Need Plumber's Expertise Help

The other day, I was doing some digging at the side of my house, and found something that I have no clue what it is or what it is for.
I was digging near the area where the sewer pipe exits the house. I always assumed that the lateral is just a simple sloped pipe that heads out to the municipal street sewer. However, after the lateral exits the house for a few feet, it inlets into a little dry-well type structure that is about 2 ft in diameter and covered with a round stone slab. Then, there is an outlet on the other side of the dry-well that acts as the continuation of the lateral. The house is about a hundred years old, and so this must have been part of the old sewer system, but I'm not sure what the purpose of that well-structure is.
Any ideas? I have some ideas on what it might be but I don't want to answer my own questions and just have people repeat it back to me. I want to hear what a professional plumber would say or someone else knowledgeable on the subject would say.
Thanks for any help provided.
--Jim
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On 7 Aug 2006 01:08:18 -0700, snipped-for-privacy@yahoo.com wrote:

I worked with plumbing for quite a few years before I retired. I did not do all that much underground work but I worked on a few sewer lines. I never saw anything like what you describe. Every one I ever saw was just a pipe going to the street which was normally clay on older houses. The only thing I can think of, would be a grease trap. However, those were normally only on septic systems. Of course, there probably was a septic 100 years ago, so what you have might be a manhole of sorts where they converted the old pipes from the septic to the city sewer. Then too, assuming you are at your foundation level, which means at least 6 feet deep, there would be little sense in having a grease trap that deep.
It would be much easier to ID if you posted a photo. Got any way to do this?
Also, tell us this......
How deep it it?
What material is it made from?
What state and / or country are you in?
What material is the lateral made from?
If you lift the rock, can you see sewerage flowing, or is this thing unused?
Are there additional pipe inlet /outlets, such as where rain gutter drops could have entered?
Is this "rock" a plain field rock or a cement casting?
Is there gravel around it?
Are there any drain tiles nearby?
Are there "drops" (pipes) under your downspouts? If yes, where do they go?
Why are you digging anyhow?
*** Is is filled with money? <lol>
Mark
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total depth from ground level (the lid is a few inches underground) to bottom is about 4 feet. There is the cover, then the well goes down 2 ft, then there is the inlet hole for the lateral on one side and the outlet hole for the lateral on the other side, and then the well continues down another 2 ft.

Clay... it's very similar to those plastic buckets that basement sump-pumps sit in. It has a bottom, the a clay wall about 4 ft tall, 2 ft diameter, and then it flanges wider at the top.

Southeast Michigan

Clay
It is being used... the bottom 2 ft (as previously described) is filled with sludge, and the top 2 ft is clear... I imagine this is a direct result of the design, since the inlet and outlet pipes are at the mid-point.

No... I saw on a website that some old city houses and buildings had a drywell for rainwater from the gutters to flow into and dissipate through holes (kind of like a seepage pit used further out from some modern septic tanks)... but there is no connection to collect any such water... there is only the inlet which is the sewer pipe coming out of the basement, and the outlet which I'm pretty sure goes to the street sewer.

The cover? It looks like a limestone slab... 2 ft diameter, 3 inch thick. It's round and covers the clay-well, but it's not a great seal... I think a previous homeowner brought it in when an older cover was perhaps broken.

Some driveway gravel, but not a special gravel section to suggest drainage if there were holes in the well.

None known.

Nope... I covered part of this answer a few questions back.

I need a good bed of limestone in the driveway before paving on top of it.

There might be a Lincoldn Wheat Cent or two at the bottom of the sludge.

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On 7 Aug 2006 15:47:08 -0700, snipped-for-privacy@yahoo.com wrote:

OK, I get a much better picture now.
That seems like an awfully shallow sewer pipe for the cold winters of Michigan. Being that shallow tells me that it originally went to a septic. Modern sewer pipes are generally below the basement floor and go deeper outside.
I would say that it's most likely a sediment separator and/or grease trap. They did things quite differently in the old days. Grease traps were used to keep heavy grease out of the septic, which tends to clog with greases.
In many ways, it sounds real handy, because if your sewer backs up, you can access it for cleaning easily, and if the sewer system backs up, you can prevent the house from filling with sewerage by opening that thing. Plus if you flush your wedding ring down the toilet, you know where to find it (yuck).
On the downside, if no one knew it exists, the poor plumber that would try to run a rotary auger from the house to the street, would be really frustrated when the auger kept stopping at that point.
I would highly recommend that you DO NOT pave over it. Make yourself a nice concrete cover or buy one. In the future there WILL come a time you'll need to get in there. I hope this is not being driven over. Walking is fine.
It might be interesting what sort of objects are in the bottom of that sludge....
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snipped-for-privacy@yahoo.com wrote:

It used to be common practice (still is in some places) to provide a "house trap" on the lateral leaving the bldg.
It could also be a "backwater valve" to prevent sewage backing up into the house; kind of a check valve.
Jim
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