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Strange exterior drain configuration

I'm stumped and I hope someone on this board can help me figure out why my exterior drain is setup the way it is.

This drain is at the bottom of an exterior stairwell. There's a wooden grate and under this grate is a trench/rectangle in the concrete. On the left and right sides of this rectangular hole are two 4" drain pipes that hook up to the main sewer line. In the middle of this is a vertical 2" pipe.



At first, I assimed this vertical drain pipe was to drain any water but it was full of dirt and rocks. When I removed this debris, it didn't drain anywhere.

Some questions:

1. What would be the puprpose of this 2" vertical drain pipe given that the horizontal (and working) drain pipes would drain any water that collected in the rectangular hole.

2. After cleaning the 2" vertical pipe, it doesn't seem to drain into the sewer line. It just seems to stopafter about 3 feet. Some people have said that older drain pipes in homes built in the 1950s drained to a small gravel pit and not to the main sewer line. Does anyone have thoughts on where this drains to?


Re: Strange exterior drain configuration

I was only able to download the 1st image of the 3---it's hard to tell exactly where these drains go without either removing the covering soil, which can be a messy, difficult & a hard job---or hiring a sewer service contractor to run a pipe video camera thru the 4" lines (expensive, & often unnecessary); it sounds like the 4" drains are there to drain the rainwater runoff from the outdoor stairwell---if your sewer system is connected to a town or city sewer system, it is illegal to connect any rainwater drainoff pipes to such a sewer system (if everyone in town did this the town sewer system would soon become overloaded during even light rains & the town sewerage wouldn't be able to be processed)---if you have a septic system (sanitary leech field) for your sewer water, it is usually also illegal to drain runoff into such a system for the same reason------ therefore, as you noted in your post, these 4" "runoff drains" almost always go into an underground drywell/french drain (often just a pile of buried rocks & gravel) where the runoff water is harmlessly drained into the soil.

My best guess for the 2" pipe with the odor of diesel/fuel oil is that it was perhaps an air vent back when your outside oil tank was being used---whenever a large 275/500/1000 gallon fuel tank is filled with oil, the oil replaces the air inside the empty tank; the tank can't be filled with oil if there is no air vent---if the vent had been placed in the front of the yard it would perhaps interfere with lawn cutting/landscaping, or perhaps there is a town ordinance that the tank air vent has to be so many feet inside the property line to minimize fuel odors, or an accidental overfill of the fuel tank--when the fuel tank was decommissioned someone no doubt cut the length to what you have now

Re: Strange exterior drain configuration

I've finally figured out what this is.

I called the company that decommissioned the tank back in 2005 and they said the old oil tank is located in the front yard. Since this drain is in the back yard, it's not a vent pipe.

The oil smell that I noticed before is now gone, when talking to the tank decommissioning company, they said this smell could have been the sewer or any decomposing material trapped in that drain when I unclogged it.

The drain actually leads to a french drain system which evidently didn't work so well because the previous owners connected this area to the sewer line via the 4" horizontal drain pipe located on the right hand side of the image. Any rain water that collects in this area makes its way to the sewer via that drain pipe on the right.

So, this vertical pipe is useless and is probably why it was filled with rocks and pebbles.

Re: Strange exterior drain configuration

Good to know, thanks.

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