Hi! I am in the process of designing a plan for installing a French Drain/Sump Pump System on the outside perimeter of my house foundation. I was hoping for some insight on a few details. Here is the background. Also, see image attached for referance.

Why-
I have moist dirt after winter rains in the crawl space underneath my house. I live in Northern California and have expansive soil which causes the soil to sink and rise. The property had foundation work done and the foundation guy said that I should install a french drain to keep moisture away from the foundation.

- I got a bid from a engineer/contractor and he suggested that I utilize the following:
A) incorporate a closed system from downspouts directing rainwater from the gutters to a gravity bubbler in the front yard.
B) Incorporate a seperate trench and sump pump 2 feet from foundation that is 4 feet deep that runs the length of my foundation.

My questions are:
1) What goes in the sump trench? Rocks on the bottom followed by pipe followed by more gravel, then a fabric liner and top soil?
2) How wide should the sump trench be? I would need to do this by hand and light machine(pull behind trencher) as their is no access for large excavators etc.
3) How close can the closed system PVC be placed next to the foundation without undermining the foundation?
3) Alternativly, Could the closed system be incorporated into the top portion of the other sump trench instead of making two trenches?

Thanks for your input.

 

Fortunately, your engineer/contractor suggested two different systems since the effects of short term rains (a day or two) are relatively quick and can be "dirty" (shingle debris, leaves, needles, etc.) and can put water (and debris that can plug it and force more water into the soil) into your french drain that that is a short term situation and is intended to carry water away, but the effects linger. A french drain is really for the long term, general soil stability and carry water away from deeper foundation to provide a way for the water to be removed and eliminate pressure on a wall that creates crack and leakage.

A real french drain system is meant to be installed just below the level of the bottom of the footing and provide continuous long term reduction of the amount of water in the soil that creates instability/settlement. Since you are planning on 24" outside the foundation wall surface, you will be able to install the perforated (not slotted) PVC at the proper elevation. If you do not go down deep as deep, the long term effectiveness will be lessened.

As far as the material to put under and around the french drain, much depends on the type of natural soil and what you can get. The simplistic and classic answer is "clean crushed rock" is not always good. It the soil around the foundation is relatively fine (dirty sand, silt or silty clay), it is better to use mixture of clean coarse aggregate(3/4" or so) and some clean sand to prevent the water from flushing/washing the surrounding fines into the aggregate filter blanket around and under the perforated PVC. This complication is only needed for severe problems.

Dick

 

Thanks for the input Dick! On the "closed system", my contractor did suggest a clean out to allow for removal of dirt and debris. He also suggested that we use that clean out to be able to discharge the sump water that accumulates in the sump barrel. The soil apears to be dirt more than sand. The contractor suggested the 24" distance from the foundation so as not to undermine it.

 

I would use rigid, perforated PVC pipe for your drain line. It's smooth wall does not clog as readily as corrugated black pipe and if it ever does clog it is much easier to rod/clean out.

I would use a drain sleeve over the pipe to prevent debris from making it inside. At an added layer of protection you can line your trench with landscape fabric, install your drain pipe and stone, and fold the landscape fabric over the top and back fill.

 

I believe my contractor had recommended SDR 35 PVC Solid Lines (not perforated) for the downspout water(see blue lines from image) that would run to my front yard sidewalk. This would be a closed system that would run the water from my gutters out to the street, not in the normal perforated fashion to make sure none of the water seeped back into the soil. I labeled this as the "closed system" in my image. The teal(see image) sump trench would utilize the perforated SDR 35 piped and I will definitely utilize your recommendation of a drain sleeve and trench fabric.

Do you see any problem with running that closed system line in the same trench I build for the sump trench? I thought I could possibly just put it on the top portion above the rocks, piping and gravel, just below the back fill.

 

No problem putting both lines in the same trench. And, I would not worry about which one goes where. Let the elevations and insuring fall determine which goes where.

 

If I plan to dig the sump trench (teal) 4 feet deep, how low will I have to dig the sump area? I would then need a sump liner basin that was at least 4+ feet deep, right? I havent been able to locate one at Home Depot or Lowes. Do I need to modify my sump trench depth to accomodate putting the sump basin at the lowest point?

 

The elevations at your site will dictate everything. At it's highest location the bottom of the French drain pipe should be at or below the top of your footer. So, the two ends of your line that would be your starting point at whatever depth your footer happens to be. Then the drain pipe must slope downhill towards your sump so the entrance point to your sump will be somewhat lower than the starting point for the lines. Then the sump goes down from there to provide a tidal reserve for the pump to work with. Yes, I know what you're thinking... it could get quite deep.

Most home centers only sell basic sump components. There are sumps and risers available but you may have to look to a more professional source. One place to check is your local septic tank supplier/manufacturer. Almost every town has one. They usually carry the risers & lids that you would need. Pre-cast concrete or plastic can be used. I like the plastic because of it's lighter weigh and they usually have knock outs which make it easier for your pipe penetrations.

This is one type that I use often. They are commonly used for septic tanks and the sections just screw together.

 

We estimated that the bottom of the footing was 18" below the surface so I will go below that to create a starting point. Thanks for your reccomendation on the riser source. I will search one out. Thanks again!

 

Normally a "drain sleeve" is not needed. Install the perforated PVC with the holes at 4:00 and 8:00 o'clock so the collection is maximized and is from the bottom the pipe filter system. With the right granular blanket around the pipe, collection at the right elevation is maximized and debris is usually not a problem. Make the perforated pipe level since water seeks it own level and eliminates excessive excavation and debris collection since it is a steady, slow system that works 24/7 even if a pump runs rarely.

Dick

 

I have started clearing out the area where the French drain will go. I haven't figured out one issue. The sump trench will lead the water to the sump basin. The pipe in the sump trench will be perforated. When the pipe gets to the sump basin how will the water be transferred to the basin? I had assumed the sump basin would be solid not perforated so how will the water only get into the bucket?

 

Often the sump is perforated because you do want the water to enter. You will also cut a hole in the side of the sump for the drain pipe to enter. The sump should have a lid to prevent debris from entering so it should only receive water, mostly from underground and the drain pipe.

 

ok makes sense. How many inches of drain rock should there be between the basin and the wall? I will use landscape fabric to insure no dirt gets into the basin.