Basement in high water table

[Gust1]

Hi, I'm an unlucky homeowner with a finished(tile) basement that has a high water table from an irrigation ditch that flows in the summer. The water table is a foot below our poured cement slab. We have a layer of 1-2" gravel under the slab with an airspace on top of gravel. We have a working exterior footer/french drain that empties to daylight (downstream) at road in front of house. The only time the basement flooded slightly was when the footer drain outlet was clogged with leaves. We have a deep interior closed sump pit in basement that is only draining an exterior stairwell and and a furnace room.The house is 50 years old.
Can anyone out there in engineering land help us brainstorm options to help us stay dry? We are thinking along the lines of drilling holes on bottom of closed sump, adding several continuous run low flow sumps, adding outside sump, taking scuba lessons.

 

[oldestguy]

What ever you do keep in mind the principles of filtering. For instance if the ground is somewhat sandy and your drain is a "French Drain" consisting of clear gravel, water flowing to an outlet or pump can carry along fines. In time that can plug the system and even undermine a building.

Assuming that ditch water is what is feeding ground water to the house, consider a man hole outside with a sump pump in it discharging well away from the building. Feeding to that manhole would be perforated pipes in a trench (dug and then filled) that intercepts that water before it gets to the house. Backfill around that perforated pipe should be a filter material. While some here may call for clear gravel that is enclosed in a filter fabric, I have 100 percent success with the following. The backfill is a filter made from the fine aggregate used in concrete, ASTM C-33. The pipe has holes on the bottom quarter 3/16" diameter. The concrete sand can be dumped in over the pipe. In case of trench sides caving in as you dig, you can keep ahead of that caving by that method. Whereas with a caving trench and fabric surrounding gravel, good luck,.

Any sump pit with holes and no filter could work, if somehow that nearby soil is well graded sand and gravel, but otherwise it may bring along soil and cause undermining.

I've used the "moat around a castle" term for the cut off drains. It does not involve messing up the inside of the building, but you have to dig sufficiently low to be effective.

 

[oldestguy]

Instead of perforated pipe, you can use wrinkled plastic flexible pipe with 1/16" slots. If you want to get that with a fabric sock on it. However, you still need a filter backfill at least around the pipe and up far enough to reach the water table. If you fill the trench with clear gravel, that may plug up and the system will fail. Also, those unfilterd fines can build up over that sock at the slots and then failure also. (That has been found at failure sites).

To construct, you can feed the pipe into the trench behind the excavator and immediately backfill as you work along. Much better in caving ground than the fabric around the rock system.

 

[hokie66]

Any chance of lining the irrigation ditch in the vicinity of your house? That might prevent a lot of the infiltration which is affecting you directly.

 

[Gust1]

Thanks for interesting ideas! I wish we could line the ditch.. The ditch company lined a portion a few years ago(bentonite) and neighbors became stressed about wells drying up(aquifer at 30ft).. The geology here(colorado) is alluvial valley with average amount of clay and fine sediment.. The water under the basement slab is clear. We opened up a radon vent (that was slurping water!)to look down. Last summer we dropped a pump hose into Radon hole and pumped water for a week before the depth of the clear water went down (from 6 inches deep to 4 inches).. And after pumping for a week the French drain outlet went from constant stong flow to no flow.. The top of water table seems to stay below the bottom of the slab by 6-8 inched.. But still makes us nervous.. We have lived here for 18 years so this problem is not new.. We would like to try something sump related.. But don't want to make things worse!

 

[Gust1]

Also the water flowing out of the French drain by road is clear and cold. This drain has been working well for 18 years.. We just had it scoped last week.. It is ceramic and in good condition ..

 

[oldestguy]

OG again.

Sounds good, but with any fix keep the filter business in mind. A reference on this may be found at the NAVAC web sites under DM (design manuals).

I'll try to attach the figure showing information on proper filters.

You may see clear water, but the proof would be seeing no"delta" at the pump discharge.