Excavation Shoring

EXCAVATION RESCUE

You are dispatched to a trench cave in with workers trapped.  As you respond, you begin a mental review of the things you learned during your trench rescue training.  Position the fire/rescue apparatus safely back from the trench.  Perform a size up.  Control hazards and people around the site.  Provide lip protection and stabilize the trench walls.  Coordinate panel team and shoring team operations.  Enter the trench, treat and extricate the patient when the benefits and risks are balanced.

     Good, it’s all coming back.  When you step off the truck you see a hole in the ground that is 12 feet deep, 24 feet wide and 40 feet long.  This is an “excavation” rescue and the shoring techniques that you learned in trench rescue school are of no use.  Now what?

HISTORY

The first time this happened to me was just over fifteen years ago when our trench rescue team was sent out on a county mutual aid request.  Wanting to help our mutual aid partners, we improvised a shoring system based on our knowledge of both trench and building collapse shoring techniques.  The walls did not collapse as we performed a recovery of a deceased underground construction worker so what we did must have been correct.   Or was it?
A few years later I responded to assist a different community perform a rescue at another excavation site.  The local fire department had no trench shoring capability and if they had, they wouldn’t have been able to stabilize the excavation walls, which were more than twenty feet apart, using typical trench shoring methods.  This incident prompted me to start experimenting with the use of raker shores at excavations.  We built rakers on the excavation lip and lowered them into the excavation.  We used the design and anchoring techniques for structural collapse shoring.   After applying pressures to the experimental rakers, I found that the typical wooden raker shores used in structural collapse were not capable of restraining pressures that would be common to excavations in soil.  The biggest difference was that the greatest forces in a leaning structural wall are at the top of the wall.  With a trench or excavation the pressures are greatest at the bottom third of the wall.  The design of a typical structural raker system is not intended to resolve those forces.

SYSTEM DESIGN
Despite the set back from the wood raker tests, we were determined to develop a working excavation shoring system.  About six years ago, we started to experiment with pneumatic raker (Paratech) systems in excavations.   In our initial tests on these systems, the weak point was the shore climbing the wall as lateral pressures increased.  It took several attempts but we have now solved that problem by bolting the aluminum Paratech wales to the Finnform panels.   We use high shear strength 1/2″ bolts that thread into T-nuts which are pre-drilled and set into the panel for quick installation.  The panels, wales and struts are then secured (pinned) into the excavation wall with pickets (1” diameter  by 36” long).   We have inch and a quarter diameter holes pre-drilled into the panels for the pickets.  These pickets are spaced across the panels to provide room for swinging sledge hammers.
With the wall climbing issue resolved, our next set of tests on the pneumatic rakers showed a failure point at the base which holds the “raker junction”.   This is the point where the upper and the lower struts come together and meet in an aluminum swivel base at the ground anchor.  Our testing resulted in large forces on the lower shore and this caused repeated failures of the base connector. (you can see one of these failures at http://www.youtube.com/watch?v=glo3hZl1yVQ   Please note that we have replaced the Paratech wall plate (seen in this video) with Paratech Wales.
Currently we have developed equipment (angled “Z” base at the raker junction) and techniques that have tested the “8×8 Excavation Raker”(8′ high x 8′ wide) to withstand forces of more than 18,000 pounds  of  lateral force.  To date we have not been able to cause a failure of this system.

SYSTEM TESTING
The MUSAR Training Foundation does destructive testing on all of the trench shoring  systems that we teach.  We commonly load the wall behind the shores by “back trenching” with a narrow bucket four feet behind the trench or excavation wall.  Then we insert large air bags into the “back trench”.  As the bags inflate they exert force in the “back trench”, pushing to earth towards the trench/excavation shoring system.  With load cells on all struts in the system we can record the forces that the shores resist during testing and compare them to calculated C-60 soil loads.  We use C-60 soil pressures as the testing standard.  We have also tested shoring systems that are being taught by so called “reputable” training companies and institutions and have failed some of them at remarkably low forces.  More on this in a future blog.

BUILDING AND INSTALLING THE EXCAVATION RAKER
The entire Excavation Raker Shore system is built on ground pads on the lip.  To provide rescuers with an eight foot safe zone in a typical (8-10 feet deep) excavation you will need two Finnform panels  (4’x8’), two 8’ aluminum wales, a Paratech 610 raker kit, nailing blocks, 2×6 braces (should be pre-cut) a bunch of steel pickets (1” diameter 36” long) a 6”x6” ground anchor, and sledge hammers.
After the raker is built, it is tied off to an anchor point well behind the excavation lip (usually a couple of pickets) and carefully lowered into the hole using ropes controlled by two rescuers.  At that point a “wall anchor team” climbs the raker down into the excavation with pickets and sledge hammers to drive pickets into the wall.  Simultaneously, a “ground anchor team” drives pickets into the excavation floor (anchor block)behind the rakers.  Since the entire excavation raker system is built and installed (lowered) from the lip, this is the only exposure time that the rescuers experience.  Their exposure risk becomes less with each picket (wall and floor) that gets placed.
This year we have added the “Deep Excavation Raker” to deal with excavations of up to 12’(see picture #4).  A second set of panels, wales and rakers is built and installed similar to the “8×8” raker system.  It rests on top of the first set of vertical wales until the struts are pressured (air system @ 175 psi)and the panels and anchors are picketed into the wall and floor.   To minimize risk, both systems have to be placed then anchored at the same time. To date the “deep excavation raker” is only a prototype  because it has not been subjected to destructive testing but the concept looks very promising.

SHORING TIME
The standard for building and setting the 8’x8′ system into the trench is 20 minutes.  It takes another 10 minutes to complete the pickets and shoot the air pressure to the shores.  The fastest I have seen the 8×8 system built and installed was 14 minutes by members of the Rochester (MI) Fire Department.  At this point we have not timed the construction and installation of the Deep Excavation Raker.  Stay tuned to the MUSAR trench blog for the testing and timing on this system.

 

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