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Merging French Drain with Solid Drain Questions

Merging French Drain with Solid Drain Questions

Merging French Drain with Solid Drain Questions

I’ve a few questions about French Drains I can not find answers to anywhere… yet. Hoping you can help.

I am running underground solid and French drains around nearly all four sides of my house. All four sides will get a solid drain for downspouts and some surface drains, but part of the front of the house, one full side and part of the back will have a French drain – the area where subsurface water accumulates due to being downhill from my neighbors. The French Drain will be 4” perforated pipe, holes down, in 12 inch wide gravel trench, appropriately deep. The pipes will then run out my front yard, about 50 feet ling, to the street. Assume I have the slope correct to gravity flow everything. My questions are:

1. Do I have to drain each pipe separately, or at some point, can I merge my perforated pipe into my solid pipe?

2. If I merge my pipes, do I have to worry about water building up in the trench after the merge? Where is the best place for the Merge?

3. Where both solid and French share the trench, do I run these pipes side-by-side or can I put the Solid on top of the Perforated pipe? Does it matter?

Thanks in advance for any info

RE: Merging French Drain with Solid Drain Questions

First, do some looking at other threads dealing with "French Drains". There you may find my comments about gravel, which is likely to plug up with any significant flow of water into them. Use ASTM C-33 fine segregate (concrete sand).

Next consider the time that there is a significant rainfall and the downspouts are running full with some pressure head on them. Connecting st any place to the perforated pipes will then be an outlet for those roof drains, in part. Then you will have water coming in backwards to your "French Drains", feeding your basement walls with water. I suspect you won't like the result.

This then tells of the requirement for never connecting the two together no matter where in the system.

In a firm where I worked, we had a case this type of thing was done to dispose of roof water, supposedly saving two systems, a decision of an architectural firm, not us. Unfortunately this was a place where high plasticity clay underlaid the building and the roof water followed backfill to utility trenches into the building and boy did those floors heave up. Very difficult job to correct.

RE: Merging French Drain with Solid Drain Questions

no, you do not want water to back up in the perf drain. but, depending on hydraulics, you can outfall the subdrain to the surface drain line as long as the storm drain line HGL is lower. the connection should be made in a catch basin or manhole where you can bring the storm drain in lower.

RE: Merging French Drain with Solid Drain Questions

The first two responses are spot on, to add:

I would consider not running the perf pipe all the way to the road. Once you get past the area near the house you are trying to drain/dewater you could consider changing to a solid pipe, with a minimum 2% slope. This will be less expensive, will increase flow out of the pipe and will make it easier to connect to downstream storm sewer (if necessary). But as stated above, you do not want to combine this pipe with the other solid pipe, but you could put in the same trench.

RE: Merging French Drain with Solid Drain Questions

Also, if you drain the perf line to a ditch, do not set the discharge level of the outfall pipe at the bottom of the ditch, but at a higher level to avoid most of any backflow in the perf system from the ditch drainage.

Mike McCann
MMC Engineering

RE: Merging French Drain with Solid Drain Questions

Thank you for all the specifics. It is very helpful!

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