How best to discourage future root growth when backfilling tunnel

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Neville Newman

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I just finished repairing a pipe separation beneath a bathroom. Not particularly relevant to my question, but for the curious, a vertical connection from a 3" sweep had pulled out of the bottom of a 4-way combo fitting which collects the last bathroom on this drain system.

The house is built on a concrete slab, the bathroom is on an outside wall, and I had to tunnel in about 3 feet from the perimeter to reach the disconnect. There are large boxwood shrubs and a couple of large crape myrtle trees in the flower bed area, 5 ft to 10 ft from the tunnel. When the pipe pulled out of the fitting (the groundswells and shrinks and generally moves around a lot in this part of Texas), a 1/4 inch gap was created and roots quickly grew into the gap and clogged up the pipe.

Now that it is repaired, and given the way it was repaired (with the movement issues in mind), I am not terribly worried about future breaks or disconnects, but I want to do what I can to mitigate any potential future problems even if their likelihood is low.

I would like to add something into the tunnel that will keep roots from growing under there, permanently .

I really don't want to use Roundup or any glyphosate-based produce and I doubt that such would work for this anyway, as they are supposed to be absorbed through the leaves of a living plant and then kill off the roots. Also, these products supposedly break down quickly in the soil. With the possible link to lymphoma, I'd just rather stay clear of this stuff.

I could salt the fill, but it seems that the salt (rock salt pellets) would need to be in the long-term presence of moisture to liquefy and spread through the fill. If roots just grow between the separated salt pellets, that doesn't help me.

When I had foundation-repair tunneling done in the distant past, the crew back filled with sand but they mixed the sand with Bentonite. The crew boss told me that when the Bentonite is exposed to moisture (which they did when the fill was almost complete) that it swells but then does not shrink back later. That part is sort of irrelevant (I don't mind the feature, but in this location I'm not too worried about thoroughness of the fill). However, if the Bentonite also discourages root growth then I could get 2 benefits at the same time by using it. In my internet searching, however, I have not found any information re. Bentonite's affects on root growth.

This seems like something that every plumber and every foundation company would want to know how to solve. But I'm striking out there.

Does anyone here have tried/tested experience to offer? I'm already awash in theories, so I don't need those.
 

Breplum

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For root intrusion prevention in planted areas, there are plastic root barrier products on the market in various sizes.
use a variety of keywords like Root Shield or tree root barrier
They come in sections but you could also just buy some good thick acrylic sheet to match the area you want to protect.

As an aside, it is only recently come to my attention how many people in Texas (I now have friends who live there) experience underslab drainage breaks and how prevalent tunneling is out there.
We virtually never tunnel under slabs out here in the west. We cut the slabs and change everything from above.
At 3 ft. in as you describe, I can see it makes some sense, but I have seen bids where whole houses get tunnels under them.
 

WorthFlorida

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At the foundation wall on the outside, dig a trench about three feet deep. Try not to dig out anything under the footing. Use a pressure treated plywood and coat it with driveway asphalt coating and sealer, then lay it up against the trench. Use tapcons to attach it to the footing. Or as breplum suggest an acrylic sheet. Most plantings do not grow roots that deep, however, Live Oak trees can. I had a neighbor who had to have a Live Oak tree removed about 18 feet from the home. The roots were lifting the slab.

If your underground pipes are clay or cast iron, they'll always move. If it's PVC with proper cemented joints it's rare for root intrusion. CI and clay pipe will get damp that can attract roots, PVC does not.

Bentonite is a natural material and if it was to state to control roots, etc, it then would need to classed as a herbicide and that opens a whole host of use regulations and EPA approval. There are a few MSDS sheets online and there are statements on where not to use the product.
 

Neville Newman

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Thank you, breplum and WorthFlorida, for your comments.

For a few different practical reasons, trenching along the perimeter beam of the slab and installing a deep physical barrier is going to be far down on the list of possible solutions. Even if I tried to list all the considerations here, I would undoubtedly leave some out. There are many mature trees close enough to this part of the house, and the plumbing for this bathroom is at the corner of the house. Stopping root intrusion at the slab perimeter would require about 30 feet of trenching (15' on each leg of the corner). We have many cottonwood and black willow trees and both run roots shockingly far. The crape myrtles do not run as far, but the roots are more numerous. These roots are all distinct from each other and I have done a lot of digging in my 2 acres (laying sprinkler systems and two low-dose septic fields) that exposes them so I know how far (and deep). If long deep external barrier ends up being my only recourse I will consider it, but I will try to find other solutions first.

Both of you live in areas where the soil is very unlike ours. Here, our houses float on top of undulating soils. The soil here has a 10:1 expansion ratio (from fully dry to fully wet). Obviously that's a lab extreme, but it can definitely pull perfectly good PVC glue joints apart, shearing the glue weld. That's why every in-ground repair I make uses telescoping couplers where possible, sprinkler heads are all mounted on flex hoses, etc. With 20 in-ground valves and 100 heads, I had to plan so as to mitigate that problem. Up under a house slab, of course, the moisture content doesn't vary nearly as much but as you near the perimeter it can become a problem. I had to replace 17 ft of 3" drain in another portion of the house (also via tunnel). Packing fill back in underneath a drain can never provide the same amount of original support, so I suspended all of it from the underside of the slab with threaded rod and clamps to keep the drain and the slab from moving relative to each other.

If I can't come up with a good chemical "barrier" to use in the tunnel, and before I opt for exterior below-grade physical barrier, I am wondering about using overlapping membrane to line the walls of the excavation before filling. I have several rolls of PVC sheet left over from a large commercial roofing project, so I could use that to at least slow and somewhat control the route of any determined roots. It's too bad the sheet material isn't TPO, because then I could air-weld the sheet pieces together. With the PVC sheet, I can't do that and so it basically would be a super-duper weed-block fabric and roots will always find their way to the edges and overlaps. This is such an off-the-beaten-path idea that I seriously doubt that there are many or any folks here that tried such a thing, but if any have then I would like to hear about the results.

Lastly, I wonder about the effectiveness of "air gapping" the area right immediately around the pipe joints. Should any cracks or separations ever happen in these drains then (unless they are major leaks) a void space around the joints would interrupt the moisture gradient and might "confuse" the roots as to which direction to grow to chase the moisture. (kind of like wiping your finger across an ant trail) Any drips would fall to the bottom of the void space and the source crack would be safely removed by at least several inches of air space. At least, that's my wild one-off theory that I'll risk opening up for comment here. Honestly, if it wasn't so close to the "outside world" at the slab perimeter, I wouldn't even bother to fill it.
 

MACPLUMB

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Get some ROOT X mix it per directions then spread though out your dirt backfill, put a lot
around the drain pipe, if it gets wet from moisture it will activate and kill any roots
 

Neville Newman

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I like that idea, MACPLUMB. The RootX tech support people officially discouraged me from this "off label" use, but privately indicated that it at least wouldn't hurt anything. That is, except me possibly :-0 . The MSDS does give me some pause. But thanks for the idea. I'll think about it.
 

Reach4

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Copper sulfate is often used to kill roots. I don't know how it would compare to RootX in this application.
 

WorthFlorida

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............. Here, our houses float on top of undulating soils. The soil here has a 10:1 expansion ratio (from fully dry to fully wet). Obviously that's a lab extreme, but it can definitely pull perfectly good PVC glue joints apart, shearing the glue weld. That's why every in-ground repair I make uses telescoping couplers ...............

One thing about central and south Florida, the ground never moves. I've had PVC joins open when tree roots push up on the pipe and separates the couplers. Usually a problem with the water mains to the home that is usually 3/4" PVC. The final part of home construction is landscaping and no one worries where the buried pipes are. Palm trees are the worst offenders.
 
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