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Perimeter French Drain

holmie | Posted in Construction Techniques on January 14, 2006 10:56am

During recent heavy rains I have had water accumulating in my crawl space and even into my basement/garage walkout. My lot is lower than the street and slopes further down hill along with the natural terrain. Water is seeping under the upslope footing in the crawl space and making its way to the basement stemwalls and footers. Have looked at the site drainage and found standing water/soggy grass days after the last rains. Am looking into digging a perimeter french drain on the upslope of my house to divert this water away. Excavation would take place about 6-8 feet from my foundation and my question is do I need to dig down at least as deep as the base of my upsope footings in the crawlspace? Common sense tells me that I do to alleviate the groundwater table but I really don’t want to dig that deep 3-4ft if I don’t have to. Any tips from one who has gone this route would be appreciated. The region is Western Nevada.

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  1. toolbear | Jan 14, 2006 12:09pm | #1

    A good French drain should solve your problems if you can dump the water elsewhere.

    Can you dump the water on the downhill side of the lot when you drain it?  Enough relief in elevation and space?

    Had one where the back yard would flood, but he was below street level.  In fact, he was the local low spot.  Laid in a drain line to a dry well.  No complaints since. Nice, considering the soils included blue clay.  Perk?  Not hardly.

    FWIW on dry wells - the way to get the mostest volume for water is to fill the well with cement blocks laid neatly on their sides.  This creates very large void areas for your water. 

    What does your soils profile look like?  You in clay?  Loam?  Got a perched watertable?  If there is hardpan down there, water from uphill is probably migrating along the top of it.  Had one that ran about 50,000 gal. per week in winter rains.

    Don't cheap out on the drain frabric.  You will need to hit up an irrigation supplier for the real stuff - or WhiteCap.  It is a thick, felt-like material and takes a knife to cut.  Doesn't tear.  Excavated an old one last month.  The rock inside was clean and full of water, i.e., working just fine.  Which was nice since it was right across my conduit trench line.  That's why I carry a bilge pump.

     

    The ToolBear

    "Never met a man who couldn't teach me something." Anon.

  2. User avater
    Matt | Jan 14, 2006 02:47pm | #2

    I'd rent a small excavator or, if you plan on moving a lot of gravel, a backhoe.   Be sure and have your utilities marked before digging - as far as I know, this is a free service - or at least it is in the few states that I ave lived/worked in.

    "Daylighted" drain tiles are always preferable to dry wells.   To me, a drywell is a last resort, and quite expensive to build a large enough one.    The thing about dry wells is the only time they fill up with water is when you need them the most.  For example, if you have 1" of rain and are draining 1 acre, you would need a pit of ~ 3600 cu ft or about 26,000 gallons to hold the runoff, and that size assumes an open pit - not one filled with gravel.

    I like using septic paper for filter fabric.  It is a white translucent fiberglass (I think) material.  Maybe $35 for a 4'x150' roll.  They sell it at home centers around here anyway.  We use ~ 1" washed bluestone for drainage systems, however what type of stone products are available is highly regional.

    As far as how deep you have to dig there is no one here who can tell you since we don't know your site and soil conditions.  Best is to go to the level of the bottom of the footings - that's why I'd rent the machine, or at least some laborers.

    As far as >> Excavation would take place about 6-8 feet from my foundation  << we call that a curtain drain.

    Also, if you have gutters and downspouts make sure the downspout water is being deposited well away from your foundation but not into you proposed drain system.  And be sure that when your project is complete you have the soil graded so that you have slope away from your house on all sides.

    BTW - it soundsl like you have a daylight basement?

    1. holmie | Jan 14, 2006 09:45pm | #3

      I do have a daylight basement and plenty of fall to get the water to daylight. It will require a pretty long stretch of pipe to get around my sidewalks and trees though. In your experience which pipe works the best and should one fill the entire trench with gravel or cap it with sod? Local soils are a mix of clay/shale and hardpans. I appreciate your help.

      1. User avater
        Matt | Jan 15, 2006 12:22am | #5

        I would cap it with probably 6 - 10" of loose dirt (not clay) and then sod for aesthetic purposes.    Or, if you didn't mind the look, you could fill all the way with gravel.

        One thing: - You say >> It will require a pretty long stretch of pipe to get around my sidewalks and trees though. <<  I hope you don't end up getting the drain so far away from the house that runoff from the roof, etc won't end up soaking down through the soil and saturating your foundation in spite of all your hard work... 

        Also, you will get a lot of fill out of the excavation, and this is an excellent opportunity to correct grading problems around the house. 

        re pipe type, as far as I'm concerned 4" black corrugated perforated pipe is fine.  Get it in the 100' rolls for about $35 ea.  IMO, in most cases, to install a bunch of rigid pipe is just a waste of money.  A good part of the waste is purchasing the fittings.  You may want to make your drain tails - the part of the pipe that actually emerges from the ground on the low end - out of Sched 30 or Sched 40 PVC, since it is much more crushproof.    The corrugated stuff won't crush form vehicle traffic, etc, as long as it has at least foot or 2 of cover gravel or soil.

        In some areas they use corrugated pipe with a fabric sleave or "sock" on it, but I have found that sometimes, especially in clay, small soil particles can clog the fabric rendering the pipe much less effective or even nearly useless.  

        The way I do footer drains is to dig the trench, line it with fabric, drop in about 2" of stone, install pipe, fill with stone, and then fold fabric over the stone and cap with dirt and sod. 

        It helps to use something to hold the pipe down to the bottom of the trench as you are covering the pipe with stone - like cinderblocks - as the pipe seems to want to "float up" as you try to cover it with stone.  So, you get maybe 4 cinderblocks, put one every 4 feet on top of the pipe, add ~6" of gravel then go down in the ditch and move the cinder blocks to the next section of pipe and repeat.

        Attached are some quick pics that show a little of how I dealt with our down slope lot with respect to grading and drainage.  This situation was a little different as I installed a good footer drain system as the house was being constructed.  It is not a basement, but does have a crawlspace that is about 2.5' toward the front of the house and 7' at the back of the house.

        Happy digging ;-)

    2. Pop | Jan 15, 2006 01:56am | #6

          Hi Matt,

         I have a similar probleml. I wasn't going to go into it until later this spring. I live on a slope with about three or four acres above me. I know you don't want a damp crawl space! "Dig Safe" is free. We all pay for the service through our bills, (elect, water, sewer, and cable TV). But don't trust them if they say, "Your ok to dig", their records are not that great! Get them out there to check anyway! I have given this much thought. My question to you Matt, why not run your roof gutters in your French drain? Why line the trench or wrap perforated drain pipe with a filtering cloth? I understand soil erosion on the surface of the ground, and you would want to cover the top of the crush stone with a filtering cloth. But doesn't the ground its self filter the water going through it? My well does not have a filter bag, or cloth to keep the silt from coming into the homes plumbing. I am also thinking of using Grace's Ice and water shield, sticky side applied to the foundation, with a four inch overlap. I have just built an addition with a three foot crawl space and boy, is it wet. With out the bilge pump running, I have had as much as 18" of standing water. My foundation for this addition goes down 4' 6". I thought of digging the trench at least 4' 6". I'm not sure this is going to cure my water problem. I don't know what else to do, (but now I understand why our home was built on a slab) LOL........Pop

      1. Yersmay | Jan 15, 2006 02:30am | #7

        I know next to nothing about excavation, so I'm curious about something. A builder taught me that it's extremely important to compact the earth when you replace it next to the foundation. He said the foundation would be stressed and possibly crack without equal pressure from either side. Is it okay to compact the earth on top of a French pipe? Is his thinking correct in the first place? Thoughts?

      2. User avater
        BillHartmann | Jan 15, 2006 06:59am | #8

        "My question to you Matt, why not run your roof gutters in your French drain?"Because if the drain ever clogs, for any reason including an animal building a nest in the discharge end, then all of the water from the gutters will back up into the french drain and do just the oposite of what it wanted."Why line the trench or wrap perforated drain pipe with a filtering cloth? I understand soil erosion on the surface of the ground, and you would want to cover the top of the crush stone with a filtering cloth."The ground is not one big lump of dirt, but rather a collection of very small particals. Those particals can and will move. Water moving through the ground will carry them with the water and after a period of time will fill the rocks and/or pipe with silt.

        1. User avater
          Matt | Jan 15, 2006 07:17am | #10

          I was typing at the same time you were...

      3. User avater
        Matt | Jan 15, 2006 07:15am | #9

        >> Why not run your roof gutters in your French drain?  <<  when you say french drain, I'm assuming you mean a footer drain, and if so the idea is to not have water up against the foundation, but if there is to get it away ASAP - hence the drain system.  By adding your roof runoff (via downspouts) you are just compounding the problem by adding more water to an area you want as dry as possible.  Further, if the foundation drain system somehow gets plugged, like by leaves from the gutter drains or silt or whatever, then you would be really sc&#$ er a I mean your house might float away :-).  If the OP is placing a curtain drain at least 10' or more from his house, I guess he could drain his gutters into there, but I still wouldn't connect directly into the french drain pipe - I'd be more apt to drain the gutter runoff on top of the gravel cover. 

        >> Why line the trench or wrap perforated drain pipe with a filtering cloth <<  I'm not saying that it is necessary to use some kind of filter cloth in every instance, but if there is already a problem, or the building project site has the right ingredients for a water problem, I want to put in a fool proof drainage system.  In the case of an open excavation for a newly built foundation, it will cost several hundred dollars to do it right - now, but several thousand dollars to fix it later.  Anyway, the filter cloth prevents soil from migrating into and filling the voids between the gravel thereby crippling it's ability to drain quickly - that is what the gravel is there for - to drain.  Since water moves laterally as well as down in the soil, it is necessary to completely contain the gravel.  And yes, I have dug up drain pipe that was full of silt/muck/sand where the pipe had been covered in gravel. 

        Your point about a well not having any kind of filter fabric is a good one; I don't know much about wells, but aren't they almost always drilled down to broken rock, sand or gravel; below the level of the soil?

        As far as using ice and water shield, I guess you could do that, but they have other similar products that are meant for that application - bichethane (sp?) is one... Why not use the right stuff?

        1. Pop | Jan 15, 2006 10:01am | #12

               Hi Matt,

          I just can't get it out of my head that the earth will filter the water. Any water movement on the surface will erode that surface. As soon as the underground water hits the gravel trench, it drops to the bottom already filtered, there by filling the bottom of the trench, through the holes in the perforated pipe and, run out. I have at this time two small drains on the property, and with the use of a few fitting, have installed some hardware cloth between the fittings to keep out the unwanted ones. I would do the same for the 4 or 6 inch drains that will go in as soon as the ground dries up, I ran out of time last fall. This drain that will go in, will be around 100 feet in length, and that is just in the front of the home. I need to control the water coming down this slope. I do worry about a plug! So just about anywhere the perforated pipe makes a turn, there will be a riser for a clean-out. I was told by an old timer, to use ridge solid plastic pipe and cut slots with an radial arm saw every four inches, and use a declivity board under the level to set the pipe. He also recomend the use of course sand around the perforated pipe, in combonation with crush rock. If the trench if filled with crush rock to the surface, use some kind of filter cloth. There is where you will find the soil particulates moving about. I have heard and read this method before. The more I hear from you folks, the smarter I get. I am thinking that a roll of filter cloth, is probably cheaper and less labor intensive than moving course sand. I like the idea of holding down the pipe with cement blocks, I will do that. ...................Pop

          1. JimB | Jan 15, 2006 03:01pm | #13

            Sounds like you're sold on the use of filter fabric.  So, I just wanted to briefly reply to your comments about soil filtering water.  Maybe I can explain a bit about why the fabric is useful.

            First, you have to realize that the mineral component of soil consists of three constituents: sand, silt, and clay (listed in order of size; clay particles are very, very much smaller than sand particles).  If sufficient water flows through the soil, over time the groundwater flow will transport the clay particles through the sand.  This occurs below the surface, as well as on top of the surface, of the soil. 

            For below surface drainage systems, filter fabric prevents these transported soil particles from being washed into the gravel and the pipe where they will settle and cause clogging.  Cleanouts are a good idea, also, but don't help if the gravel clogs.  Admittedly, the clogging isn't going to happen over night.

            You're correct that wells don't use filter fabric.  However, in some formations, clay and silt can be pulled into the well with water.  This can be prevented by using a well "screen" (a perforated section of casing) and placement of a "gravel pack" in space outside of the well screen.

             

             

          2. User avater
            Matt | Jan 15, 2006 04:21pm | #14

            >> I was told by an old timer, to use ridge solid plastic pipe and cut slots with an radial arm saw every four inches, and use a declivity board under the level to set the pipe. He also recomend the use of course sand around the perforated pipe, in combonation with crush rock. <<  You don't want to put sand in with the gravel for the same reasons discussed above - it will migrate into the perforated pipe.

            Re making perforated pipe with a radial arm saw - well, ok, but why bother when you can buy it for the same price as the solid pipe?  BTW - if you get the kind with holes only on one side, the holes go down.

            Attached is a pic of the type of filter material I like to use for crawl space drain systems.  The reason I like it is that I think it will have the least chance of getting clogged and I know from digging through drainage systems that have been installed for a number of years that it lasts good under ground - pluss it's cheap :-).  Look at the diagram on the bottom of this page: http://www.dewittcompany.com/erosion.html 

            For basements or very severe situations there are better geotextile fabrics available as well as a host of other products all for more $. 

      4. toolbear | Jan 15, 2006 08:17am | #11

        @@  Why line the trench or wrap perforated drain pipe with a filtering cloth?

        Without the whole drain wrapped in filter fabric (they call them "burritos"), the fines will migrate into the rock bed and fill the gaps between them, invade and clog the drain line, etc.  All that work and expense will turn into a plugged drain.

        At Niguel Shores the builder cheaped out, put about a foot of drain rock behind the curbs, installed periodic three inch holes in the curbs and called it a day.  The drain system is now a mixture of rock (impervious to water) and clay (just about impervious) and useless.  Water sheets over the top of it.  That French drain at the Ayres Hotel job was wrapped.  The rock inside was clean and moving water - unfortunately into my trench, which crossed the drain. 

         The ToolBear

        "Never met a man who couldn't teach me something." Anon.

  3. User avater
    SamT | Jan 14, 2006 11:43pm | #4

    Holmie.

    Have looked at the site drainage and found standing water/soggy grass days after the last rains

    To me, that is the key piece of information.

    It's alway easiest, cheapest, and most effective to control runoff before it soaks in. I would first try just putting in swales (shallow ditches, maybe 6 inches deep and 6-8 feet wide) to give the water a means of flowing around your house. I would only put pipes in where I wanted to go under a bit of landscaping I really didn't want to disturb.

    SamT

     

  4. User avater
    Nuke | Jan 15, 2006 05:05pm | #15

    Until recently, were you located in a dominant drought-stricken locality? I have to ask why this has become a problem now and not before. When I first moved into my new construction home ~5 years ago, I had problems with rainwater infiltrating my walk-out basement, which the floor was about 6-7' below the street elevation (for the street-facing side of the basement).

    Initially, water would come in where the poor foundation foots could not exactly hold up portions of the staggered poured-concrete side-walls. Over the course of 15-months they sealed the cracks, regraded appropriately, a couple of 3' retaining walls, drilled some weep holes the South American Masons didn't know to install, etc. and finally I got a dry basement.

    Of course, I then tackled water runoff into my backyard, which led me to bury the backside of the hosues gutters and run them off into storm drains, which was conveniently on one corner of my tiny 1/4 acre lot. All of this was done in an effort and grand resistence by the [so called] builder to not install a french drain.

    1. Pop | Jan 15, 2006 10:40pm | #16

              Hallelujah Brothers! I do beleive! Sometime guy's, we old farts like doing it the old way, "the hard way". Not this guy, not anymore. I like your advice, now, one more question. Would you dig the trench even with the bottom of the footing? Should I worry about moisture getting into the crawl space, by way of going under the gravel and drain pipe? The ground level in the crawl space, (with moisture barrier and gravel, what a laugh) about 16 inches above the bottom of the footing. My thinking, is that at what ever the level of moisture is in the ground, it will stay at that level and pass right under the crawl space, 16 inches below the moisture barrier. At worse condition, it will probably be damp, then its a matter of circulating fresh air in and out of the crawl space.   Many thanks for your informative information, (I will also run another pipe for the gutters) nothing like crossing the "T's" and dotting the " i's ", prior to a project, especially back braking, nad busting, digging! Thank again,..............Pop

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