Septic tank and distribution box repairs

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Hatsuwr

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The outlet bushing on my tank failed, causing the baffle to come off and probably doing some serious damage to the drain field. I have plans to install a new drain field when I get around to installing a geothermal heat pump system, but it seems functional enough still so I'm opting to just repair the components between the tank and distribution box.

r/Plumbing - Damaged tank outlet

The pipe between the tank and box is bituminized fiber (and I'm guessing the drain lines are as well unfortunately), so I'm going to replace that with 4" Schedule 40 PVC. I can't find much information regarding the correct slope for this pipe, but I figured 1/8" - 1/4" drop per horizontal foot should be good to aim for, although it looks like it might have to be slightly more due to the relative heights of the tank outlet and box inlet.

The old pipe just used dirt for the bedding and backfill, but I was thinking about adding a bit of gravel: https://www.sakrete.com/product/all-purpose-gravel/

To repair the tank wall, I'm planning on pouring concrete around the pipe and forming a flange on each side. I'll use 4 short pieces of FRP rebar parallel to and surrounding the pipe to make sure the flanges stay together since the clearance between the pipe and the hole in the tank wall is only about 1". Will aim for 2" thickness for each flange.

I've had bad experiences with Sakrete Pro-Mix Advanced before, but I think it might be the most appropriate small batch I can get for this application: https://www.sakrete.com/product/pro-mix-advanced-concrete-mix/

Here's a quick sketch of what I'm trying to describe, although the rebar isn't shown. The filter/baffle inside the tank also isn't shown.

r/Plumbing - Tank repair plan

The distribution box is similarly eroded, to the point where the back wall just crumbled away, although thankfully the side walls seem to be in pretty good condition.

r/Plumbing - Damaged distribution box

To repair this, I plan to add 2" thickness to the front and back walls and fill in any missing material from the original walls. The front wall will be formed around the new PVC pipe, and I'll add a short section of that 4" PVC coming out the back wall in case I need to add a drainage line to get me through to the field replacement. That will just stay capped in the meantime. I'll also add 2" to the top of all the walls to form a new upper surface.

For these wall repairs I'll use the same concrete mix as with the tank repair, and I'll use FRP rebar in the 2" added to the top.

Here's a model of that. Grey is the existing box, brown is the concrete that would be added. Drain pipes are not shown.

r/Plumbing - Box repair plan

Once this has cured, I plan to make a riser for easier inspection/cleanout access. Just a foot or two to reduce future digging and disturbing the area around the fragile drain pipes. I'll pour it in place, but oil the existing surface so that it should be removable in the future if needed. I'll form a lid in a similar way. I'll seal the riser to the box with the same butyl rubber sealant I used when adding risers to my tank: https://www.reddevil.com/products/product/gutter-foundation-sealant-10-1-fl-oz-cartridge-white

The riser and lid won't be exposed to the same conditions as the tank and box repairs, so a more basic concrete mix should be fine. I'm thinking this: https://www.sakrete.com/product/crack-resistant-concrete-mix/
 

Hatsuwr

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Well I discovered that the drain lines are actually what seems to be SDR 35 PVC. That's great since I can probably jet and vacuum them out to recover some drainage capacity, and I can just rebuild the distribution box rather than repairing.

I also saw that reviews for the butyl sealant I was planning on using are rather poor now, so I got some of this polyurethane sealant that seems like it might work better: https://retail.usa.sika.com/en/prod...ir/spallcrack-repair/sikaflexr-concrete-fix-0

Since I'm building my own distribution box, I'll make it a bit larger and deeper and add a tee as an inlet baffle (similar to the one on the tank). I'm also thinking about adding a vent to the lid
 

Hatsuwr

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I changed up the idea for the new distribution box. I'm instead going to size it for a Polylok riser like I have on the tank. It's just over 2' across on the inside. Base will be 2" thick and walls 3" thick. Total height will be a bit over 2', bringing it to about 1' below ground level so only one riser section will be needed.

I figured I'd add tees to all of the lines to help keep any floating matter out of the drain lines. This does mean settling matter will more easily enter the lines, but I think that type is less likely to enter the distribution box anyway, and there's still 2"+ below the baffle inlet and the bottom of the box.

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