DWP water main install, Q on compaction
There’s an DWP crew on my street this week, putting in a new water main.
Anyone familiar with modern acceptable methods of installing water mains, I’d appreciate your thoughts.
They’re digging to about five feet, leaving the disturbed clay/loam loose, placing the joints of the new main on 4×4 DF blocks, then backfilling with a slurry mix; one bag portland per cu.yd. of sand and gravel with lots of water.
According to the foreman, mechanical compaction of the disturbed soil in the trench isn’t required because four-five feet of slurry mix is enough weight to take care of that.
My experience tells me that their process doesn’t allow for inconsistencies in the sub soil, under the pipe. So if it’s softer under one pipe joint than it is under the next one, there may be leaks at one or the other joint.
Edited 9/26/2008 2:26 am by Hudson Valley Carpenter
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Some photos.
I forced the city to replace the water main on my street which they did immediately. We have sidewalks and gutters,
The first thing the company did [Ortiz] was to lay 2" pipe in the gutter using those clamp/ O ring connectors like the evil sprinkler pipe installers use. Each meter was served vis a red, air compressor-type, red hose. At driveways, they covered it with asphalt. Across the streets they cut in a shallow trench. It was fed at the end from a fire hydrant.
~Peter
Doesn't the slurry take care of any differences? I would think the slurry would bridge any off them.
The slurry is 5' deep. That's a pretty deep stiff beam.
Good question but the slurry is only ~10% portland, as opposed to 30% or more in concrete. In addition, the slurry has a high water content, which carries some of the portland away, into the porous surrounding soil. Not much stuctural integrity.
I was thinking that the shear volume of 2'X5' would make up for the weak mix.
I'm not sure at all. But it might.
Do you think they make the mix high in water so it fills voids well?
Also, If they have to remove it later you wouldn't want much more than %10.
Do you think they make the mix high in water so it fills voids well?
Well the pipe is resting on wood blocks so the answer is yes, the mix is sloppy in order to fill under and around the pipe, without using a vibrator.
the compaction fine. no problem.
Thanks BB. Now if that DWP crew of eight could lay more than four 20' lengths of pipe per day, they might make a believer out of me.
I think what you are describing is called a stabilized sand mix, there's just enough Portland to make it firm up. That way when they have to repair a leak the sand won't slough off on the workers in the hole. As for needing to mechanically tap the mix I think it will flow around and bed the pipe.
Edited 9/25/2008 11:33 pm by ottago
As for needing to mechanically tap the mix I think it will flow around and bed the pipe.
I guess my shorthand version of the proceedings wasn't as clear as it should've been. I was referring to compacting the loose soil in the bottom of the trench, before installing the pipe.
Yes on encasing the pipe in the mix. That's the stated intent.
That's different than what they do here. I was watching a private contractor run some town sewer this afternoon while eating lunch. They had a convoy of excavators moving down the street. First one with a trenching bucket busting open the trench. Second one with a larger bucket slinging spoils into a truck and generally shuttling material around. Third one with a bucket dropping fill (crushed rock and sand) in on top of the new pipe. Fourth one with a sheepsfoot compacting the fill. The whole thing moved slowly down the block along with a couple of guys dropping pipe into the ditch and a rod man on the laser rod keeping grade.