Author Topic: Backfilling with gravel - which type?  (Read 359 times)

bandito

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Backfilling with gravel - which type?
« on: December 04, 2017, 05:07:03 PM »
I have a drain pipe running close to the inside of my garage's foundation.  I would like to fill with self-compacting gravel to the underside of the drain pipe to create the correct slope and hopefully maintain it so it doesn't get effect by further back filling above.  I would then fill the rest with clay until the last six inches below the slab which would be tampered crush. My question is, which gravel is best for this scenario?  I have read a lot of different articles on self-compacting gravel and some say it isn't self compacting and others that say it is.  I also read different articles on which gravel to use. Some say 3/4 gravel, others say 3/8 gravel and even pea gravel.  I tried some of the 3/8 gravel and pea gravel.  The 3/8 gravel seam to hold together very well and feel very solid under my foot steps but the pea gravel seamed to move around a lot.  Reading the manufacturer's labels for these types of gravel and it seams they all serve the same purposes other than decorative tastes. Which one do I use?

lthenderson

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Re: Backfilling with gravel - which type?
« Reply #1 on: December 05, 2017, 07:49:57 AM »
In that situation, I always use pea gravel. It might move around a bit when you walk on it with a point load but a better test of its compactibility is to fill a five gallon bucket part way up and put a distributed load on top, i.e. another five gallon bucket bottom and push down. You won't be able to push it down even a little bit.

Not sure by your description what you are attempting, but I wouldn't backfill with clay. I would fill the entire trench up to the bottom of the slab with pea gravel.

bandito

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Re: Backfilling with gravel - which type?
« Reply #2 on: December 05, 2017, 10:18:01 AM »
Thank you for the answer. As for the clay I was hoping to use as much of the clay that I dug out when I excavated.

paddedhat

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Re: Backfilling with gravel - which type?
« Reply #3 on: December 06, 2017, 09:56:01 AM »
The important aspect of your gravel specification is that it is fairly uniform and clean. The worst choice would be known as bank run, crusher run, modified, or stone dust (these are all "street names" for the materials, your area may have different descriptions, or even state approved codes like 2A, 2B etc.... in common use) . These gravel products have a lot of fine material in them and are designed to compact in to a solid, nearly impermeable material. Any clean stone from pea to 3/4 will allow a lot of water flow through the material, which is desirable. If you need to top the gravel with clay, then you need a filter fabric, or geo-textile to keep the clay from migrating into the soil. The cheapest DIY source for this material is the weed-block fabric available by the roll in a garden center, or big box store. Just roll it out, over the gravel before you place the clay.
 In this configuration, a gravel filled trench topped with clay, you can move exponentially more water that the pipe alone can handle.

 I once relocated a high volume spring that we unearthed, a few feet from a new home foundation.  I placed a pipe in the bottom of a 2' wide trench, covered it with 2' of loose gravel, then capped with clay. The end of the pipe exited to daylight, well away from the home. It has hundreds of GPM flowing out the pipe AND surrounding stone, continually. 

Bourbon

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Re: Backfilling with gravel - which type?
« Reply #4 on: December 06, 2017, 10:04:03 AM »
The important aspect of your gravel specification is that it is fairly uniform and clean. The worst choice would be known as bank run, crusher run, modified, or stone dust (these are all "street names" for the materials, your area may have different descriptions, or even state approved codes like 2A, 2B etc.... in common use)

It's just called "dense grade" here.  I finished regrading my yard last year and used it for the base under the timber wall we built, but for drainage under the pipe I go with a crushed limestone aggregate.  allows drainage but doesn't move like pea gravel.  We have a lot of limestone here.

bandito

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Re: Backfilling with gravel - which type?
« Reply #5 on: December 07, 2017, 05:48:18 PM »
I think my explanation wasn't the greatest so I will try again. The drain is a sanitary drain connecting a toilet and sink from the garage that I am building to the sanitary drain in my house. This is not a drain for water drainage.  The trench that it is in is bit narrow, deep and awkward to get a compactor inside so I would like to fill it up beneath the drain pipe with self compacting gravel so I don't loose the correct slope when I fill above that.  I wanted to use clay above that because I a lot from when I escalated.

I was able to get some expert advice.  Apparently the only gravel that is considered self-compacting in my area is 3/4" clear.
« Last Edit: December 07, 2017, 06:32:48 PM by bandito »

paddedhat

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Re: Backfilling with gravel - which type?
« Reply #6 on: December 08, 2017, 03:11:53 AM »
I think my explanation wasn't the greatest so I will try again. The drain is a sanitary drain connecting a toilet and sink from the garage that I am building to the sanitary drain in my house. This is not a drain for water drainage.  The trench that it is in is bit narrow, deep and awkward to get a compactor inside so I would like to fill it up beneath the drain pipe with self compacting gravel so I don't loose the correct slope when I fill above that.  I wanted to use clay above that because I a lot from when I escalated.

I was able to get some expert advice.  Apparently the only gravel that is considered self-compacting in my area is 3/4" clear.

When it comes to compaction and stability, if you are using a graded, uniform gravel, the key is sharpness. As in, a gravel composed of 3/4" rounded river pebbles is never going to compact, a gravel mix of 3/4" jagged edged, randomly shaped pieces will. Once you get into mixes with various sharp pieces and sizes, all the way down to sand, this is what I spoke of earlier as bank run, crusher run, modified, or as another poster calls it, dense grade. That stuff will compact like concrete. Most sand, and or stone screenings, which is essentially course sand made from crush stone, will work fine also.

Fishindude

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Re: Backfilling with gravel - which type?
« Reply #7 on: December 12, 2017, 08:00:21 AM »
I like to bed sanitary drain lines in clean sand with about 6" over top.   This assures when you compact soils on top that you won't have a rock rub a hole in the PVC drain.
After that, your excavated clay materials should be fine, just install in 4"-6" lifts, then compact each lift.