so I am ok for half of the system. Most of the departments use 3/4" finform panels with a 10' long, 2x12 strongback installed down the center of the panel - this seems to be an offshoot from text in OSHA. They then place some struts, commonly pneumatic, between the trench walls. Paratech and Prospan seem to be pretty common. They are placed with 100 psi to 250 psi air pressure on a 2" or 3" diameter strut (say 1,000 to 2,000 pounds of force maximum pushing into the walls).
A statement that I keep running into is: "Shoring is designed to prevent soil from moving. No shoring can hold back the weight of collapsing earth." This is usually followed by the "soil arching effect" further into the piece I am reading. One manufacturer suggests that the pressure that the pneumatic strut places on the soil creates the arching effect and stabilizes the trench walls so that they don't begin to move. They further go on to say that in good and decent soils, the struts can be placed in the trench without any panels of any type, just a 8"x12" foot on the strut ends and the rest of the earth is completely unsupported. In poor soils they suggest panels with 100% contact with the trench wall which would seem impossible in a rescue situation. I also read commonly that the panels cannot be considered a structural member, although I don't understand why.
To me, shoring a trench for rescue seems somewhat similar to soldier pile with lagging in theory (i know there isn't embedment and it will behave differently), however, most guides say that even though there is soil arching between the soldier piles, there is still force (a reduced force) on the lagging between the piles. Caltrans Trenching and Shoring Manual says that this force is about 60% of the active earth and seems to max out at about 400psf. With this case, it would seem that any unsupported soil between the struts could still be at risk of collapse even if an "Arching Effect" was present.
What I am wondering, is there any sound information that would indicate what magnitude of force would be necessary to create soil arching in the walls of an excavated trench?
If so, how would you calculate how much force a strut would have to place on a trench wall in order to create the arch effect?
Even if the soil arches, is the soil between the struts adequately supported as to not have to protect it?
And lastly - even if the arching effect is reasonable, would you consider it safe to use struts without panels to enter the trench?
Thanks for any help!!
My background is structural, so I am ok for half of the system. Most of the departments use 3/4" finform panels with a 10' long, 2x12 strongback installed down the center of the panel - this seems to be an offshoot from text in OSHA. They then place some struts, commonly pneumatic, between the trench walls. Paratech and Prospan seem to be pretty common. They are placed with 100 psi to 250 psi air pressure on a 2" or 3" diameter strut (say 1,000 to 2,000 pounds of force maximum pushing into the walls).
A statement that I keep running into is: "Shoring is designed to prevent soil from moving. No shoring can hold back the weight of collapsing earth." This is usually followed by the "soil arching effect" further into the piece I am reading. One manufacturer suggests that the pressure that the pneumatic strut places on the soil creates the arching effect and stabilizes the trench walls so that they don't begin to move. They further go on to say that in good and decent soils, the struts can be placed in the trench without any panels of any type, just a 8"x12" foot on the strut ends and the rest of the earth is completely unsupported. In poor soils they suggest panels with 100% contact with the trench wall which would seem impossible in a rescue situation. I also read commonly that the panels cannot be considered a structural member, although I don't understand why.
To me, shoring a trench for rescue seems somewhat similar to soldier pile with lagging in theory (i know there isn't embedment and it will behave differently), however, most guides say that even though there is soil arching between the soldier piles, there is still force (a reduced force) on the lagging between the piles. Caltrans Trenching and Shoring Manual says that this force is about 60% of the active earth and seems to max out at about 400psf. With this case, it would seem that any unsupported soil between the struts could still be at risk of collapse even if an "Arching Effect" was present.
What I am wondering, is there any sound information that would indicate what magnitude of force would be necessary to create soil arching in the walls of an excavated trench?
If so, how would you calculate how much force a strut would have to place on a trench wall in order to create the arch effect?
Even if the soil arches, is the soil between the struts adequately supported as to not have to protect it?
And lastly - even if the arching effect is reasonable, would you consider it safe to use struts without panels to enter the trench?
Thanks for any help!!
