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Old 10-12-2011, 09:23 AM   #1
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Water under slab in basement


I read the old posting about basement water and have a variation on the problem. There is a sump hole in the floor of the laundry room. Some contractor made the hole for a sump pump, but he cut out part of the bottom of the plastic sump to go over a horizontal drain line below the slab. In a heavy rain, water flows at a very rapid rate, from beneath that horizontal drain pipe and into the hole. The hole can fill half way with 200 gals of water within a matter of minutes. Even after the rain stops, the hole continues to collect water for a day or so. When there was a pump in the hole, it would suck in silt, which ruined the pump. Question: does anyone understand why water would follow the underground pipe (which does have proper pitch) of the main stack into a sump hole? Might there be a void below the pipe which collects water in a rain? Is there any point in reinstalling a pump & sump when it is positioned right over the pipe requiring its bottom to be cut out? Any ideas for a solution to 1. where the water is coming from and 2. how to deal with it? Thanks very much. Charlie

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Old 10-12-2011, 10:21 AM   #2
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Water under slab in basement


Most likely the water table rises when it rains --up to the level of your basement floor. In other words it is likely to be ground water. I assume you have checked and corrected surface water drainage around the house and that the grading is correct so that surface water runs away from the home and that there are gutters with downspouts to direct the collected water away from the foundation.

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Old 10-12-2011, 02:00 PM   #3
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Water under slab in basement


The modern method of installing subfloor or subsurface French drains (weeping tile systems) is to surround the perforated pipe with gravel. The entire trench, pipe and all, carries water. The pipe, being an empty space, usually brings the water to the sump pit faster and acts as an air pocket to desaturate the ground in the vicinity of the pipe (about 2 feet to each side) and in effect lower the water table. (The water table in the middle of the basement floor will also be lowered a little.)

It does not matter whether the end of the drain pipe fits loosely letting water on the outside of the pipe also enter the sump (pit), other than not wanting large amounts of silt to enter the sump also. If the pipe is sealed into the side of the pit and there are no perforations further down in the pit sides or bottom then water below the level of the bottom of the pipe will stay behind in the trench and that is the lowest that the water table will be artificially lowered by the French drain system.

If the sump pump doesn't empty the pit fast enough and the drain pipe tends to remain full of water, the lowering of the water table does not happen. This can lead to not stopping any flooding at the far corner or side of the basement even if the sump is not overflowing onto the floor.
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Last edited by AllanJ; 10-12-2011 at 02:08 PM.
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Old 10-12-2011, 03:25 PM   #4
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Water under slab in basement


Place a piece of filter fabric in the bottom of the sump and add 6" of large crushed stone (minimum 1" size). Install the sump pump on top of the stone. That will prevent the pump from sucking mud.
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