french drain and surface drain in one
french drain and surface drain in one
(OP)
I came across this forum searching for information about proper french drain construction. After reading much on the subject I am left with still more questions that I was hoping someone out here could give some insight.
First off, I got a detached Garage built over 2 years ago on my property, and I live on a grade. My house and my neighbors house are both positioned higher up on this land, so I get a tremendous amount of water that comes right down to the front corner of this garage. In an attempt to divert the water, I graded as much as possible and made a swale on the side to take the rain runoff to the back. However, I'm noticing significant effervescence in my slab at the control joints, so I figure I have some underground water getting under there. Also, when I was digging a shallow 4 inch deep ditch about 10 feet uphill from the swale to further divert water, I found an underground "stream" most likely due to significant rainfall that season.
My plan is to dig a 2 foot deep "french drain" where my swale is now (about 8-10 feet from the garage) and fill it with geotextile wrapped gravel and smooth pipe. I also want to use this as a surface runoff drain, so I was thinking of covering the gravel top with 10 mil plastic sheeting, and putting river rock on top of that to prevent the large amounts of surface water from getting down in the drain closer to my footing depth (which is 18 inches).
My questions are, would this be a good way to combine drains, or are two separate drains recommended. If so, how far apart? Secondly, my soil is very sandy, probably silt in there too, but little if no clay, is there a recommended AOS for the fabric I should use to lengthen the life of this drain?
Lastly, should I just scrap this whole French Drain idea and create a deeper, wider swale? The underground water I noticed is my dilemma. The surface drain I dug to divert it worked for about a year, but now I think the stream plugged because water accumulates more in the swale now.
By the way, I want to do this as effectively as I can but money is an issue as well, so I'm not looking to get soil testing or any major engineering done. If this works for 10 years I'd be happy
First off, I got a detached Garage built over 2 years ago on my property, and I live on a grade. My house and my neighbors house are both positioned higher up on this land, so I get a tremendous amount of water that comes right down to the front corner of this garage. In an attempt to divert the water, I graded as much as possible and made a swale on the side to take the rain runoff to the back. However, I'm noticing significant effervescence in my slab at the control joints, so I figure I have some underground water getting under there. Also, when I was digging a shallow 4 inch deep ditch about 10 feet uphill from the swale to further divert water, I found an underground "stream" most likely due to significant rainfall that season.
My plan is to dig a 2 foot deep "french drain" where my swale is now (about 8-10 feet from the garage) and fill it with geotextile wrapped gravel and smooth pipe. I also want to use this as a surface runoff drain, so I was thinking of covering the gravel top with 10 mil plastic sheeting, and putting river rock on top of that to prevent the large amounts of surface water from getting down in the drain closer to my footing depth (which is 18 inches).
My questions are, would this be a good way to combine drains, or are two separate drains recommended. If so, how far apart? Secondly, my soil is very sandy, probably silt in there too, but little if no clay, is there a recommended AOS for the fabric I should use to lengthen the life of this drain?
Lastly, should I just scrap this whole French Drain idea and create a deeper, wider swale? The underground water I noticed is my dilemma. The surface drain I dug to divert it worked for about a year, but now I think the stream plugged because water accumulates more in the swale now.
By the way, I want to do this as effectively as I can but money is an issue as well, so I'm not looking to get soil testing or any major engineering done. If this works for 10 years I'd be happy





RE: french drain and surface drain in one
RE: french drain and surface drain in one
RE: french drain and surface drain in one
RE: french drain and surface drain in one
Attached is a general detail I have used in Colorado for years.
If most of the water appears to be 'high' or surface water, it may be to your advantage to not allow the collected water to seep out of the drain trench. This is a common situation in my arid to semi-arid area where wetting of lower soils could be a problem. Please note the geomembrane on one side (the house side) and that it is placed across the bottom of the trench to make a 'channel' for the drainage.
RE: french drain and surface drain in one
RE: french drain and surface drain in one
Why not separate the two, that way each can be designed and built to most effectively do its job. The underdrain can then be set or the where it will do the most good and discharge to air or a storm drain system.
What ever you do for the under-drain be sure to properly filter the drainage medium.
If you look at many of my posts, you will see I do sub-drains differently that using gravel and fabric.
You can't beat using ASTM C-33 fine aggregation (concrete sand) as a filter and drainage medium and you can't do it wrong. Can't say the same for gravel and filter fabric.
Take a look a my longer post, middle portion, in thread 193-223602
RE: french drain and surface drain in one
Either that, or do a surface drain running parallel about one foot away from the sub drain to capture the silt and cover both with river rock to make it look like a 3 foot wide dry creek.
I've been way too indecisive lately.
As for sand, I read that retains moisture and doesn't have as good of a flow as gravel. My ground is rather sandy about 6 inches down anyway, that's where the water seems to be flowing, so I figured gravel would be a better drain. I do have certain areas on the surface that are clay, but they are patchy and far apart. I was thinking of maybe putting 2-3 inches of sand over the fabric encased gravel before topping with river rock as an added filter.
RE: french drain and surface drain in one
I was also digging around today and re-exposed the concentrated area of ground water flow I have, I was thinking of an interception pit to pipe it off to daylight as well as the other french drain.
RE: french drain and surface drain in one
RE: french drain and surface drain in one
It is very likely the permeability of concrete sand is faster than your on-site sandy soil. Thus, even though gravel has higher permeability, you probably don't need it.
Width of trench should be at least slightly larger than the slotted pipe, to make sure the pipe is surrounded with sand. The theoretical thickness of filter sand over the pipe needed is very small, probably less than an inch. Also, what width shovel you use? I'd suspect that will control the width. The photo looks fine.
The beauty of the sand fill is it is a good filter, you need no fabric.
Leaving the top of a sand filled trench open won't hurt anything, but the top will be plugged in time with dirt. Of course gravel backfill will fill up with dirt unless that is surrounded all sides with a filter fabric - means more work and more cost.
It occurred to me you may be in a place where freezing can occur. Sometimes with these underdrains or sub-drains (I do not use the word French Drain because of so many interpretations), which then means the outlet can freeze up and plug the system in the winter. If serious, some people cover the outlet in winter with straw bales. That helps. Also, running water can create an icy mess in the discharge area. You fix one place and may cause a problem elsewhere.
Finally, why worry abut the slab joint effervescence? I'd only be concerned if I was getting mold or some obvious moisture problem inside the garage, or excessive heaving of the garage in winter. Are the wall sill plates treated wood? Usually the only reason we do these things is to keep water out of a house basement,minimize heaving, etc. Unless I had water coming up out of the slab, rotting of the walls, etc., I'd not worry about effervescence.
It looks like your surface water concerns are the prime concerns.
RE: french drain and surface drain in one
Also,I have a variety of shovels, but I was planning for a 12" wide trench no more than 24 inches deep due to a shallow footing. My main issue is the settlement my garage has experienced, and knowing that there is water getting under the slab, I want to remedy any future issues that might be causing.
I do have a concentrated area I have discovered sub-surface water flow that lasts 2 or more weeks after heavy rain, especially in the winter. This is headed straight to my garage. Although the water flow seems to be only 12-18 inches deep, I want to cut it off before it can potentially cause any more damage. The soil in this area is saturated and seems extremely porous, so that was my concern with thinking gravel would act as a better interceptor.
I live in Virginia, and do have cold winters, but the outlet for this is going to be in the woods behind the garage, and not interfere with anyone else's property.
Being I'm having trouble finding a good Geotextile short of buying a whole 12 foot wide roll, I think you sold me on the sand. Especially if it will not clog with only 4 inches of fill on each side of the 4" pipe. I'm assuming 2" on the bottom.
RE: french drain and surface drain in one
Years ago, Armco Steel sold a corrugated galvanized pipe for under drain use. The perforations were in the lower quarter and 3/16" diameter. Concrete sand worked well there also, since the sand did not migrate easily upwards into the pipe. If you wish to use the rigid stuff, fine, but follow the Armco method, since it worked well. I did a Master's thesis on underdrains in highways and used Armco pipe. Never saw any sand in the pipes. I also verified the use of concrete sand as best. That was recommended by US Corps of Engineers in a study in 1930's.
RE: french drain and surface drain in one
B+W Engineering and Design
Los Angeles Civil and Structural Engineering
http://bwengr.com
RE: french drain and surface drain in one