Long post - I'm an engineer, like to try and figure out what happens when stuff fails. I'll be sending something like this with a photo and drawing to a guy who knows leach fields, see if he agrees that what I'm thinking makes sense.
I've found what looks like fragments of a cap that used to be on top of that box. About 2 - 2-1/2" thick, with rebar
We did have a sinkhole pop a couple years back, a depression, in that area.
Theory:
Old original leach pipe ran N-S from the septic, then an east-west leg through the middle of that 18 x 27 man-made pan. There may have been another section to that box, the one with the pipe flanges, then the cap on top (a distribution box).
Something failed over time, the pipe going downhill (N-S) that ran along the east edge of the property.
They cut in a new piece of pipe, using a 45 connector, above the failure point.
They clean cut the existing east - west piece of pipe, removed the piece with the flanges, put the cap back on the bottom pan section.
The new piece of pipe ran diagonally over the now lower cap, connecting to another new piece of pipe going east-west. There is a clearly newer piece of thin wall just south of the box, with a connector that would have attached to the diagonal run of pipe coming from the north.
Catastrophic cap failure = little sinkhole/depression. Earth drop completely fragmented the piece running over the cap. Downward push jacked the pipe so it tried to bend - that's why it comes up on the uphill outside edge at an upward angle (trying to bend around the concrete pan lip). Pipe trying to bend was a "shock", caused the pipe caving in on the upper center line.
I've dug out further on the diagonal pipe coming in from the northeast, may have reached a non-failed piece where I can cut in a new section. Close to the fence, but there should be enough room to get a hacksaw in, with some careful digging around the pipe.
This has been a fun, moving lots of earth by hand [emoji3] Fortunately we've had some relatively cool weather for a few days, just turning hot tomorrow.
Net, best I can tell all would have been fine if they had just removed the old box completely when they cut in the patch. The failure of that concrete cap, with a 6-1/2" void to the bottom, is why the new leach pipe failed. Alternately if they filled the box with stone or dirt, no problem.
I've found what looks like fragments of a cap that used to be on top of that box. About 2 - 2-1/2" thick, with rebar
We did have a sinkhole pop a couple years back, a depression, in that area.
Theory:
Old original leach pipe ran N-S from the septic, then an east-west leg through the middle of that 18 x 27 man-made pan. There may have been another section to that box, the one with the pipe flanges, then the cap on top (a distribution box).
Something failed over time, the pipe going downhill (N-S) that ran along the east edge of the property.
They cut in a new piece of pipe, using a 45 connector, above the failure point.
They clean cut the existing east - west piece of pipe, removed the piece with the flanges, put the cap back on the bottom pan section.
The new piece of pipe ran diagonally over the now lower cap, connecting to another new piece of pipe going east-west. There is a clearly newer piece of thin wall just south of the box, with a connector that would have attached to the diagonal run of pipe coming from the north.
Catastrophic cap failure = little sinkhole/depression. Earth drop completely fragmented the piece running over the cap. Downward push jacked the pipe so it tried to bend - that's why it comes up on the uphill outside edge at an upward angle (trying to bend around the concrete pan lip). Pipe trying to bend was a "shock", caused the pipe caving in on the upper center line.
I've dug out further on the diagonal pipe coming in from the northeast, may have reached a non-failed piece where I can cut in a new section. Close to the fence, but there should be enough room to get a hacksaw in, with some careful digging around the pipe.
This has been a fun, moving lots of earth by hand [emoji3] Fortunately we've had some relatively cool weather for a few days, just turning hot tomorrow.
Net, best I can tell all would have been fine if they had just removed the old box completely when they cut in the patch. The failure of that concrete cap, with a 6-1/2" void to the bottom, is why the new leach pipe failed. Alternately if they filled the box with stone or dirt, no problem.
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