I am having this mental debate with myself. I have about 1000ft of trenching to do for various pipes, from sewer, water service, electric conduits, etc, etc max depth probably will not exceed 3ft. The issue is I have several trenches crossing each other, and will need to get under 2 structures a bit for water and sewer. I have a LOT of experience cleaning up trencher trenches with a shovel, especially intersections. I also have noticed trenchers usually don't leave a firm trench bed unless we run slow..... I have to cross a driveway where there will be heavy traffic loads at times and I am still trying to solve the riddle of depth and backfill plan to ensure all piping survives the traffic. I had considered calling for a little CLSM to go around the pipes but question whether a course sand could handle this just as good?
What we would do with shallow pipe under roads is put them in a metal sleeve (pipe).
I would go with that but we have several 3" and 4" pipes. I think I know what I "should" do, which is use standard concrete for a partial backfill, but if I ever have to repair something, this will become a nightmare! I was considering CLSM just so there is some workability there. I am currently reviewing a university study on some of this.
If you have a big enough sleeve you can cut and remove and replace a broken pipe in the sleeve .
If you are using schedule 40 pipe you should not have any issues under the driveway as long as the trench is compacted correctly. Sleeving the pipes would be ideal anyways so you don't have to tear up the driveway again. Some 6" pvc should do nicely. We spec 18" hdpe pipes with a foot and a half fill minimum cover for heavier use roads than your driveway will be.
What is CLSM? Never heard that acronym. I agree with redneckracin just use SCH 40 PVC and good clean backfill. A stone dust such as 8910 (the numbers indicate the screen size the rock can go through) is perfect for road crossings as it will not let water pass like 57 stone does which is larger rock. Sleeves under improved surfaces are also a great idea. We put in sleeves - sometimes too many, on projects that we own as it can save a ton of money in the future if you need to run back across a concrete or asphalt surface. Now back to the original question? If your sewer is not pressurized how are you going to hold grade with the trencher to get flow without a lot of shovel work?
Trenches and grade are not always fun I would go mini I guess I use 1/2 sack slurry in questionable backfill areas especially heavy traffic spots if it's good enough for the city it's good enough for me Sleeves are by far the best way to protect pipe and also the easiest to come back to after backfill and add a line
I would put an extra pvc pipe in the trench and cap it...no digging up the driveway again. Ed
I will clarify guys, the "road" surface is just gravel, which makes a big difference in applied load to underground pipe. Obviously if this was asphalt or concrete, I would have no concern. A 30K forklift with a 20K load over these areas will be no joke! Way more load than your typical semi. I would love too sleeve this but there are just too many pipes and too many sizes. It does sound like you guys have not had many issues and I really haven't either! Only a few cases, but were all very fresh excavations followed by extreme traffic, like continuous 966 traffic, loaded. As far as grade with a trencher, I guess I have done too much of this stuff..... not a biggy! Always will be a little shovel work but I can read grade, and we always have someone next to the trencher calling out depths as we move. The easy part of this project is there is already 6ft of fall between points and I only need 6". Just hold depth and roll until I need to shallow up to 18" at the destination point.
Flowable fill. Basically concrete with out gravel