We had noticed a “soft” spot in the carpet in our basement but did not think much about it until we decided to re-do the rec-room this fall. When we lifted the carpet we found a pipe opening (about 4 inches across) coming from the concrete. It’s dry, so we are not sure what it is for.
The answer to your question is largely dependent on the age of your home. You have not identified the material that the pipe was made from, which would also give me a clue to its purpose. Since the information is limited, I will do a bit of speculative detective work and offer several possibilities.
When a new home is built, one of the last items to be completed is the basement floor slab. That is because there are many embedded plumbing pipes, as well as weeping tile and often a sump pit and other items that have to be completed before pouring the concrete. These plumbing drain pipes may interconnect beneath the slab and have several vertical sections that protrude through the finished concrete. One of these is what you have found when you removed the old carpet. I have a couple of options for this if you have a newer home with ABS plastic drains and other options if you have an older home with cast iron drains.
If your home was built in the last 30 years, or so, you likely have black ABS drains. All of these vertical drains should have a small diagonal section sticking out of the floor, and have a threaded cover installed. These are “clean-outs” that allow access to the interior of the drain pipes after the floor is poured. If one of these is installed too low, and the cap has been removed, this may be your answer. This is unlikely. If your home is less than 15 years old, you will still have ABS drains, but you may also have another component installed in the system which is the more likely culprit. An inline backwater valve may be installed that has a straight vertical section, usually with a cap, which is normally flush with the top of the basement floor slab. If this cap has been removed, or the valve itself, you will see a shallow void, with little or no water at the bottom.
If either of these are the case, there may be a simple way to make that determination. Both of these situations will leave the buried drain open to the living space, without a trap at the bottom. This will allow sewer gas to enter the home, unrestricted by water in a trap, and may be a health hazard as well as giving an unpleasant smell to the area around the open drain. If you detect an outhouse odour at the top of the pipe, you will know that either a cleanout or a backwater valve has been removed, and should be immediately reinstalled by a licensed plumber to prevent further sewer gas entering your home. You can temporarily tape a plastic bag over the opening until the repairs are made.
Another possibility for a newer home is that there are “roughed-in” plumbing drains installed in the basement for a future bathroom, that were covered over. The toilet drain pipe is normally installed with a short section sticking through the floor slab, high enough to install a cap. If the cap has been removed and the pipe cut flush with the floor, then it would look similar to your description. It would not normally have water in the bottom, and the same smell test should determine if this was a possibility. If no odour is coming out of the pipe, then I have no idea what you are dealing with, but it may not be of much concern.
If your home is over 45 years old, then you may see a cast iron section of drain piping which has been cut off flush with the floor. The first and third scenarios stated for a newer home above are still a possibility, but only the clean-out is likely. This may be determined not only with the smell test, but also by looking in the top of the pipe for a female threaded end. Older homes often had the cleanouts installed flush with, or slightly embedded in the concrete floor slab. If someone had inadvertently unscrewed the cap and left it off, then this is what you have found. Same story, call a plumber and get a new cap installed before someone gets sick.
The final possibility that I can think of for an older home is an abandoned eavestrough drain. Many years ago, older eavestrough downspouts used to drain through the foundation or exterior walls and empty into a short stack sticking a few feet out of the basement floor slab. These vertical pipes typically connected under the floor slab to the catch basin, which drained the water into the floor drain. This practice was discontinued many years ago, as it often led to overburdened sewers and basement backups during heavy rainfalls. This final possibility can be determined by the smell test, which may provide a damp or even mouldy smell, but not sewage. If no sewage smell is detected, remove the cover from the floor drain/catch basin and pour some water into the hole in the floor. If this water is seen draining into the catch basin, than the mystery is solved. In this case, filling the hole with regular concrete patch will end your concerns.
I would suspect that the final scenario is the one you have discovered, but any of the above are possibilities, as well as others that I undoubtedly have overlooked. If you can’t figure this out after following my suggestions, further evaluation by a licensed plumber or CAHPI home inspector should yield the answer.