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.
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.
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.
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.
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/

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.

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.

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.

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/