I thought these are purchased items, with design values provided by the manufacturers.
I'd do a google search for trench shields, unless you've got a hankering for doing it the hard way.
Trench shields are essentially propped retaining structures. Peck gives a pressure envelope for these and you just design the members accordingly. Nothing fancy.
Thanks Jed and jayrod. The project is at a very remote overseas location so shipping something in would take months. They only need something four feet high and have materials on hand to fabricate. Jed, thanks for confirming there are no special design requirements, I just wasn't sure.
There's nothing special. However don't skimp out on the design. Soil loading is real, and permanently applied throughout the duration of the project. Temporary works like this are not the place to save a couple of bucks.
Trench shield and trench boxes have a maximum, allowable, lateral pressure rating from the manufacturer's professional engineer. The certification sheet will give you the maximum total pressure and usually several excavation depths associated with that pressure for OSHA Type A, B, and C soils. When I am asked to do a design for a trench shield or box, I check the active and surcharge pressures for the intended excavation depth and the expected soils (based on the closest soil borings). Then, I show that the expected total pressure is less than the rated allowable pressure. I do not check the shield or box sides because there is never any info given on the construction or steel strength of the sides. You may need to check the pipe spreader braces that support the opposing side walls. The wider the excavation trench, the longer the unbraced length of the pipe braces.
If you are building the shield from scratch, the entire shield needs to be designed. Is the depth 10 feet or 4 feet? You mentioned both depths.
I've probably only had maybe 1% of projects I've been involved with have trench "shields" that are ever physically in contact with the sides of the trench and providing support. Usually they're about protecting the worker so that they can get out of the trench if a failure occurs.
I would be careful about just using an apparent earth pressure chart - if you have a trench failure which is probably the only time it will actually be supporting anything the load may be higher than predicted by Peck's charts.
Do you have some guidance we could use to address your concerns here? Peck's guidance on earth retaining structures is the normal for me, but I'd love to be enlightened with new information.
I agree with geotechguy1. Is a designed excavation support system really needed for a 40 inch deep excavation? The full depth cut will have to be made just to be able to place the shield in the trench. Unless immediately next to a railroad track, I have never sheeted a 40 inch deep excavation. If there is a ground water problem expected in the 40 inch depth, a shield will not help control water.
The shield is needed due to unstable soil and a nearby pavement. Water table is about 30 inches down. Soil is stable above that. They plan to excavate 30 inches, place the shield, then excavate the remaining 10 inches from inside the shield, pushing the shield down as they go.
If the side walls of the trench are solidly frozen you may not require a trench shield but prepare to properly defend yourself when you have the presence of an OSHA inspector. You may want to discuss this method with you local OSHA office before hand. Don't forget a ladder. I did not Study the Big Dig refrigeration equipment used in Boston while excavating around train tracks but that project may provide further understanding of the stability of frozen ground.
\My guesstimate is that once this shield is made, some do-gooder will take it to other jobs and maybe add to it. So, don't pinch on strength. Over building won't hurt and maybe it won't be ruined on the jobs due to skimpy construction. They don''t get much care in use, so build it accordingly.
