Thought you may like some pics of trenches, and why trench boxes are required. At Dulles, we enforced VERY heavily the use of trench boxes in ANY trench over 3' deep. The only reason to step outside one was to pick up that last paycheck, cause if you stepped out, it would be your last. We did have a tunnel collapse while I was there, a bit different than a trench cave in, but a life was lost none the less.
You can see the type of soil we were dealing with, called fractured, or degraded rock. I've had people argue it was Class A soil just after they dug the hole, while we were arguing, the wall would collapse, END of argument.
It's about 30 miles SW of Washington, D.C. at Dulles International Airport. The soil is called degraded rock, you can see it in some of the pictures.
You literally have to beat it out, using machines called Hoe-Rams, there is no digging of this stuff initially. Once exposed to air, rain, snow, it becomes muck. You can see in the first picture the way it looks when used for backfill.
When it degrades, it becomes some very nasty clay when put back properly. We would not certify it for backfill on a good deal of the property causing contractors to import stuff from other parts of the region.
When we set manholes, or any other critical structure, we most often backfilled with a concrete (cementitious material for you afficianados) called "Flow fill" or "Slurry" mix. We made our own concrete on site most of the time. As you can see, all ductbanks had to be encased in 4,000 psi concrete and have a rebar envelope.
[quote]but do you ever find anything interesting when you go digging out that much of an area?[quote]
My brother did construction work (steelwork inspector) at the WTC when it was being built. The management said something to the effect that if someone finds something, it's yours, take it home, get it outta here. "And I dont want to know about it". They didn't want the project delayed by archeolgists digging around the site. My brother never heard if anyone ever did find anything.
ThinkGood, At Dulles, no, it was a farm for a LONG time, then it became an airport.
The Pentagon was something a bit different. It was built on an old airport, junkyard, landfill, whatever human refuse was buried there. One Steamfitter took home an INTACT model T. It was rusted, all cloth gone, but all was there.
Handmade crowbars from the original construction - they must have had someone making them on site, we had pictures of crews with them, OLD pottery, OLD china, we dug up lots of interesting stuff there.
For some reason, they were sorta funny about taking pics of the place