French Drain Question

[jacobpm]

Steve wanted me to share with all of you a few good questions someone asked him the other day.
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Steve:

From you blog it appears that you are quite knowledgeable about French drains and I would appreciate your answer to the following questions:

1) Does adding tile to the trench (either the perforated ADS or perforated PVC) really increase the flow rate so much to justify its cost and hassle compared to simply filling the trench with washed gravel alone?

2) Do you recommend lining the trench with landscape fabric? Do you recommend using the "sock" over the drainage tile?

Thanks for your help.

David
student of French drains

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David

1) while it is true that stone alone might work, it is better to use a PVC pipe with 3/4" holes facing the bottom. Surround with clean stone.

2) Depending on the soil. If it is “silt-y” sand you may want to line the trench (like a taco) putting stone around the PVC pipe. If it is stone, gravel, or clay do not use any filter.

3) NEVER use a sock around a pipe. Not a good idea.

Thanks, Steve A

[jacobpm]

Many people think that by having more holes that will allow more water through. The smaller the hole, the more the water has to push to displace air, which allows the water in.

For example: fill a 2 litter bottle with water, put the cap on. If you puncture a hole at the bottom, you might get a dribble. Puncture another hole at the top and the air replaces at the top adding pressure, which helps to press the water out the bottom hole.

That's why it's key to have 3/4" holes for entrance in your drain option. This allows for more water to flow, for air to be displaced. The more water that can get in your drain the faster it can be removed.

As mentioned above, this is why you never use a sock. It'll slow the water down from getting into the drain and allowing for a back up to occur. Mesh filters are especially designed to allow water through and keep soils out. There are a huge choice in Geotech soil filters to choose from depending on the soil content under your slab.