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I have never had foundation or ground water problems in my basement but I did have a sewar problem and as part of a recent replacment of that, I had a back flow valve installed that looks like this:

Back flow valve pit

I have been periodically checking it, and recently I found that there were a couple inches of water in the pit. It was perfectly clear with no odor. We recently had a thaw so I figured it was a little bit of ground water flowing under the foundation. I checked it for a week or so after and observed the water level drop.

Today I opened it to find the valve almost completely submerged in water, though the water level was still about 12" below the floor. We had quite a bit of rain the last week, however in order to rule out the valve itself leaking, I tried to shop-vac the water out. I filled my shop-vac completely in a few seconds and water flowed back into the pit, which leads me to beleive that there is quite a bit of water underneeth my basement floor. The water supply enters my house about a foot from this pit.

I do not see any other evidence of moisture anywhere on the basement floor. My biggest concern is that water will fill the pit completely and flood my basement.

Is it normal to find this much water under a basement floor after wet periods and is there a possibility of the pit overflowing, or am I only noticing this now because I have a big hole in my floor?

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How close is the backflow connection to an outside wall? –  HerrBag Mar 3 '13 at 2:11
    
About 2ft from the foundation wall –  Steven Mar 3 '13 at 2:22
    
Any update on the situation? Still too cold? –  HerrBag Mar 17 '13 at 17:10
    
It filled up about 4" more and has held there - I bought a pump in case - I pump it out, and it fills back up but is nowhere near overflowing –  Steven Mar 17 '13 at 19:05

2 Answers 2

up vote 1 down vote accepted

The fact that the water is seen in the pit only after a rain is a good thing, meaning your water table is not the problem, because that would mean a sump pump for sure. I think you are just seeing this because the valve is only 2' from the wall. Normally, it was dissipating. You had a good wall/slab system that wasn't leaking until a hole got poked in the slab for the repair

Some grading of the soil alongside this wall for better runoff might be in order. Any drainspouts nearby?

Some ad-hoc pump automation in the pit would give you some peace of mind until you can get outside for some landscaping.

Self contained sump pump

To verify gutters-to-storm drain are intact and not leaking down at the slab level,
Put a bottle of easter egg dye into the storm drain connection. If you see the color in the pit, I would disconnect the downspouts and direct them into the yard.

A pain for mowing, but the flip up ones work well. In our area, downspouts were connected into the floor drains and thence to the sewer. In the 1990s, we had to cap the connection with concrete or face a fine.

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Right now the water is 12" below the concrete surface, which is about 4" thick so the water is 8" below the concrete. I'm guessing it is distributed throughout the gravel under the concrete of the entire floor (also why it filled back up). What do you think the chances are that it fills the pit and overflows? Wouldn't I also have water coming up through the floor in the corners of the room and against the foundation if it got that high? –  Steven Mar 3 '13 at 3:49
    
How much bailing did you do after the hard rain? Are there localized sources (reverse pitch, downspout, etc), near the pit? –  HerrBag Mar 4 '13 at 12:51
    
It has to be ground water. The gravel is a good distributor for the water, but the pressure isn't high enough or constant enough else it would force up. If your basement slab was done properly, they laid plastic over the gravel as a vapor barrier(VB). The VB would mitigate any ground water in the gravel field. It would have been punctured by the drain work. –  HerrBag Mar 4 '13 at 13:01
    
Downspouts are connected to storm drain. grading could probably be improved but is not terrible –  Steven Mar 4 '13 at 13:08
    
Unless some factor is changed, there will be a rain long enough to overflow the pit. A sump pit would be the usual factor. I put a 4' extension on a customers down spout last fall and stopped her basement water leakage immediately. –  HerrBag Mar 4 '13 at 13:08

i have the same situation after my valve installation. I spoke to another company who said they should have cemented below the valve to stop ground water from pooling in the box as it is a (open) low point. I'm going to give this a shot.
If i were to dig a deep hole in the ground i would expect water to pool in there. So i wonder if this is the same thing happening. I only get water on heavy rain and it has never gone above the valve.

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