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kuhlman 01-19-2012 11:45 AM

Backfilling trench when full of water
 
I just replaced my water line from the meter to the house with 1" Poly. It started raining heavily the day after I completed the install and now the 25' trench is full of water, and all my fill dirt is now saturated. It looks like the rain won't be stopping here for weeks. Do I need to wait until everything dries before backfilling to properly compact the soil, or will backfilling with wet soil in a water filled trench result in good compaction (I feel like I've heard the wetting fill dirt aids in compacting it)?

kuhlman 01-19-2012 11:49 AM

The trench is 36" deep and the soil has high clay content. I also have a pump so could pump out the majority of the water first.

titanoman 01-19-2012 11:50 AM

I've never seen water stop anybody from backfilling.

joecaption 01-19-2012 11:53 AM

It's going to stay muddy and once dry will tend to settle so do not haul away extra dirt if there is any until it's back level again when it drys out.

concretemasonry 01-19-2012 11:54 AM

You are never going to compact saturated soil. A little moisture aids in compacting dry soil, but it is very easy to get too much water.

With a clay soil, you are going to have to wait quite a while to get the moisture down. Rock could provide some stability and displace the wet soil, but depending on the shape, it could damage the 1" poly.

Dick

Daniel Holzman 01-19-2012 01:09 PM

Structural fill typically consists of sand and gravel with a relatively small (less than 20% by weight) silt content. If the silt content is below 5 percent, it is possible to compact structural fill even when saturated, although it may be an unpleasant task. You state you have clay soil, which is typically unacceptable for structural backfill for commercial projects.

Typically, structural fill (meaning fill intended to be compacted) cannot contain clay, organic material, garbage, or more than 20 percent silt by weight. So if you want to backfill that trench and get reasonable results, I suggest you purchase a sand/gravel mix, or you can even use 3/4 inch crushed stone, with a small amount of topsoil (say 6 inches) on top to support seeding. You can place crushed stone/sand mix when it is saturated, and compact it even if saturated, using ordinary equipment like a mechanical tamper.

If you don't care to pay for backfill material, you can fill using the clay soil you have, understanding that it is not going to be possible to compact, so you should expect long term settlement over the trench.

Bondo 01-19-2012 02:58 PM

Quote:

Originally Posted by kuhlman (Post 827787)
I just replaced my water line from the meter to the house with 1" Poly. It started raining heavily the day after I completed the install and now the 25' trench is full of water, and all my fill dirt is now saturated. It looks like the rain won't be stopping here for weeks. Do I need to wait until everything dries before backfilling to properly compact the soil, or will backfilling with wet soil in a water filled trench result in good compaction (I feel like I've heard the wetting fill dirt aids in compacting it)?

Ayuh,... If this area sees nothing heavier than foot traffic,...
Go ahead, 'n fill it in with yer spoils...

If it runs across yer driveway, fill in a foot of sand, then finish filling with crushed stone....

Msradell 01-21-2012 10:04 PM

Quote:

Originally Posted by Bondo (Post 827970)
Ayuh,... If this area sees nothing heavier than foot traffic,...
Go ahead, 'n fill it in with yer spoils...

If it runs across yer driveway, fill in a foot of sand, then finish filling with crushed stone....

+1 ding ding ding we have a winner! :thumbsup:

kuhlman 01-23-2012 11:51 AM

Well, I did it. We had a slight break in the rain on Saturday and I pumped out all the water and filled in the trench with the saturated dirt, tamping the best I could (though tamping just created soupy mud pies). If it ever dries out, I plan on trying to compress it more. Oddly enough, I used all the dirt I had from digging the trench and I will still need another yard or so to bring it back up to grade. My only concern is I was not able to ensure the pipe was support underneath where it goes into my foundation, and where it connects to the lead coming off the meter. I'm crossing my fingers on that.

Daniel Holzman 01-23-2012 01:48 PM

Good to hear you successfully filled the trench. A couple of things to bear in mind. The soil you placed is going to settle over time, there is little point trying to compact clay or silt while it is wet, it acts like jello and never compacts, better to let it dry and settle naturally, then fill to grade in a few years to make it look pretty.

The reason clay and silt is never used in commercial backfill operations is that it exerts substantially more pressure on the pipe than sand or gravel. This may seem counterintuitive, but the sand and gravel (structural fill) bridges nicely over pipe, and reduces the pressure considerably, whereas clay and silt do not, especially if saturated. This is important in deep fill situations with relatively weak pipe, for example installation of HDPE culverts under signficant fill conditions, use of thin walled plastic irrigation pipe, etc. Unlikely to be an issue in your case, but if you ever have to run relatively lightweight pipe for any distance, you would do well to spring for some decent fill material, it will increase the probability of a successful installation.


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