I was wondering witch is the faster more efficient way to digging a deep trench. Sloping or benching? The material is like a clay mix brown shale. It has good stability when digging. Trying to increase footage. Need to spend more time hogging out material than shaping the sides. Currently using multi-bench technique( 12' deep). Laid 80' ft 24" ADS set manhole and 24" headwall plus back filled ditch with 325c excavator. 8hrs.
I would read this to cover your butt, https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10932 ..
Welcome to the Forums Willie! What are you using to compact the trench? I run a 42" bucket on a 321 to lay storm. I use the multi-bench method as well. 3-3.5' deep main trench and then start benching as needed, with each bench being 3-3.5' tall and 4' wide from there on. This gives a shelf for the pipe layer to advance tools down the ditch line and makes it easier to backfill. After initial backfill around the pipe, I put a CS533E in to compact, usually backfill with a loader as the hoe is digging more mainline. Most of our soil is class B cohesive so this method is acceptable. Most of our soil is heavy clay and rock, no sand.
At what point is it faster to pull a trench box? I can see where it would save a lot of time by only having to move half as much dirt, although its a little more work for your hoe hand, and you may need 2 buckets, one trench width and the other inside the box width.
I've only ever used trench box a few times but I'm wondering about your comment about needing a bucket to fit in the box. Would it be for pipe bedding?
I normally don't install storm sewer deep enough to need a box. Most of it is 8' or less. Pulling a box at short depths takes time and then you have to factor in your bedding/backfill material. If it's a parking lot or other non-landscaped area the backfill will need to be placed and compacted to the same spec as the rest of the fill on the project. In my area a 25 ton load of #57 stone runs $500-600 delivered. If I'm filling a 8' deep trench 42" wide with solid stone it gets expensive - like a monkey pee'ing in a cash register, it get's into money quick. It's faster to bench the sides, lay the pipe, use minimal bedding and get the 533 in the trench to compact the backfill. It just depends on the site, the pipe being laid and the equipment available. Our friends up in the far north, lay everything open when laying pipe and use big machines to do it and they have deep pipe runs.
Generally when using the box, you rough dig out for the box, pull the box and then dig to grade inside the box (no more than 18" deeper), set the pipe, stone, then back up and get digging again. As far as benching vs. sloping, I think it greatly depends on the material, I didn't read it in the link above, but I thought that all Class C had to be either sloped or shored, as well as any trench other than solid rock with water in it... thought that could just be our safety department.
If you have the room sloping is faster and much less dirt to hog out based on soil type. Benching (when done correctly) has the same exact slope as "sloping" does, but you have to hog out the sides more. Try using a V-bucket for a slope and it doesn't take long to get used to. In California we play by Cal OSHA's rules so sloping in Class C soil is 1.5' out from the edge of the trench for every 1' down. For example: 12' deep means you need to slope 1.5 x 12' = 18' on the right side and 18' on the left side + the width of the trench. So if you're using a 42" bucket then your trench needs to be 36'+ 3.5' (trench width) = 39.5' wide from one side to the other. For Class B soil its 1:1 ratio so 12' down 12' wide on each side + the width of the trench = 27.5'. Try using production rates to let you know if you're hitting your target. For example: If you have 1500' of 24" RCP to lay at 12' deep, you bid 300 man hours to complete the task, and your budget is $85,000. If you use a 3 man crew (24 man hours per day) you need to Dig 480' per day to meet a $283.33 per hour rate to meet your budget exactly. Assuming you dig all 1500' and then start to lay then you need to lay 240' per day to meet your budget rate per hour of $283.33. Backfill is the same rate as digging at 480' per day. ATTACH]125992[/ATTACH]