Clearing A Clogged French Drain


Old 03-26-07, 06:48 PM
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Question Clearing A Clogged French Drain

We have significant water infiltration into our crawl space through the retaining wall separating the crawlspace from our garage when it rains. There is also significant wetting of the concrete slab surface in the garage. I checked the drawings for our townhouse and there is supposed to be a waterproof membrane on the garage side of the retaining wall with 18 inches of drain rock and 4 inch perforated pipe to remove water from the base of the retaining wall.

Paste the picture link below to see a copy of the retaining wall drawing I color coded and scanned in.

In the driveway in front of our garage I found a pit that I cleared sand from which revealed what I believe is a cleanout for the French drain system located under the garage slab at the base of the retaining wall. Based on the age of the townhouse (25 years) and the obvious amount of hydrostatic pressure pushing against the retaining wall and concrete garage floor slab, I believe the 4-inch drainpipe in clogged from silt.

Paste the picture links below to see location of French drain cleanout pit.

View Of Pit:

Closer View:

Inside Pit:

I have no experience with French drains, but I would think there would be more than one cleanout so you could approach the clog from both sides. Can anyone explain the proper procedure to clear a French drain?

Thank you for taking the time to read my post.
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Old 03-26-07, 07:17 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
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This is not my area of expertise but that pit looks it has a 'backwater" valve. A backwater valve is a check valve that prevents a reverse flow of water in drainage systems.

The cap should unscrew. Look inside and see if it is plugged with silt. Do this the next time it rains and see if water is flowing and if it is flowing back from the street side. It is possible that it is the outlet at the street (or storm sewer if you have one) that is clogged.

Or, if this a backwater valve it might just be stuck in the closed position.
Old 03-27-07, 10:06 AM
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I did a little reading on backwater vales and I did not see any application where they are used for French drains. I see they are used for sanitary drains, but French drains are not allowed to connect to sanitary drains where I live.

Not sure how I will remove the cap. Itís to far down to get a good grip on it. Maybe I can build a tool with an extension to help me remove it.

Thank you for your input.

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