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french drain and surface drain in one

french drain and surface drain in one

french drain and surface drain in one

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

RE: french drain and surface drain in one

I would say scrap the french drain and lay 6" perforated piping. Probably 2 - 1/2" holes at 60 degrees apart at a spacing of 1 in would take care of that little stream as well drain some of the underlaying water. Its pretty cheap They pipes can also come with a geotextile sock you can just slide over which will protect the pipe for the 10 yr your looking at as long as you back fill min 6" around pipe with gravel to help filter out some of the sand before it reaches and clogs the geotextile material. You should dig to see if the watertable had risen during the recent storms. If thats the case you got a lot more then just a little problem since your house is on a grade and highly likely hood the developers did not compact the soil correctly.

RE: french drain and surface drain in one

thanks for the reply, isn't what you're describing basically a french drain without the geotextile around the gravel?  My fear if not separating the surface water from the bottom of the trench is I don't want to worsen my problem by bringing more water down closer to the footing depth.


RE: french drain and surface drain in one

Yes it is a French drain, sorry. Plastic would work assuming the river rocks don't cause interference and block the path of the runoff or get filled with sediment and back it up over sides of swale, depends on slope and amount of runoff. Perhaps, you can install an area drain near the source of runoff (if runoff can be diverted into one area) and since trenching already for perf. pipe install a pvc pipe to carry the surface runoff. Thus, no swale in yard to deal with or top river rocks to deal with.

RE: french drain and surface drain in one

I would generally agree with your shallow subsurface drain and the surface drain. They could be built in the same trench. As far as your gravel/cobble surface, I would put a filter fabric between the upper sand/gravel collector and the gravel/cobble surface to prevent clogging of the drain due to dirty surface waters. The upper gravel/cobble surface and filter fabric could be easily removed and cleaned or replaced, as needed.

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

thanks for this, this is along the lines of what I was thinking.  I had been curious if those "prefabricated" french drain geomembranes worked.  It seems like this is a good way to use them, to stop water seepage and as a redundancy to extend the life of the drain.

RE: french drain and surface drain in one

Why so complicated guy?

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

thanks, I'll take a look at your other posts.  I was thinking it would be easier to dig one trench.  I also have a lot of trees that are going to be an issue if I have more than one drain spaced too far apart.  I still think I'm going to run a gravel topped french drain 8-10 feet from the garage to intercept surface and groundwater.  I think being that far away, and that being the lowest spot of the property, it won't bring water too close to the footing.  I don't think I'll need to geomembrane the bottom or sides, I was just worried about prematurely clogging the drain due to surface runoff.
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

here's some photos, as you can, my grade to the garage is terrible due to the low door, and the only way to fix that is to cut down more trees, which my wife id dead set against, so an open topped french drain seems to be a better solution.  I drew a little picure of a possible cross section if I put a surface drain right next to it.

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

Oldestguy, If I do go with concrete sand fill, would a 12" wide drain be wide enough?  I don't have a lot of space towards the back of the garage to do a separate surface drain if I go much wider. also, would it be suitible to top with river rocks for looks, or should a sand drain be covered with soil?

RE: french drain and surface drain in one

I don't think we got into the drainage pipe enuff.  Typically these drains are done with black, 3" diam, slotted plastic pipe made from recycled plastic).   These pipes are cheap and you also can get them with no slots.  I don't see the need for a fabric sock on them when concrete sand is the backfill.

 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

I was planning on using the white 4" PVC with 3 rows of holes drilled in them to establish a more uniform grade with the pipe (also for easier clean out if necessary)  I figure if I use sand for this, I would have to use a sock do to the larger holes.  I can't seem to find 3" slotted pipe in my area.  There is 4" slotted corrugated, but it has slits all the way around.
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

I used to call for using clean outs in these systems, but have never had them used. So, not any more.  With the 4 inch pipe that will be fine and with the thin slots all the way around you will get a minimal infiltration of sand fines until coarser particles bridge the.  No need to worry about a constant grade, since some flow will be in the sand in the trench.  Just do the best that you can and it will work.  Like I say, it is difficult to do it wrong as I recommenced.

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

Wow I just posted something just like this a few minutes ago happening to my garage.  Except the path is on the neighbors property.  Very curious what you end up doing.  The neighbor doesn't want to help pay for anything, so I am going for the cheapest route myself since we would be fixing his property.

B+W Engineering and Design
Los Angeles Civil and Structural Engineering

RE: french drain and surface drain in one

Sorry for the late reply, I've been digging during every moment of free time lately.  So far I've regraded to the best of my abilities without cutting down any more trees.  And I've dug a 12" deep trench about 2 feet wide across my yard at the treeline as a surface water swale.  I got some free gabion from someone on craigslist to fill it to keep the kids from falling in it.  That has dramatically cut down the surface water getting close to the garage, but all that digging just makes me notice the groundwater issue even more.  I plan on digging a subdrain with the materials oldestguy recommended in the next few weeks.  Probably about 6 feet out from the garage.

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